format report

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FORMAT REPORT

FAKULTI KEJURUTERAAN MEKANIKALUNIVERSITI TEKNIKAL MALAYSIA MELAKA

REPORT 1. Title and Front Cover2. Objective3. Brief of project (1 page)4. Working method and procedure existing project. For CNC include

programming.5. Working method and procedure for proposed project (new project

proposed by student. (For CNC include programming)6. Discussion

-Problem encountered-Countermeasure-Suggestion

7. Safety precaution 8. Conclusion9. Appendix (if any) Report not more 30 pages comb binding; 1.5 line spacing; font size

12. Submit date: Last working day of Report Week. Penalty will be

imposed for late submission.

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FUNDAMENTAL OF MACHINING

Fundamental Of Machining

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Fundamental Of Machining


Chapter Outline

1. Mechanics of Cutting

2. Cutting Forces and Power

3. Temperatures in Cutting

4. Tool Life: Wear and Failure

5. Surface Finish and Integrity Machinability

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INTRODUCTION


Cutting processes remove material from the surface of a workpiece by producing chips. Some of the more common cutting processes are as follow:

1. Turning, in which the workpiece is rotated and a cutting tool removes a layer of material as it moves to the left.

2. Cutting-off operation, where the cutting tool moves radially inward and separates the right piece from the bulk of the blank.

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INTRODUCTION


Cutting processes remove material from the surface of a workpiece by producing chips. Some of the more common cutting processes are as follow:

3. Slab-milling operation, in which a rotating cutting tool removes a layer of material from the surface of the workpiece.

4. End-milling operation, in which a rotating cutter travels along a certain depth in the workpiece and produces a cavity.

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INTRODUCTION


Fig BELOW shows some examples of common machining operations.

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INTRODUCTION


Fig BELOW shows the schematic illustration of the turning operation showing various features.

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INTRODUCTION


Next Fig shows the schematic illustration of a two-dimensional cutting process, also called orthogonal cutting:

(a) Orthogonal cutting with a well-defined shear plane, also known as the Merchant model. Note that the tool shape, depth of cut, and the cutting speed, V, are all independent variables

(b) Orthogonal cutting without a well-defined shear plane.

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INTRODUCTION


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MECHANICS OF CUTTING


The factors that influence the cutting process are outlined in Next Table.

In order to appreciate the contents of this table, let’s now identify the major independent variables in the cutting process as follows: (a) tool material and coatings; (b) tool shape, surface finish, and sharpness; (c) workpiece material and condition; (d) cutting speed, feed, and depth of cut; (e) cutting fluids; (f) characteristics of the machine tool; and (g) workholding and fixturing.

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Dependent variables in cutting are those that are influenced by changes in the independent variables listed above, and include:

(a) type of chip produced, (b) force and energy dissipated during

cutting, (c) temperature rise in the workpiece, the

tool, and the chip, (d) tool wear and failure, and (e) surface finish and surface integrity of the

workpiece.

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Fig below (a) shows the schematic illustration of the basic mechanism of chip formation by shearing. (b) Velocity diagram showing angular relationships among the three speeds in the cutting zone.

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sin1

costan

r

r



Cutting Ratio It can be seen that the chip thickness, tc, can be

determined by knowing the depth of cut, to and α and Φ.

The ratio of to / tc is known as the cutting ratio (or chip-thickness ratio), r, and is related to the two angles by the following relationships:

cos

sin

c

o

t

tr

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Cutting Ratio The reciprocal of r is known as the chip-

compression ratio or factor and is thus a measure of how thick the chip has become compared to the depth of cut; hence, the chip-compression ratio always is greater than unity.

The depth of cut also is referred to as the undeformed chip thickness.

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Cutting Ratio The reciprocal of r is known as the chip-

compression ratio or factor and is thus a measure of how thick the chip has become compared to the depth of cut; hence, the chip-compression ratio always is greater than unity.

The depth of cut also is referred to as the undeformed chip thickness.

