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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 6
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
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
Lecture No 11 13
Summary of Independent Variables in Metal Removal
• Workpiece material - "machinability"• Cutting tools• Cutting parameters• Presence or absence of fluid• Characteristics of the machine tool
Lecture No 11 14
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
Lecture No 11 16
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 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
Lecture No 11 27
Dependent Variables
• Forces and energy dissipated• Temperature rise• Tolerances of workpiece after machining
Lecture No 11 28
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 30
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
Lecture No 11 32
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 34
Dependent Variables
• Forces and energy dissipated• Temperature rise• Tolerances of workpiece after machining• Surface finish of workpiece after
machining• Wear and failure of tool
Lecture No 11 35
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
Lecture No 11 36
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 39
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 40
Type of Chip Produced• Discontinous chips, continuous strands,
continuous serrated strands, built up edge (on tool)
Lecture No 11 41
Type of Chip Produced
• Depends on the:– machinability of the
workpiece– the design of the
cutting tool– the design of the
tool holder
Lecture No 11 42
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