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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Welcome to the Tooling University. This course is designed to be used in conjunction with the online version of this class. The online version can be found at http://www.toolingu.com. We offer high quality web-based e -learning that focuses on today's industrial manufacturing training needs. We deliver superior training content over the Internet using text, photos, video, audio, and illustrations. Our courses contain "roll -up-your-sleeves" content that offers real -world solutions on subjects such as Metal Cutting, Workholding, Materials, and CNC with much more to follow. Today's businesses face the challenge of maintaining a trained workforce. Companies must locate apprenticeship programs, cover travel and lodging expenses, and disrupt operations to cover training needs. Our web -based training offers low-cost, all -access courses and services to maximize your training initiatives.

 

Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed. Copyright © 2013 Tooling U, LLC. All Rights Reserved.

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Class Outline Objectives Setting Up a Centerless Grinder Preventing Lobing Workpiece Rounding Effect Selection of Work Rest Blade Setting the Work Rest Blade: Height Above Center Setting the Work Rest Blade: Height Above Lower Slide Throughfeed Grinding and Work Rest Blade Height Regulating Wheel Angle of Inclination Setting the Angle of Inclination Regulating Wheel and Workpiece Feed Rate Maintaining Consistent Part Diameter: Work Guides Setting Work Guides on Regulating Wheel Side Setting Work Guides on Grinding Wheel Side Truing the Grinding Wheel Setup for the Regulating Wheel Truing Angle Setup for Offsetting the Diamond Dresser Truing the Regulating Wheel Summary

 

Lesson: 1/19 Objectives

 

Lesson: 2/19 Setting Up a Centerless Grinder

 

Lesson: 3/19 Preventing Lobing

 

Lesson: 4/19 Workpiece Rounding Effect

 

Lesson: 5/19 Selection of Work Rest Blade

 

Lesson: 6/19 Setting the Work Rest Blade: Height Above Center

 

Lesson: 7/19 Setting the Work Rest Blade: Height Above Lower Slide

 

Lesson: 8/19 Throughfeed Grinding and Work Rest Blade Height

 

Lesson: 9/19 Regulating Wheel Angle of Inclination

 

Lesson: 10/19 Setting the Angle of Inclination

 

Lesson: 11/19 Regulating Wheel and Workpiece Feed Rate

 

Lesson: 12/19 Maintaining Consistent Part Diameter: Work Guides

 

Lesson: 13/19 Setting Work Guides on Regulating Wheel Side

 

Lesson: 14/19 Setting Work Guides on Grinding Wheel Side

 

Lesson: 15/19 Truing the Grinding Wheel

 

Lesson: 16/19 Setup for the Regulating Wheel Truing Angle

 

Lesson: 17/19 Setup for Offsetting the Diamond Dresser

 

Lesson: 18/19 Truing the Regulating Wheel

 

Lesson: 19/19 Summary

 

Class Vocabulary

Setup for Centerless Grinders 320

l Describe centerless grinding.

l Describe lobing.

l Describe workpiece rounding.

l Describe rest blade selection.

l Calculate work rest blade height above center.

l Calculate workpiece positioning above the lower slide.

l Describe the work rest blade height during throughfeed grinding.

l Describe the regulating wheel angle of inclination.

l List the steps for adjusting the regulating wheel angle of inclination.

l Describe the relationship between the regulating wheel and workpiece feed rate.

l Describe how work guides affect workpiece diameter.

l Explain how to set work guides on the regulating wheel side of the machine.

l Explain how to set work guides on the grinding wheel side of the machine.

l List the steps for truing a grinding wheel.

l Describe the truing angle for the regulating wheel.

l Describe offsetting the diamond dresser for the regulating wheel truing unit.

l Describe devices used to true the regulating wheel.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Figure 3. A work rest blade made of tungsten carbide, a harder material.

