Manufacturing Process - Material Removal Process

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MANUFACTURING PROCESS MANUFACTURING PROCESS (BMFG 2323) (BMFG 2323) LECTURE 8 LECTURE 8 ~ MATERIAL REMOVAL PROCESS~ Prepared and presented by: Masjuri Bin Musa @ Othman Faculty of Mechanical Engineering (Department of Innovation & Engineering Design) Universiti Teknikal Malaysia Melaka

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Material Removal Process

Transcript of Manufacturing Process - Material Removal Process

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MANUFACTURING PROCESSMANUFACTURING PROCESS(BMFG 2323)(BMFG 2323)

LECTURE 8LECTURE 8

~ MATERIAL REMOVAL PROCESS~

Prepared and presented by: Masjuri Bin Musa @ Othman

Faculty of Mechanical Engineering(Department of Innovation & Engineering Design)

Universiti Teknikal Malaysia Melaka

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FUNDAMENTAL OF CUTTINGFUNDAMENTAL OF CUTTING

- Cutting processes remove material from the surface of a w/piece and producing chips.

- Some of the common cutting processes are:

•Turning: the w/piece is rotated and the cutting tool removes a layer of material as it moves to the

left.

•Cutting-off operation: the cutting tool moves radially inward and separates the right piece from the

bulk of the blank.

•Slab-milling operation: a rotating cutting tool removes a layer of material from the surface of the

w/piece.

•End-milling operation: a rotating cutter travels along a certain depth in the w/piece and produces a

cavity.

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Fundamentals of CuttingFundamentals of CuttingFigure : Examples of cutting processes.

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- In the turning process, the cutting tool is set at a certain depth of cut (mm or inch) and travels to the left with a certain velocity as the w/piece rotates.

- The feed or feed rate is the distance the tool travels horizontally per unit revolution of the w/piece (mm/rev).

- This particular movement of the tool produces a chip, which moves up the face of the tool.

Figure: Basic principle of the turning operations.

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Factors Influencing Cutting Factors Influencing Cutting ProcessesProcesses

TABLE 20.1Parameter Influence and interrelationship

Cutting speed, depth of cut,feed, cutting fluids

Forces, power, temperature rise, tool life, type of chip, surface finish.

Tool angles As above; influence on chip flow direction; resistance to tool chipping.Continuous chip Good surface finish; steady cutting forces; undesirable in automated

machinery.Built-up edge chip Poor surface finish; thin stable edge can protect tool surfaces.Discontinuous chip Desirable for ease of chip disposal; fluctuating cutting forces; can affect

surface finish and cause vibration and chatter.Temperature rise Influences tool life, particularly crater wear, and dimensional accuracy of

workpiece; may cause thermal damage to workpiece surface.Tool wear Influences surface finish, dimensional accuracy, temperature rise, forces and

power.Machinability Related to tool life, surface finish, forces and power.

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Types of chips produced in metal cutting:

- Continuous

- Built-up edge (BUE)

- Segmented

- Discontinuous

1) Continuous chips.

• They formed with ductile materials, high cutting speeds, or high rake angles.

• It is not desirable because they cause tangled around tool holder, the fixturing, the workpiece, chip-disposal systems, as well as interfere the operator itself.

• This problem can be eliminated with chip breakers or by changing parameters, such as cutting speed, feed, doc, and by using cutting fluids.

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2) BUE (Built-up edge)

• BUE consists of layers of material from the workpiece that gradually are deposited on the

tool tip. As it grow larger, it eventually breaks apart from the tool tip.

• It affects poor surface finish, changes the geometry of the cutting edge, and dulls.

• BUE can be reduced by:

- increase the cutting speeds

- decrease the doc

- use a sharp tool

- use an effective cutting fluid

- use a cutting tool that has lower chemical affinity for the workpiece material.

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3) Segmented Chips

• Semicontinuous chips.

• Metals with low thermal conductivity and strength that decrease sharply with temperature e.g. titanium.

• The chips have a sawtooth-like appearance.

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4. Discontinuous Chips

• Consists of segments that may be attached firmly or loosely to each other.

• Formed due to:

- brittle workpiece materials or contain hard inclusions e.g. graphite flakes.

- very low or very high cutting speeds

- large doc

- low rake angle

- insufficient cutting fluid

- low stiffness of the tool holder

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CHIP BREAKERCHIP BREAKER

- Continuous and long chips are undesirable as they tend to become entangled and interfere with

machining operations and become a potential safety hazard.

