DESIGN OF JIGS, FIXTURES, PRESS TOOLS AND MOULDS

UNIT I LOCATING AND CLAMPING PRINCIPLES

Objectives of tool design- Function and advantages of Jigs and fixtures – Basic elements –

principles of location – Locating methods and devices – Redundant Location – Principles of

clamping – Mechanical actuation – pneumatic and hydraulic actuation Standard parts Drill

bushes and Jig buttons – Tolerances and materials used.

UNIT II JIGS AND FIXTURES

Design and development of jigs and fixtures for given component- Types of Jigs – Post,

Turnover, Channel, latch, box, pot, angular post jigs – Indexing jigs – General principles of

milling, Lathe, boring, broaching and grinding fixtures – Assembly, Inspection and Welding

fixtures – Modular fixturing systems- Quick change fixtures.

UNIT III PRESS WORKING TERMINOLOGIES & ELEMENTS OF CUTTING DIES

Press Working Terminologies - operations – Types of presses – press accessories – Computation

of press capacity – Strip layout – Material Utilization – Shearing action – Clearances – Press

Work Materials – Center of pressure- Design of various elements of dies –Die Block – Punch

holder, Die set, guide plates – Stops – Strippers – Pilots – Selection of Standard parts – Design

and preparation of four standard views of simple blanking, piercing, compound and progressive

dies.

UNIT IV BENDING FORMING AND DRAWING DIES

Difference between bending, forming and drawing – Blank development for above operations –

Types of Bending dies – Press capacity – Spring back – knockouts – direct and indirect –

pressure pads – Ejectors – Variables affecting Metal flow in drawing operations – draw die

inserts – draw beads- ironing – Design and development of bending, forming, drawing

reversere-drawing and combination dies – Blank development for axi- symmetric, rectangular

and elliptic parts – Single and double action dies

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UNIT V DESIGN OF MOULDS

Types of moulds and dies for various processing methods - Mould and Die Design Concept and

Materials. Injection Mould Design - Basics of mould construction - Methodical Mould Design -

Design of Feed System, Ejection System - Venting - Design of Cooling system – Mould

alignment concepts and De-moulding Techniques. Moulds with a slide core - Split cavity

moulds.

(Use of Approved Design Data Book is permitted).

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UNIT I - LOCATING AND CLAMPING PRINCIPLES

Locating and clamping are the critical functions of any workholders. As such, the

fundamental principles of locating and clamping, as well as the numerous standard components

available for these operations, must be thoroughly understood.

BASIC PRINCIPLES OF LOCATING

To perform properly, workholders must accurately and consistently position the workpiece

relative to the cutting tool, part after part. To accomplish this, the locators must ensure that the

workpiece is properly referenced and the process is repeatable.

Referencing and Repeatability

"Referencing" is a dual process of positioning the workpiece relative to the workholder, and

the workholder relative to the cutting tool. Referencing the workholder to the cutting tool is

performed by the guiding or setting devices. With drill jigs, referencing is accomplished using

drill bushings. With fixtures, referencing is accomplished using fixture keys, feeler gages, and/or

probes. Referencing the workpiece to the workholder, on the other hand, is done with locators.

If a part is incorrectly placed in a workholder, proper location of the workpiece is not

achieved and the part will be machined incorrectly. Likewise, if a cutter is improperly positioned

relative to the fixture, the machined detail is also improperly located. So, in the design of a

workholder, referencing of both the workpiece and the cutter must be considered and

simultaneously maintained.

"Repeatability" is the ability of the workholder to consistently produce parts within

tolerance limits, and is directly related to the referencing capability of the tool. The location of

the workpiece relative to the tool and of the tool to the cutter must be consistent. If the jig or

fixture is to maintain desired repeatability, the workholder must be designed to accommodate

the workpiece's locating surfaces.

The ideal locating point on a workpiece is a machined surface. Machined surfaces permit

location from a consistent reference point. Cast, forged, sheared, or sawed surfaces can vary

greatly from part to part, and will affect the accuracy of the location. 3

The Mechanics of Locating

A workpiece free in space can move in an infinite number of directions. For analysis, this

motion can be broken down into twelve directional movements, or "degrees of freedom." All

twelve degrees of freedom must be restricted to ensure proper referencing of a workpiece.

As shown in Figure 1-1, the twelve degrees of freedom all relate to the central axes of the

workpiece. Notice the six axial degrees of freedom and six radial degrees of freedom. The axial

degrees of freedom permit straight-line movement in both directions along the three principal

axes, shown as x, y, and z. The radial degrees of freedom permit rotational movement, in both

clockwise and counterclockwise radial directions, around the same three axes.

Figure 1-1. The twelve degrees of freedom.

The devices that restrict a workpiece's movement are the locators. The locators, therefore,

must be strong enough to maintain the position of the workpiece and to resist the cutting forces.

This fact also points out a crucial element in workholder design: locators, not clamps, must hold

the workpiece against the cutting forces.

Locators provide a positive stop for the workpiece. Placed against the stop, the workpiece

cannot move. Clamps, on the other hand, rely only upon friction between the clamp and the

clamped surface to hold the workpiece. Sufficient force could move the workpiece. Clamps are

only intended to hold the workpiece against the locators.

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Forms of Location

There are three general forms of location: plane, concentric, and radial. Plane locators

locate a workpiece from any surface. The surface may be flat, curved, or have an irregular

contour. In most applications, plane-locating devices locate a part by its external surfaces,

Figure 1-2a. Concentric locators, for the most part, locate a workpiece from a central axis. This

axis may or may not be in the center of the workpiece. The most-common type of concentric

location is a locating pin placed in a hole. Some workpieces, however, might have a cylindrical

projection that requires a locating hole in the fixture, as shown in Figure 1-2b. The third type of

location is radial. Radial locators restrict the movement of a workpiece around a concentric

locator, Figure 1-2c. In many cases, locating is performed by a combination of the three

locational methods.

Figure 1-2. The three forms of location: plane, concentric, and radial.

Locating from External Surfaces

Flat surfaces are common workpiece features used for location. Locating from a flat surface

is a form of plane location. Supports are the principal devices used for this location. The three

major forms of supports are solid, adjustable, and equalizing, Figure 1-3.

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Figure 1-3. Solid, adjustable, and equalizing supports locate a workpiece from a flat surface.

Solid supports are fixed-height locators. They precisely locate a surface in one axis.

Though solid supports may be machined directly into a tool body, a more-economical method is

using installed supports, such as rest buttons.

Adjustable supports are variable-height locators. Like solid supports, they will also

precisely locate a surface in one axis. These supports are used where workpiece variations

require adjustable support to suit different heights. These supports are used mainly for cast or

forged workpieces that have uneven or irregular mounting surfaces.

Equalizing supports are a form of adjustable support used when a compensating support is

required. Although these supports can be fixed in position, in most cases equalizing supports

float to accommodate workpiece variations. As one side of the equalizing support is depressed,

the other side raises the same amount to maintain part contact. In most cases adjustable and

equalizing supports are used along with solid supports.

Locating a workpiece from its external edges is the most-common locating method. The

bottom, or primary, locating surface is positioned on three supports, based on the geometry

principle that three points are needed to fully define a plane. Two adjacent edges, usually

perpendicular to each other, are then used to complete the location.

The most-common way to locate a workpiece from its external profile is the 3-2-1, or six-

point, locational method. With this method, six individual locators reference and restrict the

workpiece.

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As shown in Figure 1-4, three locators, or supports, are placed under the workpiece. The

three locators are usually positioned on the primary locating surface. This restricts axial

movement downward, along the -z axis and radially about the x and y axes. Together, the three

locators restrict five degrees of freedom.

Figure 1-4. Three supports on the primary locating surface restrict five degrees of freedom.

The next two locators are normally placed on the secondary locating surface, as shown in

Figure 1-5. They restrict an additional three degrees of freedom by arresting the axial movement

along the +y axis and the radial movement about the z axis.

Figure 1-5. Adding two locators on a side restricts eight degrees of freedom.

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The final locator, shown in Figure 1-6, is positioned at the end of the part. It restricts the

axial movement in one direction along the -x axis. Together, these six locators restrict a total of

nine degrees of freedom. The remaining three degrees of freedom will be restricted by the

clamps.

Figure 1-6. Adding a final locator to another side restricts nine degrees of freedom, completing

the 3-2-1 location.