Centerless grinding is a common type of cylindrical grinding. Rather than mounting the cylindrical part between centers or in a chuck, centerless grinding involves a grinding wheel, regulating wheel, and work rest blade. These components hold the workpiece horizontally, rotate it, and grind its surface, as shown in Figure 1. Centerless grinding is capable of achieving very tight tolerances with little difficulty. The process of generating round parts with tight tolerances depends primarily on accurate setup of the machine. An accurate setup of the main components of a centerless grinder helps ensure the following workpiece elements:

l Workpiece roundness. Roundness is the degree to which all points along the surface of a

circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder.

l Consistent workpiece diameter. Diameter is the length of a straight line as it passes from

one point on the edge of a circle through the center of the circle and terminates at a point directly opposite the beginning point.

l Accurate workpiece tolerance. Tolerance is the degree to which a workpiece meets its

specified dimensions. "Tight" tolerance involves accepting a smaller deviation from the specified dimension.

l Appropriate workpiece finish. Finish is the degree of smoothness of the part surface.

Once the grinder is set up for making one group of parts, operation is consistent and rarely changes between parts. To make a new part, the components of the centerless grinder must be set up properly. This class will teach you how to set up the main components of a centerless grinder, how to accurately position the workpiece on the work rest blade for various centerless grinding operations, and how to true the grinding and regulating wheels.

    

Figure 1. A typical centerless grinding configuration.

Workpiece roundness is a primary goal of centerless grinding. Roundness is the degree to which all points along the surface of a circular or cylindrical workpiece are equidistant from the center of the circle or the axis of the cylinder. Lobing, or deviation from part roundness, is best prevented by proper setup of the centerless grinder. To prevent lobing, as you set up the grinder:

l Monitor the position of the work rest blade. Typically, the work rest blade is placed above the

center of the grinding and regulating wheels by ½ the diameter of the part being ground. l Pay close attention to the angle of the work rest blade. Typically, the top surface of the blade

is angled at approximately 30 degrees. Figure 1 illustrates an angled work blade.

l Monitor the rotational speed of the workpiece. The rotational speed of the regulating wheel

determines work speed, or the workpiece rotational speed, as well as workpiece feed rate. Generally, the faster the regulating wheel rotates, the faster the part rounds.

    

Figure 1. Correctly angling the top surface of the work rest blade helps prevent lobing.

When the top surface of the work rest blade is horizontal and positioned so that the centerline of the workpiece is parallel to the centerline of the grinding wheel and regulating wheel, the surface of the wheels and the flat surface of the work rest blade form three sides of an imaginary square, as illustrated in Figure 1. In this configuration, as the workpiece rotates, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite low, or concave, spot to be ground by the grinding wheel, as illustrated in Figure 2. This causes workpiece lobing. To counteract the tendency of parts being produced "out of round," you should elevate the workpiece above the centerline of the grinding and regulating wheels. Elevating the workpiece reduces pressure on the workpiece during grinding so that, as the part rotates, the high and low spots produced as the part contacts the regulating and grinding wheels are not opposite each other. This means that the high spot on the workpiece is not diametrically opposite the low spot onthe workpiece and is of lesser magnitude. This constantly decreasing magnitude of error yields gradual rounding of the workpiece.

    

Figure 1. Positioning a flat work rest blade on center forms three sides of an imaginary square.

Figure 2. Under this configuration, any high spot on the periphery of the workpiece that contacts the regulating wheel causes a diametrically opposite concave spot to be ground by the grinding wheel.

The first step in setting up a centerless grinding machine is selecting the proper work rest blade. Selecting the correct blade material helps ensure part roundness, a fine finish, and the most efficient removal of workpiece material per each grinding pass. When selecting a work rest blade, consider the following factors:

l Blade material. The four most common blade materials include aluminum bronze, chilled iron,

high-speed steel, and tungsten carbide.

l Blade thickness. Generally, the thickness of the blade is slightly less than the workpiece

diameter.

l Blade length. The length of the blade is determined by the diameter of the grinding and

regulating wheels.

l Blade angle. For most centerless operations, the top surface of the blade is angled 30

degrees.