- In order to avoid this situation happened – need to break the chip intermittently with cutting tools

that have chip breaker features.

- Traditionally, chip breakers have a piece of metal clamped to the tool’s rake face, which bend and

break the chip.

- Present – there are built in chip breaker features on the cutting tools and inserts.

- Chips also can be broken by changing the tool geometry to control chip flow, as in the turning

operation (refer to the figures on the next slide).

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Examples of Chips Produced in TurningExamples of Chips Produced in TurningFigure: Various chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and breaks; (c) continuous chip moving away from workpiece; and (d) chip hits tool shank and breaks off.

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Chip BreakersChip Breakers

Figure: (a) Schematic illustration of the action of a chip breaker. Note that the chip breaker decreases the radius of curvature of the chip. (b) Chip breaker clamped on the rake face of a cutting tool. (c) Grooves in cutting tools acting as chip breakers.

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Orthogonal cutting vs Oblique cutting1. 2D cutting process.

2.The cutting edge is perpendicular to the direction of the cut. Thus, the chip slides directly up to the

face of the tool.

3.The forces (thrust force, Ft and cutting force, Fc) involved are perpendicular to each other. The Fc

acts in the direction of the cutting speed, while the Ft acts in the direction normal to the cutting speed

and both of this forces produce the resultant force, R.

4.The cutting tool has a positive rake angle and a clearance angle.

•vs

1. 3D cutting process.

2.The chip is helical at angle i0 and the chip moves sideways and away from the cutting zone and

doesn’t obstruct the cutting zone.

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Orthogonal CuttingOrthogonal Cutting

Figure: Schematic illustration of a two-dimensional cutting process, also called orthogonal cutting. Note that the tool shape and its angles, depth of cut, to, and the cutting speed, V, are all independent variables.

Figure: Forces acting on a cutting tool in two-dimensional cutting. Note that the resultant force, R, must be colinear to balance the forces.

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Cutting With an Oblique ToolCutting With an Oblique Tool

Figure: (a) Schematic illustration of cutting with an oblique tool. (b) Top view showing the inclination angle, i. (c) Types of chips produced with different inclination.

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TOOL LIFETOOL LIFE

- The conditions which induce the tool wear:• high localized stresses at the tip of the tool.• high temperatures, especially along the rake face.• sliding of the chip along the rake face.• sliding along the newly cut w/piece surface.

- Tool wear adversely:• affect tool life.• the quality of the machined surface.• its dimensional accuracy.• economics of the cutting operations.

- The rate of tool wear depends on:• tool and w/piece materials.• tool geometry.• process parameters.• cutting fluids.• characteristics of the machine tools.

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Tool-life curves

Figure shows the effect of workpiece microstructure and hardness on tool life in turning ductile cast iron. Note the rapid decrease in tool life as the cutting speed increases. Tool materials have been developed that resist high temperatures such as carbides, ceramics, and cubic boron nitride.

Figure below shows tool-life curves for a variety of cutting-tool materials.

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- From previous figures, we can conclude that:• tool life decreases rapidly as the cutting speed increases.• the condition of the w/piece material has a strong influence on tool life.• there is a large difference in tool life for different w/piece material microstructure.

- Cutting tools need to be replaced or resharpened when:• the surface finish of the machined w/piece begins to deteriorate.• cutting forces increase significantly.• temperature rises significantly.

- The allowable wear land for various machining conditions is given in the table below.

TABLE 20.4 Allowable Average Wear Land (VB)for Cutting Tools in Various Operations

Allowable wear land (mm)Operation High-speed Steels Carbides

TurningFace millingEnd millingDrillingReaming

1.51.50.30.4

0.15

0.40.40.30.4

0.15Note: 1 mm = 0.040 in.

- Note that for the improvement of the dimensional accuracy, tolerances, and surface finish, the allowable wear land may be smaller than the values given in the table.

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Examples of Wear and Tool Examples of Wear and Tool FailuresFailures

Figure (a) Schematic illustrations of types of wear observed on various types of cutting tools. (b) Schematic illustrations of catastrophic tool failures. A study of the types and mechanisms of tool wear and failure is essential to the development of better tool materials.

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Surface Finish and IntegritySurface Finish and Integrity• Surface finish describes the geometric features of a surface e.g. dimensional accuracy,

properties, and performance.

• Surface integrity pertains to material properties, i.e. fatigue life and corrossion resistance.

• BUE has the greatest influence on surface finish.