Although cylindrical rest buttons are the most-common way of locating a workpiece from its

external profile, there are also other devices used for this purpose. These devices include flat-sided

locators, vee locators, nest locators and adjustable locators.

Locating from Internal Surfaces

Locating a workpiece from an internal diameter is the most-efficient form of location. The

primary features used for this form of location are individual holes or hole patterns. Depending

on the placement of the locators, either concentric, radial, or both-concentric-and-radial location

are accomplished when locating an internal diameter. Plane location is also provided by the plate

used to mount the locators.

The two forms of locators used for internal location are locating pins and locating plugs.

The only difference between these locators is their size: locating pins are used for smaller holes

and locating plugs are used for larger holes.

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As shown in Figure 1-7, the plate under the workpiece restricts one degree of freedom. It

prevents any axial movement downward, along the -z axis. The center pin, acting in conjunction

with the plate as a concentric locator, prevents any axial or radial movement along or about the x

and and y axes. Together, these two locators restrict nine degrees of freedom. The final locator,

the pin in the outer hole, is the radial locator that restricts two degrees of freedom by arresting

the radial movement around the z axis. Together, the locators restrict eleven degrees of freedom.

The last degree of freedom, in the +z direction, will be restricted with a clamp.

Figure 1-7. Two locating pins mounted on a plate restrict eleven-out-of-twelve degrees of

freedom.

Analyzing Machining Forces

The most-important factors to consider in fixture layout are the direction and magnitude of

machining forces exerted during the operation. In Figure 3-8, the milling forces generated on a

workpiece when properly clamped in a vise tend to push the workpiece down and toward the

solid jaw. The clamping action of the movable jaw holds the workpiece against the solid jaw and

maintains the position of the part during the cut.

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Figure 1-8. Cutting forces in a milling operation should be directed into the solid jaw and base

of the vise.

Another example of cutting forces on a workpiece can be seen in the drilling operation in

Figure 1-9. The primary machining forces tend to push the workpiece down onto the workholder

supports. An additional machining force acting radially around the drill axis also forces the

workpiece into the locators. The clamps that hold this workpiece are intended only to hold the

workpiece against the locators and to maintain its position during the machining cycle. The only

real force exerted on the clamps occurs when the drill breaks through the opposite side of the

workpiece, the climbing action of the part on the drill. The machining forces acting on a

correctly designed workholder actually help hold the workpiece.

Figure 1-9. The primary cutting forces in a drilling operation are directed both downward and

radially about the axis of the drill.

An important step in most fixture designs is looking at the planned machining operations to

estimate cutting forces on the workpiece, both magnitude and direction. The "estimate" can be a

rough guess based on experience, or a calculation based on machining data. One simple formula

for force magnitude, shown in Figure 1-10, is based on the physical relationship:

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Please note: "heaviest-cut horsepower" is not total machine horsepower; rather it is the

maximum horsepower actually used during the machining cycle. Typical machine efficiency is

roughly 75% (.75). The number 33,000 is a units-conversion factor.

Figure 1-10. A simple formula to estimate the magnitude of cutting forces on the workpiece.

The above formula only calculates force magnitude, not direction. Cutting force can have

x-, y-, and/or z-axis components. Force direction (and magnitude) can vary drastically from the

beginning, to the middle, to the end of the cut. Figure 1-11 shows a typical calculation.

Intuitively, force direction is virtually all horizontal in this example (negligible z-axis

component). Direction varies between the x and y axes as the cut progresses.

Figure 1-11. Example of a cutting force calculation.

LOCATING GUIDELINES

No single form of location or type of locator will work for every workholder. To properly

perform the necessary location, each locator must be carefully planned into the design. The

following are a few guidelines to observe in choosing and applying locators.

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Positioning Locators

The primary function of any locator is to reference the workpiece and to ensure

repeatability. Unless the locators are properly positioned, however, these functions cannot be

accomplished. When positioning locators, both relative to the workholder and to the workpiece,

there are a few basic points to keep in mind.

Whenever practical, position the locators so they contact the workpiece on a machined

surface. The machined surface not only provides repeatability but usually offers a more-stable

form of location. The workpiece itself determines the areas of the machined surface used for

location. In some instances, the entire surface may be machined. In others, especially with

castings, only selected areas are machined.

The best machined surfaces to use for location, when available, are machined holes. As

previously noted, machined holes offer the most-complete location with a minimal number of

locators. The next configuration that affords adequate repeatability is two machined surfaces

forming a right angle. These characteristics are well suited for the six-point locational method.

Regardless of the type or condition of the surfaces used for location, however, the primary

requirement in the selection of a locating surface is repeatability.

To ensure repeatability, the next consideration in the positioning of locators is the spacing

of the locators themselves. As a rule, space locators as far apart as practical. This is illustrated in

Figure 1-12. Both workpieces shown here are located with the six-point locating method. The

only difference lies in the spacing of the locators. In the part shown at (b), both locators on the

back side are positioned close to each other. In the part at (a), these same locators are spaced

further apart. The part at (a) is properly located; the part at (b) is not. Spacing the locators as far

apart as practical compensates for irregularities in either the locators or the workpiece. Its also

affords maximum stability.

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Figure 1-12. Locators should be spaced as far apart as practical to compensate for slight

irregularities and for maximum stability.

The examples in Figure 1-13 show conditions that may occur when locators are placed too

close together if the center positions of the locators are misaligned by .001". With the spacing

shown at (a), this condition has little effect on the location. But if the locating and spacing were

changed to that shown at (b), the .001" difference would have a substantial effect. Another

problem with locators placed too close together is shown at (c). Here, because the locators are

too closely spaced, the part can wobble about the locators in the workholder.

Figure 1-13. Positioning locators too close together will affect the locational accuracy.

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Controlling Chips

The final consideration in the placement of locators involves the problem of chip control.

Chips are an inevitable part of any machining operation and must be controlled so they do not

interfere with locating the workpiece in the workholder. Several methods help minimize the chip

problem. First, position the locators away from areas with a high concentration of chips. If this is

not practical, then relieve the locators to reduce the effect of chips on the location. In either case,

to minimize the negative effects of chips, use locators that are easy to clean, self-cleaning, or

protected from the chips. Figure 1-14 shows several ways that locators can be relieved to reduce

chip problems.

Figure 1-14. Locators should be relieved to reduce locational problems caused by chips.

Coolant build-up can also cause problems. Solve this problem by drilling holes, or milling

slots, in areas of the workholder where the coolant is most likely to build up. With some

workholders, coolant-drain areas can also act as a removal point for accumulated chips.

When designing a workholder, always try to minimize the chip problem by removing areas

of the tool where chips can build up. Omit areas such as inside corners, unrelieved pins, or

similar features from the design. Chip control must be addressed in the design of any jig or

fixture.

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Avoiding Redundant Location

Another condition to avoid in workholder design is redundant, or duplicate, location.

Redundant locators restrict the same degree of freedom more than once. The workpieces in

Figure 1-15 show several examples. The part at (a) shows how a flat surface can be redundantly

located. The part should be located on only one, not both, side surfaces. Since the sizes of parts

can vary, within their tolerances, the likelihood of all parts resting simultaneously on both

surfaces is remote. The example at (b) points out the same problem with concentric diameters.

Either diameter can locate the part, but not both.

The example at (c) shows the difficulty with combining hole and surface location. Either

locational method, locating from the holes or locating from the edges, works well if used alone.

When the methods are used together, however, they cause a duplicate condition. The condition

may result in parts that cannot be loaded or unloaded as intended.

Figure 1-15. Examples of redundant location.

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Always avoid redundant location. The simplest way to eliminate it is to check the shop

print to find which workpiece feature is the reference feature. Often, the way a part is

dimensioned indicates which surfaces or features are important. As shown in Figure 1-16, since

the part on the left is dimensioned in both directions from the underside of the flange, use this

surface to position the part. The part shown to the right, however, is dimensioned from the

bottom of the small diameter. This is the surface that should be used to locate the part.

Figure 1-16. The best locating surfaces are often determined by the way that the part is

dimensioned.

Preventing Improper Loading

Foolproofing prevents improper loading of a workpiece. The problem is most prevalent

with parts that are symmetrical or located concentrically. The simplest way to foolproof a

workholder is to position one or two pins in a location that ensures correct orientation, Figure 1-

17. With some workpieces, however, more-creative approaches to foolproofing must be taken.

Figure 1-17. Foolproofing the location prevents improper workpiece loading.