When choosing blade material, always start with a hard material. Figure 1 shows a work rest blade composed of tungsten carbide, a hard material. If the blade is too hard for the operation, scoring, or pick-up, may occur. Scoring occurs when excessive pressure from the work rest blade on the workpiece causes tiny chips to fuse to the blade. If you notice scoring during the operation, choosea blade made of a softer material. This should help eliminate scoring. Additionally, when choosing the blade angle, make sure you choose a smaller blade angle for larger workpiece diameters.

    

Figure 1. A work rest blade made of tungsten carbide, a harder material.

To ensure roundness of a part, you must place the work rest blade so that the centerline of the workpiece is above the centerlines of the grinding and regulating wheels. Grinding too high above the wheels' centerline causes the wheels to squeeze the workpiece upward, pushing it off the work rest blade, often resulting in chatter marks. Using a grinding wheel with a softer grade helps eliminate the chatter by reducing the cutting pressure exerted on the workpiece. Grinding too far below the wheels' centerline exerts excessive pressure on the periphery of the workpiece, which usually results in a triangular workpiece, as illustrated in Figure 1. The diameter of the workpiece is a key factor in accurately positioning the work rest blade for a specific centerless operation. For smaller workpieces that have a diameter of approximately 1 in. (2.5 cm) or less, the work rest blade should be positioned so that the centerline of the workpiece isabove the centerlines of the grinding wheel and regulating wheel by a distance equal to approximately 1/2 of the workpiece diameter. For example, for a workpiece with a diameter of 1 in., the work rest blade should be positioned 0.5 in. (1.27 cm) inch above the wheels' centerline. For larger workpieces that have a diameter of greater than 1 in., the work rest blade is typically set 1/2 in. above the centerline of the wheels.

    

Figure 1. A part's triangular shape can result from grinding too far below the centerline of the grinding and regulating wheels.

Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. As a result, in addition to calculating the distance the centerline of the workpiece must be positioned above the centerline of the wheels, you must also calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide. Figure 1 illustrates the lower slide on a centerless grinder. Most manufacturers of centerless grinders specify the distance from the centerline of the regulatingand grinding wheels to the top of the lower slide in a technical manual. If you cannot find this information, ask your supervisor. Once you know the distance from the centerline of the wheels to the top of the lower slide, the next step in correctly positioning the work rest blade is to add the workpiece centerline height above the centerline of the regulating and grinding wheels to the distance from the centerline of thewheels to the top of the lower slide. For example, if the workpiece adjustment height above center is 0.5 in. and the distance from the centerline of the wheels to the top of the lower slide is 9.275 in., the centerline of the workpiece must be 9.775 in. from the top of the lower slide.

    

Figure 1. You must calculate the distance that the centerline of the workpiece must be positioned above the top of the lower slide.

Throughfeed grinding sometimes requires additional adjustments to various machine components to ensure accurate grinding of a part. For example, when grinding long work of a single diameter, such as long steel bars, the part sometimes has kinks or bends along its length, asshown in Figure 1. To straighten the part by eliminating these kinks or bends, you must often position the work rest blade so that the centerline of the part is below, rather than above, the centerline of the grinding and regulating wheels. Positioning the part below center increases the pressure exerted on the part by the grinding and regulating wheels. This increased pressure holds the part firmly on the blade, which assists in straightening the part. Straightening the part helps to eliminate whipping, or chattering, which is often caused by kinks or bends in a part.

    

Figure 1. Positioning the work rest blade below center helps to eliminate kinks or bends in a part.