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CUTTING-TOOL MATERIALSCUTTING-TOOL MATERIALS

The characteristicsThe characteristics

• Hot hardness: the hardness, strength, and wear resistance of the tool are maintained at the room temperatures encountered in machining operations. This is because to ensures that the tool does not undergo plastic deformation and retains its shape and sharpness.

• Toughness and impact strength/mechanical shock: the impact forces on the tool encountered repeatedly in interrupted cutting operations, and this will not fracture the tool.

• Thermal shock resistance: to withstand the rapid temperature cycling encountered in interrupted cutting.

•Wear resistance: an acceptable tool life is obtained before the tool has to be replaced.

• Chemical stability and inertness: with respect to the material being machined, to avoid or minimize any adverse reactions, adhesion, and tool chip diffusion that would contribute to tool wear.

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Categories of cutting tool materials

1.High-speed steel (HSS)

2.Cast-cobalt alloys

3.Carbides

4.Coated tools

5.Alumina-based ceramics

6.Cubic boron nitride (CBN)

7.Silicon-nitride-based ceramics

8.Diamond

9.Whisker-reinforced materials and nanomaterials

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Carbide InsertsCarbide Inserts

Figure: Typical carbide inserts with various shapes and chip-breaker features. The holes in the inserts are standardized for interchangeability.

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Carbide Inserts (cont.)Carbide Inserts (cont.)

Figure: Methods of attaching inserts to toolholders: (a) Clamping, and (b) Wing lockpins. (c) Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws.

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INSERTSINSERTS

- High speed tools are shaped in one piece and ground to impact various geometry features; such

tools include drill bits and milling and gear cutters.

- After the cutting edge wears, the tool has to be removed from its holder and reground.

- A square insert has eight cutting points, and a triangular insert has six.

- Inserts are usually clamped on the tool holder with various locking mechanisms.

- Clamping type is the most preferred method of securing the inserts because each insert has a

number of cutting points, and after one edge is worn, it is indexed to make available another cutting

point.

- Carbide inserts have a variety of shapes – square, triangle, diamond, round.

- The strength of the cutting edge of an insert depends on its shape – the smaller the included angle,

the lower the strength of the edge.

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Figure: Relative edge strength and tendency for chipping and breaking of insets with various shapes. Strength refers to the cutting edge shown by the included angles.

Shape of inserts and its anglesShape of inserts and its angles

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Cutting FluidsCutting FluidsUsed to achieve:Used to achieve:

1. Reduce friction and wear, thus improving tool life and surface finish.

2. Cool the cutting zone, thus improving tool life and reducing the temperature and thermal distortion.

3. Reduce forces and energy consumption.

4. Flush away the chips from cutting zone, thus prevent the chips from interfering cutting operations.

5. Protect the machined surface from environmental corrosion.

- Depending on the type of machining operation, the cutting fluid needed may be a coolant, a lubricant, or might need both.

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- The effectiveness of the cutting fluids depends on a number of factors, such as the type of machining operations, tool and workpiece materials, cutting speed, and the method of application.

- Example: water is an excellent coolant and can reduce effectively the high temperatures developed in the cutting zone. However, on the other hand, water is not an effective lubricant, hence it does not reduce the friction and can causes rusting of workpieces and machine tool components.

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Types of cutting fluidsTypes of cutting fluidsTypes:

1. Oils: mineral, animal, vegetable, compounded, and synthetic oils. Use for low speed operations.

2. Emulsions: mixture of oil and water and additives. Use for high speed operations.

3. Semisynthetics: chemical emulsions containing little mineral water, diluted in water, and

additives.

4. Synthetics: chemical with additives, diluted in water, and contain no oil.

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Effects of cutting fluidsEffects of cutting fluids-The selection of a cutting fluid also should include considerations such as its effects on:

• Workpiece material and machine tools.Consideration should be made whether the machined component will be subjected to stress and adverse effects, which possibly leading to stress, corrosion or cracking. This concern particularly for cutting fluids with the assistance of sulfur and chlorine additives. The machine parts should be cleaned and washed in order to remove any cutting fluid residue.

• Biological considerations. This is related to the health effects of the operator contact with fluids.Mist, fumes, smoke, and odors from cutting fluids can cause severe skin reactions and respiratory problems, especially for those cutting fluids which contains with chemical constituents such as sulfur, chlorine, phosphorus, hydrocarbons, biocides and other various additives.

• The environment. The cutting fluids may undergo chemical changes as they are used repeatedly over time. These changes may be due to environmental effects or caused contamination from various sources.