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Figure 1-18 shows ways to foolproof part location. In the first example, shown at (a), an

otherwise-nonfunctional foolproofing pin ensures proper orientation. This pin would interfere

with one of the tabs if the part were loaded any other way. In the next example, shown at (b), a

cavity in the workpiece prevents the part from being loaded upside-down. Here, a block that is

slightly smaller than the opening of the part cavity is added to the workholder. A properly

loaded part fits over the block, but the block keeps an improperly loaded part from entering the

workholder.

Figure 1-18. Simple pins or blocks are often used to foolproof the location.

Using Spring-Loaded Locators

One method to help ensure accurate location is the installation of spring-loaded buttons or

pins in the workholder, Figure 1-19. These devices are positioned so their spring force pushes

the workpiece against the fixed locators until the workpiece is clamped. These spring-loaded

accessories not only ensure repeatable locating but also make clamping the workpiece easier.

Figure 1-19. Spring-loaded locators help ensure the correct location by pushing the workpiece

against the fixed locators.17

Determining Locator Size and Tolerances

The workpiece itself determines the overall size of a locating element. The principle rule to

determine the size of the workpiece locator is that the locators must be made to suit the MMC

(Maximum-Material Condition) of the area to be located. The MMC of a feature is the size of

the feature where is has the maximum amount of material. With external features, like shafts, the

MMC is the largest size within the limits. With internal features, like holes, it is the smallest size

within the limits. Figure 1-20 illustrates the MMC sizes for both external and internal features.

Figure 1-20. Locator sizes are always based on the maximum-material condition of the

workpiece features.

Sizing cylindrical locators is relatively simple. The main considerations are the size of the

area to be located and the required clearance between the locator and the workpiece. As shown

in Figure 1-21, the only consideration is to make the locating pin slightly smaller than the hole.

In this example, the hole is specified as .500-.510" in diameter. Following the rule of MMC, the

locator must fit the hole at its MMC of .500". Allowing for a .0005 clearance between the pin

and the hole, desired pin diameter is calculated at .4995". Standard locating pins are readily

available for several different hole tolerances, or ground to a specific dimension. A standard 1/2"

Round Pin with .4995"-.4992" head diameter would be a good choice.

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Figure 1-21. Determining the size of a single locating pin based on maximum-material

conditions.

The general accuracy of the workholder must be greater than the accuracy of the

workpiece. Two basic types of tolerance values are applied to a locator: the first are the

tolerances that control the size of the locator; the second are tolerances that control its location.

Many methods can be used to determine the appropriate tolerance values assigned to a

workholder. In some situations the tolerance designation is an arbitrary value predetermined by

the engineering department and assigned to a workholder without regard to the specific

workpiece. Other tolerances are assigned a specific value based on the size of the element to be

located. Although more appropriate than the single-value tolerances, they do not allow for

requirements of the workpiece. Another common method is using a set percentage of the

workpiece tolerance.

The closer the tolerance value, the higher the overall cost to produce the workpiece.

Generally, when a tolerance is tightened, the cost of the tolerance increases exponentially to its

benefit. A tolerance twice as tight might actually cost five times as much to produce.

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The manufacturability of a tolerance, the ability of the available manufacturing methods to

achieve a tolerance, is also a critical factor. A simple hole, for example, if toleranced to ±.050",

can be punched. If, however, the tolerance is ±.010", the hole requires drilling. Likewise, if the

tolerance is tightened to ±.002", the hole then requires drilling and reaming. Finally, with a

tolerance of ±.0003", the hole must be drilled, reamed, and lapped to ensure the required size.

One other factor to consider in the manufacturability of a tolerance is whether the tolerance

specified can be manufactured within the capability of the toolroom. A tolerance of .00001" is

very easy to indicate on a drawing, but is impossible to achieve in the vast majority of

toolrooms.

No single tolerance is appropriate for every part feature. Even though one feature may

require a tolerance of location to within .0005", it is doubtful that every tolerance of the

workholder must be held to the same tolerance value. The length of a baseplate, for example,

can usually be made to a substantially different tolerance than the location of the specific

features.

The application of percentage-type tolerances, unlike arbitrary tolerances, can accurately

reflect the relationship between the workpiece tolerances and the workholder tolerances.

Specification of workholder tolerances as a percentage of the workpiece tolerances results in a

consistent and constant relationship between the workholder and the workpiece. When a straight

percentage value of 25 percent is applied to a .050" workpiece tolerance, the workholder

tolerance is .0125". The same percentage applied to a .001" tolerance is .00025". Here a

proportional relationship of the tolerances is maintained regardless of the relative sizes of the

workpiece tolerances. As a rule, the range of percentage tolerances should be from 20 to 50

percent of the workpiece tolerance, usually determined by engineering-department standards.

CLAMPING GUIDELINES

Locating the workpiece is the first basic function of a jig or fixture. Once located, the

workpiece must also be held to prevent movement during the operational cycle. The process of

holding the position of the workpiece in the jig or fixture is called clamping. The primary

devices used for holding a workpiece are clamps. To perform properly, both the clamping

devices and their location on the workholder must be carefully selected.

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Factors in Selecting Clamps

Clamps serve two primary functions. First, they must hold the workpiece against its

locators. Second, the clamps must prevent movement of the workpiece. The locators, not the

clamps, should resist the primary cutting forces generated by the operation.

Holding the Workpiece Against Locators. Clamps are not intended to resist the primary

cutting forces. The only purpose of clamps is to maintain the position of the workpiece against

the locators and resist the secondary cutting forces. The secondary cutting forces are those

generated as the cutter leaves the workpiece. In drilling, for example, the primary cutting forces

are usually directed down and radially about the axis of the drill. The secondary forces are the

forces that tend to lift the part as the drill breaks through the opposite side of the part. So, the

clamps selected for an application need only be strong enough to hold the workpiece against the

locators and resist the secondary cutting forces.

The relationship between the locators and clamps can be illustrated with a milling-machine

vise. In Figure 1-22, the vise contains both locating and clamping elements. The solid jaw and

vise body are the locators. The movable jaw is the clamp. The vise is normally positioned so that

the locators resist the cutting forces. Directing the cutting forces into the solid jaw and vise body

ensures the accuracy of the machining operation and prevents workpiece movement. In all

workholders, it is important to direct the cutting forces into the locators. The movable vise jaw,

like other clamps, simply holds the position of the workpiece against the locators.

Figure 1-22. A vise contains both locating and clamping elements.21

Holding Securely Under Vibration, Loading, and Stress. The next factors in selecting a

clamp are the vibration and stress expected in the operation. Cam clamps, for example, although

good for some operations, are not the best choice when excessive vibration can loosen them. It is

also a good idea to add a safety margin to the estimated forces acting on a clamp.

Preventing Damage to the Workpiece. The clamp chosen must also be one that does not

damage the workpiece. Damage occurs in many ways. The main concerns are part distortion and

marring. Too much clamping force can warp or bend the workpiece. Surface damage is often

caused by clamps with hardened or non-rotating contact surfaces. Use clamps with rotating

contact pads or with softer contact material to reduce this problem. The best clamp for an

application is one that can adequately hold the workpiece without surface damage.

Improving Load/Unload Speed. The speed of the clamps is also important to the

workholder's efficiency. A clamp with a slow clamping action, such as a screw clamp,

sometimes eliminates any profit potential of the workholder. The speed of clamping and

unclamping is usually the most-important factor in keeping loading/unloading time to a

minimum.

Positioning the Clamps

The position of clamps on the workholder is just as important to the overall operation of the

tool as the position of the locators. The selected clamps must hold the part against the locators

without deforming the workpiece. Once again, since the purpose of locators is to resist all

primary cutting forces generated in the operation, the clamps need only be large enough to hold

the workpiece against the locators and to resist any secondary forces generated in the operation.

To meet both these conditions, position the clamps at the most-rigid points of the workpiece.

With most workholders, this means positioning the clamps directly over the supporting elements

in the baseplate of the workholder, Figure-1-23a.

In some cases the workpiece must be clamped against horizontal locators rather than the

supports, Figure 1-23b. In either case, the clamping force must be absorbed by the locating

elements.

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Figure 1-23. Clamps should always be positioned so the clamping force is directed into the

supports or locators.

For workholders with two supports under the clamping area of the workpiece, two clamps

should be used — one over each support, Figure 1-24a. Placing only one clamp between the

supports can easily bend or distort the workpiece during the clamping operation. When the

workpiece has flanges or other extensions used for clamping, an auxiliary support should be

positioned under the extended area before a clamp is applied, Figure 1-24b.