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Shear Strain The shear strain, , that the material undergoes can

be expressed as

Note that large shear strains are associated with low shear angles or with low or negative rake angles.

tancotOC

OB

OC

AO

OC

AB

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Shear Strain The shear angle has great significance in the

mechanics of cutting operations. It influences force and power requirements, chip

thickness, and temperature. This analysis yielded the expression

where β is the friction angle and μ is related to the coefficient of friction, at the tool–chip interface by the expression μ = tan β .

2245

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Shear Strain Among the many shear–angle relationships developed,

another useful formula that generally is applicable is

The coefficient of friction in metal cutting generally ranges from about 0.5 to 2, indicating that the chip encounters considerable frictional resistance while moving up the tool’s rake face.

45

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Velocities in the cutting zone Since mass continuity has to be maintained,

A velocity diagram also can be constructed, where from trigonometric relationships, we obtain the equation

cos

sin

or

VV

VVtVVt

c

rccco

sincoscoscs VVV

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Velocities in the cutting zone

where Vs is the velocity at which shearing take place in the shear plane.

Note also that

These velocity relationships will be utilized further when describing power requirements in cutting operations.

V

V

t

tr c

c

o

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Types of chips produced in metal cutting The four main types are:

1. Continuous

2. Built-up edge

3. Serrated or segmented

4. Discontinuous

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Types of chips produced in metal cuttingContinuous chips Continuous chips usually are formed with ductile

materials, machined at high cutting speeds and/or high rake angles.

The deformation of the material takes place along a narrow shear zone called the primary shear zone.

Continuous chips may develop a secondary shear zone because of high friction at the tool–chip interface; this zone becomes thicker as friction increases.

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Types of chips produced in metal cuttinga) Continuous chip in

cutting brassb) Secondary shear

zone in cutting copper

c) Built Up edge in cutting sintered tungsten

d) Serrated chip in cutting S.steel

e) Discontinuous chip in cutting brass

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Types of chips produced in metal cuttingContinuous chips Deformation in continuous chips also may

take place along a wide primary shear zone with curved boundaries.

This problem can be alleviated with chip breakers (to follow) or by changing parameters, such as cutting speed, feed, depth of cut, and by using cutting fluids.

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Types of chips produced in metal cuttingBuilt-up edge chips A built-up edge (BUE) consists of layers of

material from the workpiece that gradually are deposited on the tool tip—hence the term built-up.

Built-up edge commonly is observed in practice. It is a major factor that adversely affects surface

finish, however, a thin, stable BUE usually is regarded as desirable because it reduces tool wear by protecting its rake face.

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Types of chips produced in metal cuttingBuilt-up edge chips The tendency for BUE formation can be reduced

by one or more of the following means:

1. Increase the cutting speeds

2. Decrease the depth of cut

3. Increase the rake angle

4. Use a sharp tool

5. Use an effective cutting fluid

6. Use a cutting tool that has lower chemical affinity for the workpiece material

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Types of chips produced in metal cuttingSerrated chips Serrated chips are semicontinuous chips with

large zones of low shear strain and small zones of high shear strain, hence the latter zone is called shear localization.

Metals with low thermal conductivity and strength that decreases sharply with temperature (thermal softening) exhibit this behavior, most notably titanium.

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Types of chips produced in metal cuttingDiscontinuous chips Discontinuous chips consist of segments that may be

attached firmly or loosely to each other. Discontinuous chips usually form under the following

conditions:

1. Brittle workpiece materials, because they do not have the capacity to undergo the high shear strains involved in cutting.

2. Workpiece materials that contain hard inclusions and impurities or have structures such as the graphite flakes in gray cast iron.

3. Very low or very high cutting speeds

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Types of chips produced in metal cuttingDiscontinuous chips

4. Large depths of cut.

5. Low rake angles.

6. Lack of an effective cutting fluid.

7. Low stiffness of the toolholder or the machine tool, thus allowing vibration and chatter to occur.

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Types of chips produced in metal cuttingDiscontinuous chips Because of the discontinuous nature of chip

formation, forces continually vary during cutting.

Consequently, the stiffness or rigidity of the cutting-tool holder, the workholding devices, and the machine tool are important in cutting with serrated chips as well as with discontinuous chips.