During throughfeed grinding, the part must both rotate and move in an axial direction. The regulating wheel angle of inclination determines workpiece feed rate by providing the thrust that causes through feed, or axial workpiece movement. The regulating wheel angle helps provide workpiece axial thrust. This angle forms as the regulating wheel swivels about a horizontal axis relative to the axis of the grinding wheel spindle. This angle drives the part past the wheels. Increasing the regulating wheel angle increases workpiece feed rate, while decreasing the angle decreases workpiece feed rate. For most throughfeed grinding operations, the regulating wheel is set at a 3° angle. Unlike throughfeed grinding, infeed grinding requires very little axial workpiece movement. During infeed grinding, the angle of inclination holds the workpiece firmly against the end stop. As a result, the angle of inclination for infeed grinding operation rarely exceeds 0.25 degrees. Figures 1 and 2 compare regulating wheel angles for throughfeed grinding and infeed grinding.

    

Figure 1. The regulating angle for throughfeed grinding is typically set at 3°.

Figure 2. The regulating wheel angle for infeedgrinding rarely exceeds 0.25°.

To adjust the angle of inclination for both throughfeed and infeed grinding, follow these steps:

1. Loosen the regulating wheel housing clamp bolt. 2. Loosen the regulating wheel housing clamp screw. 3. Adjust the angle of inclination using the regulating wheel housing swivel screw, located at the

rear of the grinding machine.

Following these steps enables you to set the angle of inclination at any angle between 0° and 8 degrees. Increasing workpiece feed rate assists in straightening longer parts of a single diameter. Increasingthe rate at which a part travels past the grinding and regulating wheels helps to eliminate bends in the part. Figure 1 shows a bend in a longer part. To straighten a longer part by increasing workpiece feed rate, set the angle of inclination between 5° and 6 degrees.

    

Figure 1. To straighten a longer part by increasing workpiece feed rate, set the regulating wheel angle of inclination between 5° and 6°.

The regulating wheel rotational speed helps determine workpiece rotational speed and feed rate. Forboth throughfeed grinding and infeed grinding, the regulating wheel rotational speed typically ranges from 12 to 100 revolutions per minute (rpm). The average regulating wheel speed is typically set between 22 and 39 rpm. To adjust the regulating wheel rotational speed, move the lever located on the gear box. Throughfeed grinding requires axial workpiece movement. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. When d equals regulating wheel diameter, N equals regulating wheel rotational speed, and α equals regulating wheel angle of inclination, use the following formula to calculate workpiece feed rate (Figure 1): Workpiece feed rate = pi x d x N x sin α For example, if d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm, as shown in Figure 2.

    

Figure 1. This formula calculates workpiece feed rate.

Figure 2. If d equals 3, N equals 30, and α equals 2, then workpiece feed rate equals 9.8649 ipm.

Once the work rest blade is positioned at the necessary height prior to throughfeed grinding, work guides ensure that the workpiece travels in a straight line. Work guides prevent the workpiece from moving too far toward either the regulating or grinding wheel during the operation. Work guides are mounted to the work rest and located at the front and rear sides of both the regulating wheel and grinding wheel. To ensure that a workpiece maintains a consistent diameter, the work guides must remain accurately aligned with the work rest blade. Incorrectly setting the work guides may cause a tapered workpiece, a hollow-shaped workpiece, or a barrel-shaped workpiece. The placement of the work guides affects workpiece diameter in the following ways:

l When work guides at the entrance side push the workpiece toward the grinding wheel, the

workpiece is tapered at the front end.

l When the work guides at the exit side push the workpiece toward the regulating wheel, the

workpiece is tapered at the back end.

l When the work guides at both the entrance and exit sides push the workpiece toward the

regulating wheel, the workpiece tends to be barrel-shaped.

l When the work guides at either or both the entrance and exit sides push the workpiece

toward the grinding wheel, the workpiece is hollow-shaped (Figure 1).

During throughfeed grinding, the regulating wheel face must have the correct shape. That is, the wheel face must be flat, rather than concave or convex. Generally, if the regulating wheel has a concave face, the workpiece is barrel-shaped. If the regulating wheel has a convex face, the workpiece is hollow-shaped. Figure 2 illustrates the hollowing effect a convex regulating wheel face would have on a workpiece.