Figure 1-24. The number and position of clamps is determined by the workpiece and its

supports.

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Another consideration in positioning clamps is the operation of the machine tool

throughout the machining cycle. The clamps must be positioned so they do not interfere with the

operation of the machine tool, during either the cutting or return cycle. Such positioning is

especially critical with numerically controlled machines. In addition to the cutters, check

interference between the clamps and other machine elements, such as arbors, chucks, quills,

lathe carriages, and columns.

When fixturing an automated machine, check the complete tool path before using the

workholder. Check both the machining cycle and return cycle of the machine for interference

between the cutters and the clamps. Occasionally programmers forget to consider the tool path

on the return cycle. One way to reduce the chance of a collision and eliminate the need to

program the return path is simply to raise the cutter above the highest area of the workpiece or

workholder at the end of the machining cycle before returning to the home position.

Most clamps are positioned on or near the top surface of the workpiece. The overall height

of the clamp, with respect to the workpiece, must be kept to a minimum. This can be done with

gooseneck-type clamps, Figure 1-25. As shown, the gooseneck clamp has a lower profile and

should be used where reduced clamp height is needed.

Figure 1-25. Using gooseneck clamps is one way to reduce the height of the clamps.

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The size of the clamp-contact area is another factor in positioning a clamp. To reduce

interference between the clamp and the cutter, keep the contact area as small as safely possible.

A small clamping area reduces the chance for interference and also increases the clamping

pressure on the workpiece. The overall size of the clamp is another factor to keep in mind. The

clamp must be large enough to properly and safely hold the workpiece, but small enough to stay

out of the way.

Once again, the primary purpose of a clamp is to hold the workpiece against the locators.

To do this properly, the clamping force should be directed into the locators, or the most-solid

part of the workholder. Positioning the clamping devices in any other manner can easily distort

or deform the workpiece.

The workpiece shown in Figure 1-26 illustrates this point. The part is a thin-wall ring that

must be fixtured so that the internal diameter can be bored. The most-convenient way to clamp

the workpiece is on its outside diameter; however, to generate enough clamping pressure to hold

the part, the clamp is likely to deform the ring. The reason lies in the direction and magnitude of

the clamping force: rather than acting against a locator, the clamping forces act against the

spring force of the ring resisting the clamping action. This type of clamping should only be used

if the part is a solid disk or has a small-diameter hole and a heavy wall thickness.

Figure 1-26. Directing the clamping forces against an unsupported area will cause this

cylindrical part to deform.

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To clamp this type of part, other techniques should be used. The clamping arrangement in

Figure 1-27 shows the workpiece clamped with four strap clamps. The clamping force is

directed into the base plate and not against the spring force of the workpiece. Clamping the

workpiece this way eliminates the distortion of the ring caused by the first method.

Figure 1-27. Strap clamps eliminate deformation by directing the clamping forces into the

supports under the part.

A similar clamping method is shown at Figure 1-28. Here the workpiece has a series of

holes around the ring that can be used to clamp the workpiece. Clamping the workpiece in this

manner also directs the clamping force against the baseplate of the workholder. This type of

arrangement requires supports with holes that permit the clamping screws to clamp through the

supports.

Figure 1-28. When possible, part features such as holes can be used to clamp the part.

26

If the part can be clamped only on its outside surface, one other method can be used to hold

the part: a collet that completely encloses the part. As shown in Figure 1-29, the shape of the

clamping contact helps control distortion. Depending on the size of the part, either a collet or

pie-shaped soft jaws can be used for this arrangement.

Figure 1-29. When the part can only be clamped on its outside surface, pie-shaped chuck jaws

can be used to hold the part and reduce deformation.

Selecting Clamp Size and Force

Calculations to find the necessary clamping force can be quite complicated. In many

situations, however, an approximate determination of these values is sufficient. The table in

Figure 1-30 shows the available clamping forces for a variety of different-size manual clamp

straps with a 2-to-1 clamping-force ratio.

Figure 1-30. Approximate clamping forces of different-size manual clamp straps with a 2-to-1

clamping-force ratio.

27

Alternatively, required clamping force can be calculated based on calculated cutting forces.

A simplified example is shown in Figure 1-31. The cutting force is entirely horizontal, and no

workpiece locators are used, so frictional forces alone resist the cutting forces.

Figure 1-31. A simplified clamping-force calculation with the cutting force entirely horizontal,

and no workpiece stops (frictional force resists all cutting forces).

When workpiece locators and multi-directional forces are considered, the calculations

become more complicated. To simplify calculations, the worst-case force situation can be

estimated intuitively and then treated as a two-dimensional static-mechanics problem (using a

free-body diagram). In the example shown in Figure 1-32, the cutting force is known to be 1800

lbs, based on a previous calculation. The workpiece weighs 1500 lbs. The unknown forces are:

28

FR = Total force from all clamps on right side

FL = Total force from all clamps on left side

R1 = Horizontal reaction force from the fixed stop

R2 = Vertical reaction force from the fixed stop

R3 = Vertical reaction force on the right side

N = Normal-direction force = FL + FR + 1500

µ = Coefficient of friction = .19

Figure 1-32. A more-complicated clamping-force calculation, using a two-dimensional free-

body diagram.

29

The following equations solve the unknown forces assuming that for a static condition:

1. The sum of forces in the x direction must equal zero.

2. The sum of forces in the y direction must equal zero.

3. The sum of moments about any point must equal zero.

At first glance, this example looks "statically indeterminate," i.e., there are five variables

and only three equations. But for the minimum required clamping force, R3 is zero (workpiece

barely touching) and FL is zero (there is no tendency to lift on the left side). Now with only three

variables, the problem can be solved:

Solving for the variables, FR = 1290 lbs; R1 = 1270 lbs ;R2 = 2790 lbs

In other words, the combined force from all clamps on the right side must be greater than

1290 lbs. With a recommended safety factor of 2-to-1, this value becomes 2580 lbs. Even

though FL (combined force from all the clamps on the left side) equals zero, a small clamping

force may be desirable to prevent vibration.

Another general area of concern is maintaining consistent clamping force. Manual

clamping devices can vary in the force they apply to parts during a production run. Many factors

account for the variation, including clamp position on the workpiece, but operator fatigue is the

most-common fault. The simplest and often-best way to control clamping force is to replace

manual clamps with power clamps.

The force generated by power clamps is not only constant but also adjustable to suit

workpiece conditions. Another benefit of power clamps is their speed of operation: not only are

individual power clamps faster than manual clamps, every clamp is activated at the same time.

30

UNIT 2 - JIGS AND FIXTURES

Introduction

Jigs and fixtures are devices used to facilitate production work, making interchangeable

pieces of work possible at a savings in cost of production. Both terms are frequently used

incorrectly in hops. A jig is a guiding device and a fixture a holding device.

Jigs and fixtures are used to locate and hold the work that is to be machined. These

devices are provided with attachments for guiding, setting, and supporting the tools in such a

manner that all the workpieces produced in a given jig or fixture will be exactly

alike in every way.

The employment of unskilled labor is possible when jigs and fixtures can be used in

production work. The repetitive layout and setup (which are time-consuming activities and

require considerable skill) are eliminated. Also, the use of these devices can result in such a

degree of accuracy that workpieces can be assembled with a minimum amount of fitting.

A jig or fixture can be designed for a particular job. The form to be used depends on the

shape and requirement of the workpiece to be machined.

Jigs and fixtures

Jigs and fixtures are devices used to locate and hold the work that is to be machined. A

jig is a guiding device, and a fixture is a holding device. A jig or fixture can be designed for a

particular job. The form to be used depends on the shape and requirements of the workpiece that

is to be machined.

There are generally two types of jigs used: the clamp jig and the box jig. Various names

are applied to jigs (such as drilling, reaming, and tapping) according to the operation to be

performed. Clamp jigs are sometimes called open jigs. Frequently, jigs are named for their

shape, such as plate, ring, channel, and leaf.

31

A fixture anchors the workpiece firmly in place for the machining operation, but it does

not form a guide for the tool. It is sometimes difficult to differentiate between a jig and a fixture,

since their basic functions can overlap in the more complicated designs.

A plate jig consists of a plate, which contains the drill bushings, and a simple means of

clamping the work in the jig, or the jig to the work. Where the jig is clamped to the work, it

sometimes is called a clamp-on jig.

