Lecture No 11 1

Fundamentals of Metal removal processes

Dr. Ramon E. GoforthAdjunct Professor of Mechanical

EngineeringSouthern Methodist University

Lecture No 11 2

Outline of Lecture• Basic information on material removal• Factors involved in material removal• Independent variables• Dependent variables• Machining Processes• Machining Economics• Machines

Lecture 10

Lecture 11

Lecture 12

Lecture No 11 3

Independent variables in cutting• Workpiece material - "machinability"• Cutting tools• Cutting parameters• Presence or absence of fluid

Lecture No 11 4

Functions of Cutting Fluids

• Reduce friction and wear• Reduce forces and energy consumption

– 30% of total energy can go into friction and heat generated

• Cooling the cutting zone• Wash away chips• Protect new surfaces from corrosion

Lecture No 11 5

Cutting fluids

• Basically four types– Oils– Emulsions– Semisynthetic– Synthetics

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Dry Machining Scenario

• Turn off fluid– Friction increases– Shear angle decreases– Shear strain increases– Chip is thicker– Can form built up edge

• Friction increases further– Total energy increases

• Temperature increases– Dimensions change

» machining inaccurate– Surface finish likely to deteriorate– Tool wear increases

Lecture No 11 7

Dry Machining Scenario

• HOWEVER– Recent studies are taking a hard look at dry

machining to minimize environmental impact of waste fluids

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Good cooling practice

Lecture No 11 9

Independent variables in cutting• Workpiece material - "machinability"• Cutting tools• Cutting parameters• Presence or absence of fluid• Characteristics of the machine tool

Lecture No 11 10

Characteristics of the Machine• The machine provides the power and

ensures that the tool is maintained in the chosen location relative to the workpiece– Stiffness

• Deflection under load - inaccurate cuts– Dynamic response

• Vibrations - chatter - rough surfaces– Horse power available

• Determines the maximum material removal rate– Gearing

• Determines the speeds and feeds available

Lecture No 11 11

Independent variables in cutting• Workpiece material - "machinability"• Cutting tools• Cutting parameters• Presence or absence of fluid• Characteristics of the machine tool• Fixture design

Lecture No 11 12

Fixture Design

• Fixtures hold the workpiece fixed while the cutting tool acts on it– Movement under the cutting force not

desirable– Deflection of the workpiece under cutting force

not desirable• Not truly independent

– Vibration of the workpiece undesirable

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Summary of Independent Variables in Metal Removal

• Workpiece material - "machinability"• Cutting tools• Cutting parameters• Presence or absence of fluid• Characteristics of the machine tool

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Dependent Variables

• Forces and energy dissipated• Temperature rise• Tolerances of workpiece after machining• Surface finish of workpiece after

machining• Wear and failure of tool• Type of chip produced

Lecture No 11 15Relationships among the variables

Forces

Power

TempRise

Tolerances

SurfaceFinish

Chip Type

ToolDegradation

ToolChoice

Workpieceproperties

Cutting fluid

Machine Tool Characteristics

Fixture design

ProcessVariables

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Force and Energy Consumption• Important to know force to

– Avoid excessive distortion in workpiece, tools• Distortion gives rise to inaccuracies - tolerances

– Allow adequate fixturing to be designed– Determine the work done by force which

ends up as heat• Important to know Power to

– Choose a machine with adequate power capabilities

– Estimate how long it will take to machine a part

– Estimate the rate at which heat is generated

Lecture No 11 17

The Common Cutting Diagram

Kalpakjian p 595/546

Chip

Tool

Workpiece

RFn

V

Shear Zone

Friction Zone

Lecture No 11 18Forces

F = R sin N= R cos Fs = F cos Ft cosFs = F sin Ft sin F/N = (Ft + Fc tan )/(Ft - Fc tan

is rake angle: is shear angle: is friction angle

Kalpakjian p 608/546

Lecture No 11 19ForcesR is the resultant force consisting of

– thrust force, Ft and – cutting force, Fc

– OR– F, Frictional force, – N normal force

perpendicular to – friction force

Ft can be + or - depending on rake angle and frictionR is balanced by an equal and opposite force which resolves into

– Shear force, Fs and– Shear normal force Fn

Kalpakjian p 608

Lecture No 11 20

Force Diagram

Ft

N

R F

Fc

Fn

R

Fs

Workpiece

tool

Shear Zone

V

chipFriction Zone

But, forces usually computed from measured machine power or measured with sensors