    

Figure 1. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 2. A convex regulating wheel face produces hollow-shaped parts.

The setup of the work guides in relation to the regulating wheel plays a key role in successful throughfeed grinding operations. The work guides must be accurately aligned with the regulating wheel face. Rough grinding operations involve cutting the part with little regard for part surface finish. Duringrough grinding, the bearing surface of the work guide by the front, or entrance side, of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to1/2 the amount of stock to be removed in one grinding pass, as illustrated in Figure 1. The work guide at the rear, or exit side, of the regulating wheel must be squared, or directly aligned with, the regulating wheel, as illustrated in Figure 2. Finish grinding operations emphasize tight tolerances and smooth surface finish. To ensure accurate tolerances and proper finish, less workpiece material is removed during each pass. Generally, between 0.0002 in. and 0.0003 in. (0.0051 mm and 0.0076 mm) of material is removed per finish grinding pass. To prevent grinding of excess workpiece material during finish grinding operations, the work guidesmust be set so that the workpiece slides past the regulating and grinding wheels without touching either the regulating wheel or the work guide. Use a standard feeler gauge to ensure that there is sufficient clearance for the part to pass through without touching the regulating wheel or work guide.

    

Figure 1. During rough grinding, the bearing surface of the work guide by the front of the regulating wheel must be set behind the regulating wheel face by an amount approximately equal to 1/2 the amount of stockto be removed.

Figure 2. During rough grinding, the work guide at the rear of the regulating wheel must be squared with the regulating wheel.

During throughfeed grinding, you must set work guides located on the side of the grinding wheel as well as the regulating wheel. Since the grinding wheel actually grinds the part, work guides on the grinding wheel side have less of an effect on workpiece diameter and surface finish than do work guides on the regulating wheel side. Work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a consistent diameter. To allow the workpiece sufficient clearance during a pass, the work guides must be set slightly behind the grinding wheel face. Typically, both the front and rear work guides on the grinding wheel side are set the same distance behind the grinding wheel face. The distance the front and rear work guides are set behind the grinding wheel face depends on the workpiece diameter before grinding. Typically, the work guides on the grinding wheel side are set between 0.02 in. to 0.03 in. (0.5mm to 0.76mm) behind the grinding wheel face, as illustrated in Figure 1.

    

Figure 1. Typically, the work guides on the grinding wheel side are set between 0.02 in. to0.03 in. behind the grinding wheel face.

For grinding straight, cylindrical workpieces, a standard truing unit is used to true a grinding wheel. Figure 1 shows the standard truing unit used to true a grinding wheel. For form grinding, aprofile truing attachment is typically used to true a grinding wheel. Both a standard truing unit and a profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. Figure 2 shows a diamond used to true a grinding wheel. Follow these steps to true a grinding wheel:

1. Pull the lever downward to make the diamond pass over the wheel toward you. 2. Stop the diamond in the middle of its first pass across the wheel. 3. Adjust the dial on the truing device until the diamond just barely touches the wheel. You can

view the diamond through a hole on the wheel guard cover. 4. Turn on the coolant.

For each pass of the diamond over the wheel during truing, do not remove more than 0.001 inch ofwheel material. The truing tool rate of traverse, or rate at which the diamond or metallic dresser travels across the wheel, is typically controlled by a hydraulic feeding mechanism. The accurate rate of traversedepends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for rough grinding is 10 to 20 inches per minute (ipm), while the rate of traverse for finish grinding is 4 to 7 ipm. When using a metallic dresser, the rate of traverse for rough grinding is 40 to 60 ipm, while the rate of traverse for finish grinding is 10 to 20 ipm. Figure 3 summarizes these guidelines.

    

Figure 1. A standard truing unit for a grinding wheel.

Figure 2. A diamond used to true a grinding wheel.

Figure 3. Truing tool rates of traverse for a grinding wheel.