An indexing fixture can be used for machining operations that are to be performed in

more than one plane. It facilitates location of the given angle with a degree of precision.

Jigs

The two types of jigs that are in general use are (1) clamp jig and (2) box jig. A few

fundamental forms of jigs will be shown to illustrate the design and application of jigs. Various

names are applied to jigs (such as drilling, reaming, and tapping) according to the operation to

be performed.

Clamp Jig

This device derives its name from the fact that it usually resembles some form of clamp.

It is adapted for use on workpieces on which the axes of all the holes that are to be drilled are

parallel.

Clamp jigs are sometimes called open jigs. A simple example of a clamp jig is a design

for drilling holes that are all the same size—for example, the stud holes in a cylinder head

(Figure 2-1).

32

As shown in Figure 2-1, the jig consists of a ring with four lugs for clamping and is frequently

called a ring jig. It is attached to the cylinder head and held by U-bolt clamps. When used as a

guide for the drill in the drilling operation, the jig makes certain that the holes are in the correct

locations because the holes in the jig were located originally with precision. Therefore, laying

out is not necessary.

Figure 2-1 A plain ring-type clamp jig without bushings.

A disadvantage of the simple clamp jig is that only holes of a single size can be drilled. Either

fixed or removable bushings can be used to overcome this disadvantage. Fixed bushings are

sometimes used because they are made of hardened steel, which reduces

wear. Removable bushings are used when drills of different sizes are to be used, or when the

drilled holes are to be finished by reaming or tapping.

A bushed clamp jig is illustrated in Figure 2-2. In drilling a hole for a stud, it is evident that the

drill (tap drill) must be smaller in size than the diameter of the stud. Accordingly, two sizes of

twist drills are required in drilling holes for studs. The smaller drill (or tap drill) and a drill

slightly larger than the diameter of the stud are required for drilling the holes in the cylinder

head. A bushing can be used to guide the tap drill.

33

Figure 2-2 A clamp jig, with the tap drill guided by a bushing, designed for drilling holes in

the cylinder (top); the operation for a hole for the cylinder head (bottom).

The jig is clamped to the work after it has been centered on the cylinder and head so that the

axes of the holes register correctly. Various provisions (such as stops) are used to aid in

centering the jig correctly. The jig shown in Figure 2-2 is constructed with four lugs as a part of

the jig. As the jig is machined, the inner sides of the lugs are turned to a diameter that will

permit the lugs to barely slip over the flange when the jig is applied to the work.

A reversible clamp jig is shown in Figure 2-3. The distinguishing feature of this type of jig is the

method of centering the jig on the cylinder and head. The position of the jig for drilling the

cylinder is shown at the top of Figure 2-3. An annular projection on the jig fits

closely into the counterbore of the cylinder to locate the jig concentrically with the cylinder

bore.

34

The jig is reversed for drilling the cylinder head. That is, the opposite side is placed so that the

counter bore or circular recessed part of the jig fits over the annular projection of the cylinder

head at the bottom of Figure 2-3.

Figure 2-3 Note the use of a reversible clamp jig for the tap drill operation (top), and reversing

the jig to drill the hole for the stud in the cylinder head (bottom).

This type of jig is often held in position by inserting an accurately fitted pin through the jig

and into the first hole drilled. The pin prevents the jig from turning with respect to the cylinder

as other holes are drilled.

A simple jig that has locating screws for positioning the work is shown in Figure 2-4. The

locating screws are placed in such a way that the clamping points are opposite the bearing points

on the work. Two setscrews are used on the long side of the work, but in this instance, because

the work is relatively short and stiff, a single lug and setscrew (B in Figure 2-4) is sufficient.

35

This is frequently called a plate jig since it usually consists of only a plate that contains the drill

bushings and a simple means of clamping the work in the jig, or the jig to the work. Where the

jig is clamped to the work, it sometimes is called a clamp-on jig.

Figure 2-4 A simple jig that uses locating screws to position the work.

Diameter jigs provide a simple means of locating a drilled hole exactly on a diameter of a

cylindrical or spherical piece (Figure 2-5).

36

Figure 2-5 Diameter jig.

Another simple clamp jig is called a channel jig and derives its name from the cross-sectional

shape of the main member, as shown in Figure 2-6. They can be used only with parts having

fairly simple shapes.

Figure 2-6 Channel jig.

37

Box Jig

Box jigs (sometimes called closed jigs) usually resemble a boxlike structure. They can be used

where holes are to be drilled in the work at various angles. Figure 2-7 shows a design of box jig

that is suitable for drilling the required holes in an engine link. The jig is built in the form of a

partly open slot in which the link is moved up against a stop and then clamped with the clamp

bolts A, B, and C.

Figure 2-7 Using the box jig for drilling holes in an engine link.

The bushings D and E guide the drill for drilling the eccentric rod connections, and the bushing

F guides the drill for the reach rod connections. The final hole, the hole for lubrication at the top

of the link, is drilled by turning the jig 90°, placing the drill in the

bushing G.

This type of jig is relatively expensive to make by machining, but the cost can be reduced by

welding construction, using plate metal. In production work, the pieces can be set and released

quickly.

A box jig with a hinged cover or leaf that may be opened to permit the work to be inserted and

then closed to clamp the work into position is usually called a leaf jig (Figure 2-8). Drill

bushings are usually located in the leaf. However, bushings may be located in other surfaces to

permit the jig to be used for drilling holes on more than one side of the work. Such a jig, which

requires turning to permit work on more than one side, is known as a rollover jig.

38

Figure 2-8 Leaf jig.

A box jig for angular drilling (Figure 2-9) is easily designed by providing the jig with legs of

unequal length, thus tilting the jig to the desired angle. This type of jig is used where one or

more holes are required to be drilled at an angle with the axis of the work. As can be seen in

Figure 2-9, the holes can be drilled in the work with the twist drill in a vertical position.

Sometimes the jig is mounted on an angular stand rather than providing legs of unequal length

for the jig. Figure 2-10 shows a box jig for drilling a hole in a ball.

In some instances, the work can be used as a jig (Figure 2-11). In the illustration, a bearing and

cap are used to show how the work can be arranged and used as a jig. After the cap has been

planed and fitted, the bolt holes in the cap are laid out and drilled. The cap is clamped in

position, and the same twist drill used for the bolt holes is used to cut a conical spot in the base.

This spotting operation provides a starting point for the smaller tap drill (A and B in Figure 2-

11).

39

Figure 2-9 A box jig with legs of unequal length, used for drilling holes at an angle.

Figure 2-10 A box jig used for drilling a hole in a ball.

Also, both parts can be clamped together and drilled with a tap drill (C in Figure 2-11). Then,

the tap drill can be removed and the holes for the bolts enlarged by means of a counterbore (D in

Figure 2-11). Following are some factors of prime importance to keep in mind with jigs:

40

• Proper clamping of the work

• Support of the work while machining

• Provision for chip clearance

Figure 2-11 Using the work as a jig. In (A) the same drill used for the bolt holes is used to cut a

conical spot in the base.This forms a starting point for the smaller tap drill, as shown in (B). In

(C), the cap and bearing are clamped together and drilled by means of a tap drill, after which the

tap drill is removed and a counterbore is used to enlarge the holes for the bolts, as shown in (D).

When excessive pressure is used in clamping, some distortion can result. If the distortion is

measurable, the result is inaccuracy in final dimensions. This is illustrated in an exaggerated

way in Figure 2-12. The clamping forces should be applied in such a way that will not produce

objectionable distortion.

Figure 2-12 Effects of excessive pressure.

41

It is also important to design the clamping force in such a way that the work will remain in the

desired position while machining, as shown in Figure 2-13.

Figure 2-13 Effects of clamping force.

Figure 2-14 shows the need for the jig to provide adequate support while the work is being

machined. In the example shown in Figure 2-12, the cutting force should always act against a

fixed portion and not against a movable section. Figure 2-13 illustrates the need to keep the

points of clamping as nearly as possible in line with the cutting forces of the tool. This will

reduce the tendency of these forces to pull the work from the clamping jaws. Support beneath

the work is necessary to prevent the piece from distorting. Such distortion can result in

inaccuracy and possibly a broken tool.

42

Figure 2-14 Support for work during machining.

Adequate provision must be made for chip clearance, as illustrated in Figure 2-15. The first

problem is to prevent the chips from becoming packed around the tool. This could result in

overheating and possible tool breakage. If the clearance is not great enough, the chips cannot

flow away. If there is too much clearance, the bushing will not guide the tool properly.