Lecture No 11 21

PowerPower = FcV

where Fc is the cutting force and V is the tool

velocity

Specific Energy = power/volume= shearing energy + friction energy= FsV/wt0V + F/wto (Vc/V)

where t0 is the depth of cut, w is the width of cut

Example on page 611 illustrates that 30% of the energy can go into frictionEnergy can also go into rubbing friction if tool is dull

Lecture No 11 22

Specific Energy

Kalpakjian p 611/548

Lecture No 11 23

Where the Energy Goes• Overcoming

friction• Shearing the

metal

Temperature rise in theWorkpieceToolChip

Creating chips

Higher temperatures• Cause dimensional changes in the workpiece• Induce thermal damage in the machine surface• Affect strength, hardness and wear resistance of the

cutting tool• Eventually distort the machine tool itself

Shearing normally gives good surfacesPoor tool/feed/speed selection can produce poor

surfacesDull tools also generate heat through rubbing of the workpiece surface

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Dependent Variables

• Forces and energy dissipated• Temperature rise

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Temperature Distribution

Kalpakjian p 613/550

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Temperature Distribution

Kalpakjian p 613/550

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Dependent Variables

• Forces and energy dissipated• Temperature rise• Tolerances of workpiece after machining

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Tolerances

• Tolerances on a machine part depend on– Forces generated

• Distort the part and its fixturing– Software Tools

• Distort the tool and its holder– Depends on machine and tool design

• Distort the machine itself– Depends on the machine design

– Temperature generated• Thermal induced expansion of all components

in the system results in machining errors

Lecture No 11 29

Tolerances

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Dependent Variables

• Forces and energy dissipated• Temperature rise• Tolerances of workpiece after machining• Surface finish of workpiece after

machining

Lecture No 11 31

Surface Finish and Integrity

• Surface finish describes the geometry• Surface Integrity pertains to the

mechanical properties– Fatigue life, corrosion resistance

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Surface Finish and Integrity

• Factors affecting surface integrity include– Workpiece temperature during processing– Residual stresses induced by the shearing– Metallurgical effects (phase transformations)– Plastic deformation– Tearing– Built up edge on chip

• Table on Page 685/616 provides surface roughness for various processes

Lecture No 11 33Machining Processes and Surface Finish

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Dependent Variables

• Forces and energy dissipated• Temperature rise• Tolerances of workpiece after machining• Surface finish of workpiece after

machining• Wear and failure of tool

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Tool Life• Very important economic factor

– Cost of tools– Cost of damaged workpiece– Cost of rework due to inaccurate

machining• Machinability of part has direct influence

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Tool Life• Abrasion and high temperature cause wear

on – The face

• mostly craters– The flank

• High forces and shocks (interrupted cutting)cause chipping – Fracture of the tool– Produces holes and gouges in part

• Poorly machinable materials can give a built up edge– Material adheres to edge of tool and causes

inaccuracies and extra friction

Lecture No 11 37

Tool Life

• Formula for tool life due to wear and abrasion– VTn = C

• C, n are constants• V is cutting speed, T is time in minutes

– VTn dx fy = C• d is depth of cut, f is feed rate, x and y are

constants– Tool life, T given by

• T = C1/nV-1/nd-x/nf-y/n

– For n=0.15, x= 0.15, y=.06• T=C7V-7d-1f-4

Lecture No 11 38

Tool Life for Different Materials

Kalpakjian P 617/553

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Dependent Variables

• Forces and energy dissipated• Temperature rise• Tolerances of workpiece after machining• Surface finish of workpiece after

machining• Wear and failure of tool• Type of chip produced

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Type of Chip Produced• Discontinous chips, continuous strands,

continuous serrated strands, built up edge (on tool)

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Type of Chip Produced

• Depends on the:– machinability of the

workpiece– the design of the

cutting tool– the design of the

tool holder

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Summary

• The results of choosing a specific set of the independent variables influences:– Forces and energy dissipated– Temperature rise– Tolerances of workpiece after machining– Surface finish of workpiece after machining– Wear and failure of tool– Type of chip produced

• Forces and power are important for choice of machine for a job

• Next week we discuss some of the individual processes