During a centerless grinding operation, the workpiece must make contact with the entire width of the regulating wheel face as the workpiece passes between the grinding and regulating wheels. The regulating wheel truing device creates this complete line of contact by truing the regulating wheel sothat the truing tool follows the same path across the regulating wheel as the workpiece. The truingangle and centerline of the workpiece in relation to the centerline of the grinding and regulating wheels are two key factors that create the workpiece complete line of contact with the regulating wheel. During truing of the regulating wheel, the truing tool swivels to form an angle that matches the angle of inclination. For example, if the regulating wheel is angled so that it is tilted 4° in the clockwise direction, the truing tool must also be angled so that it tilts 4° in the clockwise direction. To create a complete line of contact between the regulating wheel face and workpiece, the truing device swivels to form an angle that matches the angle of inclination. This truing angle creates a slightly concave regulating wheel surface, as shown in Figure 1. The concave regulating wheel surface compensates for the regulating wheel angle of inclination, which creates a complete straightline of contact between the face of the regulating wheel and workpiece.

    

Figure 1. The truing angle creates a slightly concave regulating wheel surface, which compensates for the regulating wheel angle of inclination.

To create true roundness during centerless grinding, the workpiece is positioned above the centerline of the grinding and regulating wheels. Due to the above center position of the workpiece,the truing angle creates a line of contact only on the front portion of the regulating wheel. To raise the line of contact made during truing so that it matches the line of contact made during grinding, the truing tool should be offset by an amount approximately equal to the distance of the part above the centerline of the wheels. Offsetting the diamond dresser in this way shifts the deepest part of the concave regulating wheel surface to the front of the regulating wheel, so that the front of the regulating wheel is smaller than the back of the regulating wheel, as illustrated in Figure 1. With the front of the regulating wheel smaller than the back, the line of contact during truing moves up to match the line of contact during grinding. When D equals regulating wheel diameter, d equals workpiece diameter, and H equals height of workpiece centerline above the centerline of the regulating and grinding wheels, use the following formula to calculate the diamond offset (Figure 2): Diamond offset = D x H / D+d For example, if D equals 4, H equals 7, and d equals 3, the diamond offset should be 4 inches, as shown by the equation in Figure 3.

    

Figure 1. Accurately offsetting the diamond dresser shifts the deepest part of the concave regulating wheel face to the front of the regulating wheel.

Figure 2. Formula for calculating the diamond offset.

Figure 3. If D equals 4, H equals 7, and d equals 3, then the diamond should be offset 4 inches.

After you have set the truing angle and offset the diamond dresser, you must true the regulating wheel. Truing the regulating wheel serves three main purposes:

l Controlling workpiece size.

l Controlling workpiece rotational speed.

l Controlling workpiece feed rate.

The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece. Figure 1 shows an angled truing unit on a regulating wheel. When grinding straight, cylindrical workpieces, a standard truing unit is used to true a regulating wheel. Figure 2 shows a standard truing unit used to true a regulating wheel. For form grinding, a profile truing attachment is typically used to true a regulating wheel. Both the standard truing unit and profile truing unit typically uses a diamond dresser to true the face of a regulating wheel. Metallic dressers are usually not used to true regulating wheels. The standard truing unit to true a regulating wheel is manually operated. For each pass of the diamond over the wheel during truing, do not remove more than 0.001 in. of wheel material. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism. The accurate rate of traverse depends on both the type of truing tool and desired workpiece finish. When using a diamond dresser, the rate of traverse for both rough grinding and finish grinding is 1 to 2 ipm. Figure 3 shows these guidelines.

    

Figure 1. When the truing tool for the regulating wheel is properly angled, the diamond follows the same path as that of the workpiece.

Figure 2. A standard truing unit for a regulating wheel.

Figure 3. Truing tool rates of traverse for a regulating wheel.