Figure 2-15 Provision for chip clearance.

The second factor in chip clearance is to prevent the chips from interfering with the proper

seating of the work in the jig, as shown in Figure 2-16.

43

Figure 2-16 Provision for chip clearance.

Fixtures

As mentioned previously, a fixture is primarily a holding device. A fixture anchors the

workpiece firmly in place for the machining operation, but it does not form a guide for the tool.

It is sometimes difficult to differentiate between a jig and a fixture, since their basic functions

can overlap in the more complicated designs. The best means of differentiating between the two

devices is to apply the basic definitions, as follows:

• The jig is a guiding device.

• The fixture is a holding device.

A typical example of a fixture is the device designed to hold two or more locomotive

cylinders in position for planing (Figure 2-17).

This fixture is used in planing the saddle surfaces. In the planning operation, two or more

cylinders are placed in a single row, the fixture anchoring them firmly to the planer bed.

44

Figure 2-17 A fixture used to hold locomotive cylinders in position

The fixture consists of heavy brackets or angles, with conical projections that permit the bores of

the cylinders to be alignedaccurately with each other. The end brackets are made with a single

conical flange; the intermediate brackets are made with double conical flanges. A bolt through

the center of the flanges aligns the cylinder bores when it is tightened. The legs of the 90°-angle

brackets at the ends are bolted firmly to the planer table. The intermediate brackets are also

bolted to the planer table and aid in holding the assembly in firm alignment for the machining

operation.

The use of fixtures can result in a considerable saving in the time required to set the work, and

they also ensure production of accurate work.

An indexing fixture can be used for machining operations that are to be performed in more than

one plane (Figure 2-18). It facilitates location of the given angle with a degree of precision.

45

Figure 2-18 A simple type of indexing fixture that can be used to facilitate machining at

accurately spaced angles.

A disc in the indexing fixture is held in angular position by a pin that fits into a finished hole in

the angle iron and into one of the holes in the disc. The disc is clamped against the knee by a

screw and washer while the cut is being taken. Since the holes are properly spaced in the disc

(index plate), the work attached to the disc can be rotated into any desired angular position.

Radial drilling operations can be performed when a projecting plate is provided with a jig hole.

The same general principles concerning clamping support while machining, and chip clearance

as covered in jigs apply as well to fixtures.

46

UNIT III PRESS WORKING TERMINOLOGIES & ELEMENTS OF CUTTING DIES

Introduction

A press is a machine tool used to shape or cut metal by applying force. There are three

different ways of working sheet metal in presses.

Shearing.

In this operation metal is deformed to a shear failure in order to cut various contours

from the metallic sheet. Bending It is a localized deformation within the plastic range Drawing

In drawing there is a deformation within plastic range involving considerable change of

shape.

Types of Presses

The presses are classified as follows:

Based on source of power: -

Hand press

Power press.

Based on type of frame.-

Pillar press

Horn press

Inclinable

Straight side press.

Presses are exclusively intended for mass production.

Method of actuation of press

Number of slides incorporated Intended use.

The frame of a press is fabricated by casting or by welding, heavy steel plates. Cast frames

are quite stable and rigid but expensive.

The general classification by frame includes:

gap frame

Straight side.

47

The gap frame is cut back below the ram to form the shape of letter C. This allows feeding

the strip from the side. back by gravity. The three position inclinable press is frequently referred

to as an open Back Inclinable (O.B.I.) press.

Fig. shows an OBI press. It has the following parts.

Rectangular bed

It acts as a table to which a bolster plate is mounted.

Bolster Plate

It is used for locating and supporting the die assembly.

Ram

To ram the punch or upper the assembly is fastened.

Fly Wheel

To supply energy to the work piece.

Knockout

48

It is used to eject the work piece.

Pitman

It is used to transfer motion and pressure from the main shaft or eccentric to press slide.

OBI press is quite commonly used for blanking and piercing operations on relatively

small work pieces although bending, forming and drawing operations can also are

carried out.

The straight side frame press incorporates a slide or ram which travels up and down

between two straight sides or housing and is commonly used for heavy and large work.

According to sources of power majority of presses receive their power mechanically or

hydraulically.

Manually operated presses called hand presses are used for making very small

components.

Mechanically operated presses use a flywheel driven system to obtain ram movement.

The slide actuation may be achieved by two methods.

By using a crank shaft.

By using an eccentric.

Selection of Press

Press tool designer has to make proper selection of the type of press to be used and also

the kind of press tools to be provided.

The design of the tools should be simplest possible and the method of operation the most

efficient one.

Hydraulic presses are used primarily forging and coining opera tions where a

comparatively slow stroke is advisable.

While selecting a press the following points should be considered.

Force required to cut the metal

Size and type of die

Stroke length

Shut height

Method of feeding and size of sheet blank

Type of operation.

In progressive die punching is carried out atone station and blanking is carried out at49

Another station. Progressive dies can be used for many variations of piercing, blanking, forming,

lancing and so forth.

They have the advantage of being fairly simple to construct and are

economical to repair because a broken punch or die does not necessitate

the replacement of the entire set.

However if highly accurate alignment of various operations is required,

they are not as satisfactory as compound dies.

The compound dies are more accurate but they usually are more expensive

to construct and repair and are more subject to breakage.

Clearance between Die and Punch

Clearance is the intentional space between the punch cutting edge and the die cutting

edge. It is expressed as the amount of clearance per side.Clearance is necessary to allow the

fractures to meet when break occurs. The amount of

clearance depends upon the following factors:

Type of material

Thickness of material

Hardness of material

Type of operation.

In case of normal clearance the fracture lines will start (ideally) at the cutting edge of the

punch and also at the die as shown in Fig. They will proceed toward each other until they meet as

shown in Fig

The edge of the blank will appear as shown in Fig. There will be minimum bun.

50

Excessive clearance allows a large edge radius and excessive plastic deformation.

The edge of the material tends to be drawn or pulled in the direction of the working force and

the break is not smooth.

Blanking.

In blanking the cut out piece is the desired part. In this case punch is made smaller in size

and the die is made of exact size clearance is thus provided on punch as shown in Fig.

S = Size of work piece

C = Clearance

Die size S = Size of work piece

Punch size = S—2C

Piercing

In case of piercing the cut out part is waste and left out piece is of importance. In

Piercing the punch is made of exact size and die is made of bigger size as shown in Fig...

51

Punch size = size

Die size = size + 2C

Thus clearance is given on the die.

Where C = Clearance

= 3% to 8% of material thickness.

Press Working Operations

The commonly used press work operations are as follows

Blanking

In this operation the cut out part from the material strip is the required component

Piercing

In this operation the cut out part from the material strip is waste, hole left in the strip

being required

52

Notching

This operation is used to remove small amount of metal from the edges of the metallic

strip.

Lancing

It is a combined bending and cutting operation.

53

Drawing: It is used to manufacture cup-shaped components.

Cutting off:

This operation is used to separate the work material along a straight line in a single line cut).

Work material

Cutting force is a function of the area of the cut edge being sheared at any instant and the

shearing strength of the work piece material.

Cutting force calculations help in selecting press of proper tonnage (capacity), in order to

avoid overloading the press or failure to use it to capacity.

P = Cutting force (tonnes) = itDtfsfor round holes = L. t. For other contours

Where

D = Hole diameter (mm)

= Material thickness (mm).

L = Shear length (mm)

Die Block Design

The various dimensions of the die block are calculated as follows

Dies must be adequately supported on a flat die plate or die plate or die holder (shoe).

Two and only two dowels should be provided in each block or element that requires

Accurate and permanent positioning.

They should be located as far apart as possible for maximum locating effect usually near

diagonally opposite corners.

Two or more screws should be used depending on the size of die block.

Where p = Blanking perimeter.

A grinding allowance up to 3 mm to 5 mm should be added to calculate die thickness.

There should be minimum of 30 mm margin around the opening in the die block. Die

block should never be thinner than 7 to 10 mm54

For securing the die block to the die holder following rules should be used:

On die blocks up to 150 mm square use 10 mm Allen screws and two 10 mm dowel

pins.

On sections up to 250mm square use three Allen screws and two dowel pins.

For heavier stock and still bigger blocks use screws and pins of 12 mm diameter.

Counter

bore the block to accommodate Allen heads at least 3 mm deeper than length heads to

Compensate for the sharpening.