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Types of chips produced in metal cuttingChip Curl In all cutting operations performed on metals,

as well as nonmetallic materials such as plastics and wood, chips develop a curvature (chip curl) as they leave the workpiece surface.

Among factors affecting the chip curl are:1. The distribution of stresses in the primary and

secondary shear zones.

2. Thermal effects.

3. Work-hardening characteristics of the workpiece material.

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Types of chips produced in metal cuttingChip Curl4. The geometry of the cutting tool.

5. Cutting fluids. Generally, as the depth of cut decreases, the radius

of curvature decreases; that is, the chip becomes curlier.

Also, cutting fluids can make chips become more curly (the radius of curvature decreases), thus reducing the tool–chip contact area and concentrating the heat closer to the tip of the tool. As a result, tool wear increases.

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Types of chips produced in metal cuttingChip Breakers Continuous and long chips are undesirable as

they tend to become entangled and severely interfere with machining operations and also become a potential safety hazard.

Next Fig(a) shows the schematic illustration of the action of a chip breaker. Note that the chip breaker decreases the radius of curvature of the chip and eventually breaks it. (b) Chip breaker clamped on the rake face of a cutting tool. (c) Grooves in cutting tools acting as chip breakers. Most cutting tools used now are inserts with built-in chip-breaker features.

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Types of chips produced in metal cutting

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Types of chips produced in metal cuttingChip Breakers Chip breakers have traditionally been a piece of

metal clamped to the tool’s rake face, which bend and break the chip.

However, most modern cutting tools and inserts now have built-in chip-breaker features of various designs.

Next Fig shows the chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and breaks; (c) continuous chip moving radially away from workpiece; and (d) chip hits tool shank and breaks off.

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38




Chip Breakers In interrupted-cutting operations (such as

milling), chip breakers generally are not necessary, since the chips already have finite lengths.

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Types of chips produced in metal cuttingControlled contact on tools Cutting tools can be designed so that the tool–chip

contact length is reduced by recessing the rake face of the tool some distance away from its tip.

This reduction in contact length affects chip-formation mechanics.

Primarily, it reduces the cutting forces and, thus, the energy and temperature.

Determination of an optimum length is important as too small a contact length would concentrate the heat at the tool tip, thus increasing wear.

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Types of chips produced in metal cuttingCutting nonmetallic materials A variety of chips are encountered in cutting

thermoplastics, depending on the type of polymer and process parameters, such as depth of cut, tool geometry, and cutting speed.

Many of the discussions concerning metals also are applicable generally to polymers.

Because they are brittle, thermosetting plastics and ceramics generally produce discontinuous chips.

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Oblique Cutting The majority of machining operations involve tool

shapes that are three-dimensional, thus the cutting is oblique.

Whereas in orthogonal cutting, the chip slides directly up the face of the tool, in oblique cutting, the chip is helical and at an angle i, called the inclination angle.

Next Fig(a) shows the schematic illustration of cutting with an oblique tool. Note the direction of chip movement. (b) Top view, showing the inclination angle, i. (c) Types of chips produced with tools at increasing inclination angles.

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Oblique Cutting

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Oblique Cutting Note that the chip in previous Fig flows up the rake

face of the tool at angle (chip flow angle), which is measured in the plane of the tool face.

Angle αi is the normal rake angle, and it is a basic geometric property of the tool.

This is the angle between line oz normal to the workpiece surface and line oa on the tool face.

The effective rake angle, αe is

ie ii sincossinsin 221

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Oblique Cutting Next Fig shows the schematic illustration of a

right-hand cutting tool. The various angles on these tools and their effects on machining.

Although these tools traditionally have been produced from solid tool-steel bars, they have been replaced largely with Next Fig inserts made of carbides and other materials of various shapes and sizes.

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Oblique Cutting

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Oblique CuttingShaving and skiving Thin layers of material can be removed from

straight or curved surfaces by a process similar to the use of a plane to shave wood.

Shaving is useful particularly in improving the surface finish and dimensional accuracy of sheared parts and punched slugs.