Workpiece roundness is a primary goal of centerless grinding. To ensure workpiece roundness, monitor the position and angle of the work rest blade, as well as the rotational speed of the workpiece. When selecting a work rest blade, consider the blade material, thickness, length, and angle. Both the part's position above the centerline of the grinding and regulating wheels and the part's position above the top of the lower slide are key factors that assist in part roundness. During throughfeed grinding, position the work rest blade so that the centerline of the part is below the centerline of the regulating and grinding wheels. The regulating wheel angle of inclination helps provide workpiece axial thrust. The average regulating wheel speed is typically set between 22 and 39 rpm. During throughfeed grinding, the regulating wheel diameter, rotational speed, and angle of inclination interact according to a specific formula to determine workpiece feed rate. To ensure that a workpiece maintains a consistent diameter during the throughfeed grinding operation, the work guides must remain accurately aligned with the work rest blade. On the regulating wheel side, the work guides must be accurately aligned with the regulating wheel face. The work guides on the grinding wheel side of the machine must be set so that the workpiece has enough clearance to be sufficiently ground by the grinding wheel during a grinding pass while still maintaining a constant diameter. Both the standard truing unit and profile truing attachment typically uses either a diamond dresser or metallic dresser to true the face of a grinding wheel. To true the regulating wheel, you must first set the truing angle and offset the diamond. The function of any truing unit for the regulating wheel is to ensure that the diamond follows the same path as that of the workpiece.

    

Figure 1. A part's triangular shape that can result from grinding too far below the grinding and regulating wheels' centerline.

Figure 2. Work guides that push the workpiecetoo far toward the grinding wheel produce a hollow-shaped workpiece.

Figure 3. Formula for calculating the diamond offset.

Term Definition

Angle Of Inclination The angle formed as the regulating wheel swivels to allow the part to pass. The regulating wheel angle of inclination provides axial thrust that feeds the workpiece past the grinding wheel during operation.

Axial Along or parallel to the axis of the workpiece. During throughfeed grinding the part must move in an axial direction.

Bearing Surface The circular underside of the bolt head that makes contact with the part. During centerless grinding, the bearing surface of the work guide on the entrance side of the regulating wheel must be set behind the regulating wheel face by a specific amount.

Center A hardened, pointed, cylindrical component used to hold one end of a workpiece during center-type cylindrical grinding. The tip of a center is inserted into a matching hole on the end of a workpiece.

Centerless Grinding A common grinding operation during which a cylindrical part is supported on a work rest blade and guided between a grinding wheel and a regulating wheel. Centerless grinding is a type of cylindrical grinding.

Chatter Marks Surface imperfections on the workpiece caused by vibrations of the grinding wheel. Positioning the work rest blade too high above the centerline of the grinding and regulating wheels often results in chatter marks.

Chattering Also called whipping, the occasional unwanted vibration between components. Chattering is often caused by kinks or bends in a part.

Chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

Cylindrical Grinding A common grinding process during which a cylindrical part is held on each end and rotated as a grinding wheel is guided along its length. Centerless grinding is a type of cylindrical grinding.

Diameter The distance from one edge of a circle to the opposite edge that passes through the center. Accurate setup of a centerless grinder ensures consistent workpiece diameter.

End Stop Device that halts or prevents workpiece motion once the workpiece is ground to a specified location and depth. During throughfeed grinding, the regulating wheel angle holds the workpiece firmly against the end stop.

Face In grinding, the part of the wheel that contacts the workpiece. The truing angle ensures that the workpiece makes a straight line of contact with the regulating wheel face.

Feed Rate The rate at which the grinding wheel and the workpiece move in relation to one another. The regulating wheel angle of inclination provides the axial thrust that determines workpiece feed rate.

Feeler Gauge A device sometimes used to determine if there is proper clearance between components on a machine. A feeler gauge is often used to ensure sufficient clearance between the regulating wheel and work guide for the workpiece to pass through.

Finish The degree of "smoothness" of a workpiece surface. Accurate setup of a centerless grinder ensures appropriate workpiece finish.