Die block thickness (T) is calculated as follows 15 mm for p= 75 mm

= 25mm for p = 75—250mm

= 30 for larger values

h = Die opening should be straight

A = Critical distance

= Distance between cutting edge and the die border

= 1.5T—2T for small dies

= 2T — 3T for larger dies

a = Angular clearance = ½° to 2°, the overall design of die block depends on

Part size and shape, Type of die. = 3 mm

Die block is generally made up of cast iron.

Punch Design

The maximum allowable punch length (Lm) is calculated using the following formula:

55

Punch material should have sufficient compressive strength. Piercing punch should not

be smaller in diameter than the thickness of the stock they are to pierce.

Where a small unguided puncher must pierce stock thicker than the punch diameter, the

Punch shank should be at least twice the hole size in diameter and the cutting face should be

ground to the hole size for a distance of about twice the stock thicknes. Always avoid designing

punches that would have more than 4 in of unguided length. A spacer block should be used

between the punch and punch holder (or punch plate) in order to shorten punches if more than a

4-in, length is necessary.In rare cases the punches are over 100 mm’ in length. An allowance of

6 mm to 12.5 mm for sharpening should be provided whenever possible in the length of

punches.

Punch Holder: Punch holder thickness varies between 25 to 75 mm

Punch Plate: The thickness of punch plate = 1.5 x D

Where D = Diameter of punch.

Both punch holder and punch plate are made of cast iron.

While designing a punch following factors should be considered

Easier fitting and removal of punch.

Alignment of punch and die.

Adequate provision of screws to overcome stripping force.

Prevention of rotation of punch.

The punch dimensions should be checked for

Where i = Diameter of punched hole is = shear stress

t = Material thickness

E = Modulus of elasticity of punch material

56

(a)Strength

(b) Deflection.

Methods of Reducing Cutting Forces

The cutting forces can be reduced by following methods:

Stepped punches. Punches or groups of punches progress become shorter by about one

stock.Material thickness as in Fig.

h = t + 0.4 mm

Where t = Material thickness

Centre of Pressure

It is the centre of gravity of the line i.e. the perimeter of the blank. It is not the centre of

gravity of the area. The press tool is so designed that the centre of pressure will lie on the axis of

the press ram when the tool is mounted on the press.

Centre of pressure is determined by using the following procedure.

Draw the outline of the actual cutting edges.

Draw XX and YY axes at right angle in convention position.

Divide the cutting edges into line elements such as straight lines, arcs etc. Find the

lengths Li, LzL etc. of these elements.

Mark the centre of gravity of these elements say C C etc.

Find x the distances of centre of gravity C from YY axis and flats respectively. Similarly

find XY2 of C and X, Y of c etc.

Determine distance X and Y of centre of pressure. Centre of Pressure

Divide the cutting edges into line elements such as straight lines, arcs etc. Find the

length.Li, Lz L etc. of these elements.Mark the centre of gravity of these elements say C

etc.

Find x the distances of centre of gravity C from YY ,C ,C ,C axis and flats respectively.

Similarly find X Y2 of C

57

58

UNIT IV BENDING FORMING AND DRAWING DIES

Introduction

Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental

forms used in metalworking, and can be cut and bent into a variety of different shapes.

Countless everyday objects are constructed of the material. Thicknesses can vary significantly,

although extremely thin thicknesses are considered foil or leaf, and pieces thicker than 6 mm

(0.25 in) are considered plate.

Sheet metal processing

The raw material for sheet metal manufacturing processes is the output of the rolling

process. Typically, sheets of metal are sold as flat, rectangular sheets of standard size. If the sheets

are thin and very long, they may be in the form of rolls. Therefore the first step in any sheet metal

process is to cut the correct shape and sized ‘blank’ from larger sheet.

Sheet metal forming processes

Sheet metal processes can be broken down into two major classifications and one minor

classification

Shearing processes -- processes which apply shearing forces to cut, fracture, or separate

the material. 

Forming processes -- processes which cause the metal to undergo desired shape changes

without failure, excessive thinning, or cracking. This includes bending and stretching. 

Finishing processes -- processes which are used to improve the final surface

characteristics.

Shearing Process

1. Punching: shearing process using a die and punch where the interior portion of the

sheared sheet is to be discarded.

2. Blanking: shearing process using a die and punch where the exterior portion of the

shearing operation is to be discarded.

3. Perforating: punching a number of holes in a sheet

4. Parting: shearing the sheet into two or more pieces

59

5. Notching: removing pieces from the edges

6. Lancing: leaving a tab without removing any material

Fig.1Shearing Operations: Punching, Blanking and Perforating

Forming Processes

Bending: forming process causes the sheet metal to undergo the desired shape change by

bending without failure. Ref fig.2 & 2a

Stretching: forming process causes the sheet metal to undergo the desired shape change

by stretching without failure. Ref fig.3

Drawing: forming process causes the sheet metal to undergo the desired shape change

by drawing without failure. Ref fig.4

Roll forming: Roll forming is a process by which a metal strip is progressively bent as it

passes through a series of forming rolls. Ref fig.5

Common Die-Bending Operations

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V

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ou

s

Bending Operations

Schematic illustration of a stretch-forming process.

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Schematic of the Drawing process.

Eight-roll sequence for the roll forming of a box channel

Finishing processes

Material properties, geometry of the starting material, and the geometry of the desired final

product play important roles in determining the best process

Equipments

Basic sheet forming operations involve a press, punch, or ram and a set of dies

Presses

Mechanical Press - The ram is actuated using a flywheel. Stroke motion is not uniform.

Ref fig.6

Hydraulic Press - Longer strokes than mechanical presses, and develop full force

throughout the stroke. Stroke motion is of uniform speed, especially adapted to deep

drawing operations. Ref fig.7

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Dies and Punches

Simple- single operation with a single stroke

Compound- two operations with a single stroke

Combination- two operations at two stations

Progressive- two or more operations at two or more stations with each press stroke,

creates what is called a strip development

Tools and Accessories

The various operations such as cutting, shearing, bending, folding etc. are performed by

these tools.

Marking and measuring tools

Steel Rule - It is used to set out dimensions.

Try Square - Try square is used for making and testing angles of 90degree

Scriber – It used to scribe or mark lines on metal work pieces.

Divider - This is used for marking circles, arcs, laying out perpendicular lines, bisecting

lines, etc

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Marking and measuring tools

Cutting Tools

Straight snip - They have straight jaws and used for straight line cutting. Ref fig.10

Curved snip - They have curved blades for making circular cuts. Ref fig.10a

Straight snip

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Curved Snip

Striking Tools

Mallet - It is wooden-headed hammer of round or rectangular cross section. The

striking face is made flat to the work. A mallet is used to give light blows to the

Sheet metal in bending and finishing. Ref fig.11

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Merits

High strength

Good dimensional accuracy and surface finish

Relatively low cost

Demerits

Wrinkling and tearing are typical limits to drawing operations

Different techniques can be used to overcome these limitations

o Draw beads

o Vertical projections and matching grooves in the die and blank holder

Trimming may be used to reach final dimensions

Applications

Roofings

Ductings

Vehicles body buildings like 3 wheelers, 4 wheelers, ships, aircrafts etc.

Furnitures, House hold articles and Ra

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UNIT V DESIGN OF MOULDS

The processing of polymeric materials -plastics, elastomers and composites- is

characterized by a wide variety of distinct methods or techniques. Techniques involving the

continuous manufacture of a product basically have uniform cross section, which include

extrusion, extrusion covering, film blowing and calendering; techniques involving the shaping of

a deformable polymer perform against a mold surface, which involve coating and rotational

molding; and, finally, techniques which involve the complete filling of a mold cavity, and

include casting, compression molding, transfer molding, injection molding and reaction injection

molding.

Fundamental to the choice of polymer processing technique is the question of whether to

use a high molecular mass starting material or a system that polymerizes in the mold.

In the liquid state, most monomers and low molecular mass polymers flow in much the

same way as molten metals in that the shear stress needed to make them flow is directly

proportional to the shear strain rate- they are Newtonian fluids. As their molecular masses

increase their viscosities increase but at some point the long thin chains begin to rearrange

themselves under the applied shear stressed to line up in the direction of flow, and the

proportionality between stress and strain rate starts to change- the polymer has become non-

Newtonian.