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CUTTING FORCES & POWER


Knowledge of the cutting forces and power involved in machining operations is important for the following reasons:• Data on cutting forces is essential so that:a. Machine tools can be properly designed to minimize distortion of the machine components, maintain the desired dimensional accuracy of the machined part, and help select appropriate toolholders and workholding devices.b. The workpiece is capable of withstanding these forces without excessive distortion.• Power requirements must be known in order to enable the selection of a machine tool with adequate electric power.

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Fig Below (a) shows the Forces acting in the cutting zone during two-dimensional cutting. Note that the resultant force, R, must be colinear to balance the forces. (b) Force circle to determine various forces acting in the cutting zone.

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The thrust force, acts in a direction normal to the cutting speed.

These two forces produce the resultant force, R, as can be seen from the force circle.

Note that the resultant force can be resolved into two components on the tool face: a friction force, F, along the tool-chip interface and a normal force, N, perpendicular to it. It can also be shown that

cos

sin

RN

RF

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Note also that the resultant force is balanced by an equal and opposite force along the shear plane and is resolved into a shear force, and a normal force.

It can be shown that these forces can be expressed as follows:

cossin

sincos

tcn

tcs

FFF

FFF

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Because the area of the shear plane can be calculated by knowing the shear angle and the depth of cut, the shear and normal stresses in the shear plane can be determined.

The ratio of F to N is the coefficient of friction, μ, at the tool–chip interface, and the angle β is the friction angle.

The magnitude of μ can be determined as

tan

tan

tc

ct

FF

FF

N

F

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Thrust force A knowledge of the thrust force in cutting is important

because the tool holder, the workholding devices, and the machine tool must be sufficiently stiff to support this force with minimal deflections.

We also can show the effect of rake angle and friction angle on the direction of thrust force by noting from previous Fig. that

tan

sin

ct

t

FF

RF

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Thrust force A knowledge of the thrust force in cutting is important

because the tool holder, the workholding devices, and the machine tool must be sufficiently stiff to support this force with minimal deflections.

We also can show the effect of rake angle and friction angle on the direction of thrust force by noting from previous Fig. that

tan

sin

ct

t

FF

RF

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Power Power is the product of force and velocity. Thus, the

power input in cutting is

This power is dissipated mainly in the shear zone (due to the energy required to shear the material) and on the rake face of the tool (due to tool–chip interface friction).

The power dissipated in the shear plane is

VFcPower

ssVFshearingforPower

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Vwt

VFu

o

sss

cFVfrictionforPower



Power Letting w be the width of cut, the specific energy for

shearing, us, is given by

Similarly, the power dissipated in friction is

and the specific energy for friction, uf is

oo

cf wt

Fr

Vwt

FVu

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Power The total specific energy, ut thus is

Because of the many factors involved, reliable prediction of cutting forces and power still is based largely on experimental data, such as those given in Next Table.

The sharpness of the tool tip also influences forces and power. Because it rubs against the machined surface and makes the deformation zone ahead of the tool larger, duller tools require higher forces and power.

fst uuu

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Measuring cutting forces and power Cutting forces can be measured using a force

transducer (typically with quartz piezoelectric sensors), a dynamometer or a load cell (with resistance-wire strain gages placed on octagonal rings) mounted on the cutting-tool holder.

It is possible to calculate cutting force from the power consumption during cutting, provided that the mechanical efficiency of the machine tool is known or can be determined.

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Example :Relative energies in cutting

In an orthogonal cutting operation, to = 0.13 mm, V = 120 m/min, α = 10º and the width of cut=6 mm. It is observed that tc = 0.23 mm, Fc = 500 N and Ft = 200 N. Calculate the percentage of the total energy that goes into overcoming friction at the tool-chip interface.

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Solution The percentage of the energy can be expressed as

%32or32.0

500

565.0286Percentage

N28632sin539

32

10539cos500

N539500200

cos

sin

565.0

22

23.013.0

energyTotalenergyFriction

F

FFR

RF

RF

r

ct

c

tt

FFr

VFFV

c

o

cc

c

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THE END


THANK YOU

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cutting speed

rotating cutting tool

common cutting processes

tool material

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