Finish Grinding An abrasive process that improves the surface of the part. Finish grinding emphasizes tight tolerances and smooth surface finish.

Form Grinding Type of centerless grinding that uses a specialized edge, or profile, that is added to the face of a wheel. The "form" or profile is then imparted into the workpiece during grinding.

Grade The strength of the bond in an abrasive wheel. Using a wheel with a softer grade during centerless grinding helps to prevent chatter marks.

Grinding Wheel A wheel made of a bonded abrasive used to remove material from a workpiece surface. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine surface finishes.

Hydraulic Feeding Mechanism Device that typically controls the rate at which the truing tool travels across the grinding wheel.

Infeed Grinding Similar to plunge grinding, a method of centerless grinding in which the workpiece is held stationary while the grinding wheel is fed into the workpiece at a specified location and depth.

Lobing Deviation from workpiece roundness. Lobing is prevented by proper setup of the centerless grinder.

Lower Slide The device attached to the swivel plate that moves the regulating wheel toward or away from the workrest blade. The correct positioning of the workpiece above the top of the lower slide assists in part roundness.

Periphery The outer edge of a workpiece. During centerless grinding, any high spot on the periphery of the workpiece causes workpiece lobing.

Pick-Up Also called scoring, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Pick-up often occurs when the blade is too hard for the grinding operation.

Profile Truing Attachment A system used to true a form grinding wheel for centerless grinding. The profile truing attachment is used to impart a particular shape into the form grinding wheel, which is then used to impart a particular shape into the workpiece.

Rate Of Traverse Speed at which the truing tool travels across the grinding wheel. The truing tool rate of traverse is typically controlled by a hydraulic feeding mechanism.

Regulating Wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation. The regulating wheel controls workpiece rotational speed and feed rate.

Rough Grinding Relatively aggressive cutting or grinding done with little regard for surface finish.

Roundness The quality of a cylindrical workpiece characterized by the entire length of the workpiece having the same diameter relative to a common axis. All points on the exterior surface of a perfectly round cylindrical workpiece are equidistant from the axis of the workpiece.

Scoring Also called pick-up, a centerless grinding problem that occurs when excessive pressure from the work rest blade causes tiny chips to fuse to the blade. Scoring often occurs when the blade is too hard for the grinding operation.

Squared Directly aligned with the surface of another object. The work guide at the rear of the regulating wheel must be squared with the regulating wheel.

Standard Truing Unit A system used to true a grinding wheel in centerless grinding. This system uses a diamond dresser to true the face of a grinding wheel for straight cylindrical parts.

Stock Raw workpiece material that is removed during a grinding pass or operation.

Through Feed In grinding, the movement of a workpiece as it travels through an operation.

Throughfeed Grinding A method of centerless grinding in which the regulating wheel and work guides feed the workpiece past the grinding wheel in a straight line. Throughfeed grinding is used primarily to grind straight cylindrical workpieces with no obstructive features.

Tolerance An unwanted but acceptable variation from a specified dimension. Accurate setup of a centerless grinder ensures accurate workpiece tolerance.

Truing Angle The angle formed by the truing device that matches the regulating wheel angle of inclination. The truing angle creates a slightly convex regulating wheel face, which enables a complete line of contact between the regulating wheel and workpiece during centerless grinding.

Whipping Also called chattering, the occasional unwanted vibration between components. Whipping is often caused by kinks or bends in a part.

Work Guide A device used in centerless grinding that prevents the workpiece from moving too far toward either the regulating wheel or grinding wheel. Work guides help maintain consistent workpiece diameter during throughfeed grinding.

Work Rest A part of a centerless grinding machine that supports the workpiece as it is ground. Work guides are mounted to the work rest.

Work Rest Blade A device, usually with an angled edge, that supports cylindrical parts during centerless grinding. The above-center positioning of the work rest blade produces concentricity of the workpiece.

Work Speed The rotational speed of the workpiece. The regulating wheel rotational speed determines work speed.

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