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Application Application of Cutting of Cutting

FluidsFluidsFigure: Schematic illustration of proper methods of applying cutting fluids in various machining operations: (a) turning, (b) milling, (c) thread grinding, and (d) drilling.

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Machining Processes Used to Produce Round Shapes

Cutting Cutting OperationsOperations

Figure: Various cutting operations that can be performed on a late. Not that all parts have circular symmetry.

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Components of a LatheComponents of a Lathe

Figure: Components of a lathe.

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General Characteristics of Machining ProcessesGeneral Characteristics of Machining ProcessesTABLE: General Characteristics of Machining Processes Process Characteristics Commercial tolerances(±mm) Turning Turning and facing operations on all types of materials; uses single-point or form tools; requires skilled

labor; low production rate, but medium to high with turret lathes and automatic machines, requiring less-skilled labor.

Fine: 0.05–0.13 Rough: 0.13 Skiving: 0.025–0.05

Boring Internal surfaces or profiles, with characteristics similar to turning; stiffness of boring bar important to avoid chatter.

0.025

Drilling Round holes of various sizes and depths; requires boring and reaming for improved accuracy; high production rate; labor skill required depends on hole location and accuracy specified.

0.075

Milling Variety of shapes involving contours, flat surfaces, and slots; wide variety of tooling; versatile; low to medium production rate; requires skilled labor.

0.13–0.25

Planing Flat surfaces and straight contour profiles on large surfaces; suitable for low-quantity production; labor skill required depends on part shape.

0.08–0.13

Shaping Flat surfaces and straight contour profiles on relatively small workpieces; suitable for low-quantity production; labor skill required depends on part shape.

0.05–0.13

Broaching External and internal flat surfaces, slots, and contours with good surface finish; costly tooling; high production rate; labor skill required depends on part shape.

0.025–0.15

Sawing Straight and contour cuts on flat or structural shapes; not suitable for hard materials unless saw has carbide teeth or is coated with diamond; low production rate; requires only low labor skill.

0.8

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Schematic Illustration of a Turning OperationSchematic Illustration of a Turning Operation

Figure (a) Schematic illustration of a turning operation showing depth of cut, d, and feed, f. Cutting speed is the surface speed of the workpiece at the Fc, is the cutting force, Ft is the thrust or feed force (in the direction of feed, Fr is the radial force that tends to push the tool away from the workpiece being machined.

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Summary of Turning Parameters and Summary of Turning Parameters and FormulasFormulas

N = Rotational speed of the workpiece, rpm f = Feed, mm/rev or in/rev v = Feed rate, or linear speed of the tool along workpiece length, mm/min or in/min =fN V = Surface speed of workpiece, m/min or ft/min = p Do N (for maximum speed) = p Davg N (for average speed) l = Length of cut, mm or in. Do = Original diameter of workpiece, mm or in. Df = Final diameter of workpiece, mm or in. Davg = Average diameter of workpiece, mm or in. = (Do +Df ) /2 d = Depth of cut, mm or in. = ( Do +Df ) /2 t = Cutting time, s or min =l/f N MRR = mm

3/min or in

3/min

= p Davg d fN Torque = Nm or lb ft = ( Fc )( Davg /2 ) Power = kW or hp = (Torque) (w , where w=2p radians/min Note: The units given are those that are commonly used; however, appropriate units must be used and checked in the formulas.

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Material Removal Rate (MRR)- Material removal rate in turning is the volume of material removed per unit time with the units of

mm3/min.

- For each revolution of the w/piece, a ring-shaped of layer of material is removed, which has a cross sectional area that equals the product of distance the tool travels in one revolution (feed, f), and the depth of cut, d.

- The volume of this ring is the product of the cross sectional area (f)(d), and the average circumference of the ring, πDavg, where:

Davg = (Do + Df )/2

- The rotational sped of the w/piece is N, and the material removal rate per revolution is (π)(Davg)(d)(f)

- Since there are N revolutions per minute, the removal rate is :MRR = πDavgdfN

- This equation can also be written as:MRR = dfV

- Where V is the cutting speed and MRR has the same unit of mm3/min.

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- The cutting time for , t, for a w/piece of length, l, can be calculated by noting the tool travels at a feed rate of fN= (mm/rev)(rev/min) = mm/min.

- Since the distance traveled is l mm, the cutting time ist = l / fN

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Examples of Parts Produced Using the Machining Processes

Figure: Typical parts and shapes produced with the machining processes described in this chapter.