The consequences of the much higher pressures needed to cast high molecular mass

polymers are not difficult to appreciate. In addition, the arrangements for keeping the mold

closed will need to be more robust since the pressures applied to the mold tend to force the two

mold halves open during filling and feeding. And the molds themselves must be made from

stronger materials to withstand being repeatedly exposed to these pressures.

Reducing the viscosity of the polymer will clearly allow higher flow rates at the same

applied pressures, or permit the use of substantial machinery and tooling. The polymer chains

tangle around each other and form so-called ‘mechanical cross links’, effectively strengthening

the material in the solid state but making it more difficult to cast in the fluid state. So the grade

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polymer, which is easy to cast, is going to give inferior performance in the end product and the

best performance will be obtained from a material that is more difficult to cast.

Extrusion

The extrusion process basically is continuously shaping a fluid polymer through the

orifice of a suitable tool (die), and subsequently solidifying it into a product (extrudate of

constant cross section). In the case of thermoplastics, the feed material, in powder or pellet form,

is most commonly heated to a fluid state and pumped into the die, through a screw extruder; it is

then solidified by cooling after exiting from the die.

Figure 1. Extrusion dies can have complex shapes to (a) compensate for die swell,

(b) distribute material across the width of a sheet, or (c) coat a wire.

Extrusion products are often subdivided into groups that include filaments of circular

cross-section, profiles of irregular cross section, axissymmetric tubes and pipes, and flat

products such as films or sheets.

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Film Blowing

The film blowing process basically consists of a extruding a tube of molten thermoplastic

and continuously inflating it to several times initial diameter, to form a thin tubular product that

can be used directly, or slit to form a flat film.

Figure 2. Film Blowing.

Films produced by the film blowing process are widely used for agricultural,

construction, and industrial applications, including covers for silage, greenhouses,

chemical/solar ponds, flat cars, etc., or for a variety of packing applications, with include wrap,

can lining, fabricated bags such as garbage bags, gusseted (V-tucks, W-folds) fashion bags, and

T-shirt bags (sacks) for groceries.

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Sheet Thermoforming

Sheet thermoforming, or simply thermoforming, involves the heating of a flat

thermoplastic sheet to a softened state (above the glass transition temperature Tg for non-

crystallizing thermoplastics or near the melting temperature Tm for crystallizing ones), followed

by the deformation (forming) of the softened sheet into a desired shape by pneumatic or

mechanical means, and finally its solidification into this shape by cooling.

Figure 3. (1) A flat plastic sheet is softened by heating; (2) the softened sheet is placed

over a concave mold cavity; (3) a vacuum draws the sheet into the cavity; and (4) the plastic

hardens on contact with the cold mold surface, and the part is removed and subsequently

trimmed from the web.

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Products made by sheet thermoforming include skin and blisterpacks, individual

containers for jelly or cream, vials, cups, tubs, trays and lids. As many as millions of parts per

day can be produced with a tool featuring several hundred cavities. Larger products are

generally made from cut sheets at much shower rates; the heating stage often is the limiting

factor. Transparent products, such as contoured windows, skylights and cockpit canopies, are

often made by this method.

Blow Molding

The basic principle of the blow molding process is to inflate a softened thermoplastic

hollow preform against the cooled surface of a closed mold, where the material solidifies into a

hollow product.

Figure 4. (1) Injection molding of parison, (2) stretching, and (3) blowing.

Packing is the major area of application of small to medium-size disposable blow molded

products. Liquid foodstuffs are increasingly packaged in narrow neck plastic PET bottles. Blow

molded containers are also used for cosmetics, toiletries, pharmaceutical and medical packaging

and a variety of household products.

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Compression Molding

The compression molding process is used for temperature activated thermosetting

polymers. Compression molding basically involves the pressing of a deformable material charge

between the two halves of a heated mold, and its transformation into a solid product under the

effect of the elevated mold temperature. Compression molding temperatures are often in the

range 140-200 0C; mold pressures can vary from 35 atm to 700 atm. Material charges are often

pre-heated to speed up the initial softening stage. Compression molding is characterized by the

show and moderate flow of the very viscous material charge to fill the cavity, and it is not

normally suitable for making complicated parts, or parts featuring fragile inserts.

Figure 5. (1) Charge is loaded, (2) and (3) charge is compressed and cured, and (4) part

is ejected and removed.

Transfer Molding

Transfer molding is often associated with compression molding, because it is used with

the same two classes of materials, temperature-activated thermosets, and vulcanizable rubbers.

In transfer molding a softened temperature-activated material is transferred through a narrow

gate into the closed cavity of a heated mold, where it cures to a solid state.

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Transfer molding is normally used with materials that have fairly high pre-curing

fluidity, facilitating the flow from the loading area to cavities. This also permits the molding of

complex parts, parts featuring fragile inserts.

Injection Molding

The injection molding process involves the rapid pressure filling of a specific mold

cavity with a fluid material, followed by the solidification of the material into a product. The

process is used for thermoplastics, thermosetting resins, and rubbers.

Figure 6. (a) Reciprocating plunger and (b) reciprocating rotating screw.

The injection molding of thermoplastics can be subdivided into a several stages. At the

plasticity stage, the feed unit operates pretty much as an extruder, melting and homogenizing the

material in the screw/barrel system. The screw however is allowed to retract, to make room for

the molten material reservoir. At the injection state, the screw is used as a ram for the rapid

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transfer of the molten material from the reservoir to the cavity between the two halves of the

closed mold. Since the mold is kept at a temperature below the solidification temperature of the

material, it is essential to inject the molten material rapidly ensure complete filling of the cavity.

A high holding packing pressure is normally exerted, to partially compensate for the thermal

contraction of the material upon cooling. After the cooling stage, the mold can be opened and

the solid product removed.

Figure 7. Typical cavity pressure variations over the entire injection molding

cycle is shown here.

The cavity pressure rises rapidly during the filling stage, which is followed by the

holding or the packing stage. Once the gate freezes off, the cavity pressure decays with time till

the part is ejected. At high pressures, a polymer melt is compressible, allowing additional

material to be packed in the mold cavity after mold filling is complete. This is necessary to

reduce non-uniform part shrinkage, which leads to part warpage. Excessive packing results in a

highly stressed part and may cause ejection problems whereas insufficient packing causes poor

surface, sink marks, welds and non-uniform shrinkage.

All thermoplastics are, in principle, suitable for injection molding, but since fast flow

rates are needed, grades with good fluidity are normally preferable. Significant differences in

ease of molding, and the resulting structure and properties of products are found between

amorphous and crystallizing thermoplastics; they concern problems such as shrinkage, warpage,

sink marks, flashing, and short shots.

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Figure 8. Pressure history varies with position.

The pressure distribution inside the mold cavity changes with distance from the inlet

gate. The figure 8 shows a simple part geometry with pressure variations among the points one,

two and three respectively. Further away from the gate, pressure rises slowly and it decays

quicker than at the points closer to the gate. The pressure in the mold cavity should be more

uniform to minimize part warpage.

A major disadvantage of injection molded products is the incorporation of fine details

such as bosses, locating pins, mounting holes, ribs, flanges, etc., which normally eliminates

assembly and finishing operations.

Figure 9. The figure shows a typical temperature history in the cavity during

the injection molding cycle.

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The gate freezes off first because it is thinner than the cavity. Once the part temperature

is well below the polymer solidification temperature, the part is ejected.

PROBLEMS ENCOUNTERED IN INJECTION MOLDING

There are many details to pay attention in injection molding which affects the physical

properties, they may even cause to the failure of the molding.

Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas,

such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the

mold wall. This leads to part weakness, surface blemishes, and a multiplicity of internal defects.

An air trap is air that is caught inside the mold cavity. It becomes trapped by converging

polymer melt fronts or because it failed to escape from the mold vents, or mold inserts, which

also act as vents.

A short shot is a molded part that is incomplete because insufficient material was

injected into the mold. It can be caused by entrapped air, insufficient machine injection pressure

(resulting from high melt resistance and a restricted flow path), pre-mature solidification of the

polymer melt, and machine defects.

A sink mark is a local surface depression and a void is a vacuum bubble in the core. Sink

marks and voids are caused by localized shrinkage of the material at thick sections without

sufficient compensation when the part is cooling.

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A weld line (also called a weld mark or a knit line) is formed when separate melt fronts

traveling in opposite directions meet. The formation of weld lines can be caused by holes or

inserts in the part, multi-gate cavity systems, or variable wall thickness where hesitation or race

tracking occurs. The weld lines are undesirable when the strength and the surface quality are

important.

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