Download lecture_11 - Lyle School of Engineering

Lecture No 11
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Fundamentals of Metal removal
processes
Dr. Ramon E. Goforth
Adjunct Professor of Mechanical
Engineering
Southern Methodist University
Lecture No 11
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Outline of Lecture
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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
Independent variables in cutting
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Workpiece material - "machinability"
Cutting tools
Cutting parameters
Presence or absence of fluid
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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
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Cutting fluids
• Basically four types
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Oils
Emulsions
Semisynthetic
Synthetics
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Dry Machining Scenario
• Turn off fluid
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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
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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
Independent variables in cutting
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Workpiece material - "machinability"
Cutting tools
Cutting parameters
Presence or absence of fluid
Characteristics of the machine tool
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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
Independent variables in cutting
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Workpiece material - "machinability"
Cutting tools
Cutting parameters
Presence or absence of fluid
Characteristics of the machine tool
Fixture design
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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
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Workpiece material - "machinability"
Cutting tools
Cutting parameters
Presence or absence of fluid
Characteristics of the machine tool
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Dependent Variables
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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
Relationships among the variables
Workpiece
properties
Tool
Choice
Cutting
fluid
Process
Variables
Fixture
design
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Machine Tool
Characteristics
Tolerances
Forces
Surface
Finish
Temp
Rise
Chip
Type
Power
Tool
Degradation
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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
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The Common Cutting Diagram
Friction Zone
a
Chip
V
Shear Zone
Tool
Fn
Workpiece
Kalpakjian p
595/546
f
R
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Forces
Kalpakjian
p 608/546
a is rake angle:
f is shear angle:
b is friction angle
F = R sin b :
N= R cos b
Fs = F cos f - Ft cos f
Fs = F sin f - Ft sin f
m = F/N = (Ft + Fc tan a)/(Ft - Fc tan a)
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Lecture No 11
Forces
R 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
friction
R is balanced by an equal and opposite force
Kalpakjian p 608
which resolves into
– Shear force, Fs and
– Shear normal force Fn
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Force Diagram
Friction Zone
chip
Fs
R
b
N
R
Fc
F
a
Ft
tool
f
Fn
Shear Zone
Workpiece
But, forces usually computed from measured machine
power or measured with sensors
V
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Power
Power = 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 friction
Energy can also go into rubbing friction if tool is
dull
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Specific Energy
Kalpakjian
p 611/548
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Where the Energy Goes
• Overcoming
friction
• Shearing the
metal
Temperature rise in the
Workpiece
Tool
Chip
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 surfaces
Poor tool/feed/speed selection can produce poor
surfaces
Dull 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
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Tolerances
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Dependent Variables
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Forces and energy dissipated
Temperature rise
Tolerances of workpiece after machining
Surface finish of workpiece after
machining
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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
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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
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Machining Processes and
Surface Finish
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Dependent Variables
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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
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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
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Tool Life for Different Materials
Kalpakjian
P 617/553
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Dependent Variables
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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:
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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