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Examples of Milling Cutters and OperationsExamples of Milling Cutters and OperationsFigure: Some of the basic types of milling cutters and milling operations.

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Face MillingFace MillingFigure: Face-milling operation showing (a) action of an insert in face milling; (b) climb milling; (c) conventional milling; (d) dimensions in face milling. The width of cut, w, is not necessarily the same as the cutter radius.

Figure: A face-milling cutter with indexable inserts.

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Summary of Milling Parameters and FormulasSummary of Milling Parameters and Formulas N = Rotational speed of the milling cutter, rpm f = Feed, mm/tooth or in./tooth D = Cutter diameter, mm or in. n = Number of teeth on cutter v = Linear speed of the workpiece or feed rate, mm/min or in./min V = Surface speed of cutter, m/min or ft/min =D N f = Feed per tooth, mm/tooth or in/tooth =v /N n l = Length of cut, mm or in. t = Cutting time, s or min =( l+lc ) v , where lc =extent of the cutter’s first contact with workpiece MRR = mm

3/min or in.

3/min

=w d v , where w is the width of cut Torque = N-m or lb-ft ( Fc ) (D/2) Power = kW or hp = (Torque) ( ), where = 2 N radians/min Note: The units given are those that are commonly used; however, appropriate units must be used in the formulas.

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Face Milling

- In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the

w/piece surface and removes material.

- The cutter rotates at rotational speed, N, and the w/piece moves along a straight path at a linear

speed, v.

- The cutting teeth, such as carbide inserts are mounted on the cutter body of the spindle.

- Due to the relative motion between the cutter teeth and the w/piece, face milling leaves feed

marks on the machined surface, similar like turning operations.

- The lead angle of the insert in face milling has a direct influence on the underformed chip

thickness.

- As the lead angle increases, the underformed chip thickness decreases (the decreases of chip

thickness), and the length of the contact increases (the width of the chip increases).

- The lead angle also influences the force in milling.

- The range of lead angles for most milling cutters is typically from 00 to 450.

- The ratio of the cutter diameter, D, to the width of cut, w, should be not less than 3:2.

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Effect of Lead AngleEffect of Lead AngleFigure shows the effect of lead angle on the undeformed chip thickness in face milling. Note that as the lead angle increase, the chip thickness decreases, but the length of contact (i.e., chip width) increases. The insert in (a) must be sufficiently large to accommodate the contact length increase.

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Cutters for Different Types of MillingCutters for Different Types of Milling

Figure shows: Cutters for (a) straddle milling, (b) form milling, (c) slotting, and (d) slitting with a milling cutter.

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Other Milling Operations and CuttersOther Milling Operations and Cutters

Figure shows: (a) T-slot cutting with a milling cutter. (b) A shell mill.

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Horizontal- and Vertical-Spindle Column-and-Knee Horizontal- and Vertical-Spindle Column-and-Knee Type Milling MachinesType Milling Machines

Left figure shows a schematic illustration of a horizontal-spindle

column-and-knee type milling machine.

Right figure shows a schematic illustration of a vertical-spindle column-and-knee type milling machine (also called a knee miller).

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Milling parameters

- The cutting speed, V, in milling is the surface speed of the cutter:V = πDN

- Where D is the cutter diameter and N is the rotational speed of the cutter.

- Feed per tooth is determined by:f = v/Nn

- Where v is the linear speed (feed rate) of the w/piece and n is the number of teeth on the cutter.

- The cutting time, t, is given by:t = (l + 2lc)/v

- Where l is the length of the workpiece and lc is the extent of the cutter’s first contact with w/piece.

- Therefore, the material removal rate MRR is:MRR = wdv

Where w is the width of the cut.

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Exercise:Exercise:

A face milling operation is being carried out on a mild steel block with D = 300 mm, w =

120 mm, l = 1000 mm, d = 6.0 mm, v = 1.2 m/min, and N = 200 rpm. The cutter has 20

inserts, and the work piece material is a high strength aluminum alloy. Calculate the

material removal rate in m3/min and cutting time in second.

[MRR = wdv; t = l + 2lc / v]

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Cross section of the cut, wd = (120) (6) = 720 mm2

Work piece speed, v = 1.2 m/min = 1200 mm3/minThus,

MRR = (720) (1200) = 0.84 m3/min

The cutting time, t = ( l + 2lc ) / v

Where, lc = D/2 = 300/2 = 150 mm

Thus, Cutting time = 1000 + 300 = 65 s

Answer:Answer:

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