Lecture No 12
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Metal Removal Processes
Dr. Ramon E. Goforth
Adjunct Professor of Mechanical
Engineering
Southern Methodist University
Lecture No 12
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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 12
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Basic Cutting Processes
• Rotating part - turning
– Creates round shapes
• Stationary part - milling, drilling, sawing,
etc
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Basic Turning
• Part of cylindrical cross section clamped
in a "chuck" so that it can rotate about its
axis
• Part is rotated at fixed speed
• A cutting tool is brought to bear on the
moving surface of the part cutting of
material
• The "chuck" is a kind of vice which has
rotational symmetry
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Turning Process Parameters
f
d
N
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Turning Parameters
•Tool Geometry
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Rake angles
Side rake angle - more important than
Back rake angle
Cutting edge angles
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Turning Parameters
• Tool Geometry
• Tool Materials
• Feeds and speeds, N,d,f
– (see table 22.4 for recommendations)
• Cutting fluids
• Material Removal rates
– = p Davg d f N
• Where Davg is the average diameter, d is the depth of
cut, f is the feed rate and N the rotational speed
• Forces and power used
• Surface finish (scallops)
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Power used
• Power used is the material removal rate,
MRR, times the specific energy
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Feed Marks in Turning
• Scallops created
• The depth depends on the feed rate, surface
velocity and tool shape
Scallops
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Machining Processes for Round
Shapes
• Turning
• Facing
• Boring
– Produces circular internal profiles in hollow
workpieces
• Drilling
– Produces round holes
• Reaming
– Produces more accurate holes than drilling
• Parting
• Threading
• Knurling
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Lecture No 12
Machining Processes for Round
Shapes
Kalpakjian p 663
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Turning Guidelines
• Avoid long skinny parts
• Request wide accuracy and surface finish
parameters
• Avoid sharp corners and tapers
• Avoid major dimensional changes
• Design blanks to be as close to final
dimensions as possible
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Turning Guidelines
• Allow for travel of tools across surfaces of
workpiece
• Design features so that standard tools can be
used
• Choose machinable materials
• Minimize overhang of tool
• Support workpiece
• Use machines with high rigidity
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Non Round Machining Processes
• The operation
– Clamp the workpiece onto a stationary bed
or one that can move in multiple directions
slowly
– Bring a rotating tool to bear on the surface
to be shaped
– Move the rotating tool over the part or
move the part past the rotating tool to
shape it
Non Round Machining - Slab
Milling
Lecture No 12
• Milling
– Slab/Peripheral
– Cutter rotation axis parallel to
workpiece surface
• Conventional/up
– Maximum chip thickness
at end of cut
– Low impact of tool with workpiece
• Climb/down
– Maximum chip thickness at beginning
of cut
– High low impact of tool with workpiece
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Non Round Machining - Face
milling
– Axis of rotation
perpendicular to
workpiece surface
– Large multi-insert cutter
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Non Round Machining - Face
Milling
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• Difference between climb and
conventional face milling
Action of an insert in face milling
Climb Milling
Parameters in face milling
Conventional milling
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Non Round Machining
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Generic Milling formula
• Cutting (peripheral) speed,
– V =pDN
– where D is the cutter diameter and N its
rotational speed
• Feed per tooth,
– f = v/Nn
– where v is the linear speed or feed rate of
the workpiece, and n is the number of teeth
• Undeformed chip thickness, (chip depth of
cut),
– tc = 2 f (d / D)
– Where f is the feed per tooth, d is the depth
of cut
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Generic Milling formula
• Cutting time, t = (l + 2lc)/ v
– where v is the feed rate of the workpiece, l is the length
of the workpiece and lc is the extent of the cutter’s first
contact with the workpiece
• Material removal rate, MRR
– MRR = lwd/t = wdv
– assuming the lc<<l and where w is the width of the cut
– Power is equal to the MRR times the specific energy
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Feed Marks from Milling
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Design Guidelines for Milling
• Design for standard cutters
• Use chamfers instead of radii
• Avoid internal cavities and pockets with sharp
corners
• Design workpieces with sufficient rigidity
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Other Non Round Machining
Processes
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Drilling
Straddle milling
Planing
Broaching
Sawing
– Generally used for cutting off pieces to be
worked on by other processes
• Filing and finishing
• Gear machining
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Drilling Practices
• Type of drill bit, drill point geometry
• Type of machine
– Drill, press, radial drills, gang drills, NC
controlled
– Capabilities of drilling and boring operations
(p 633)
– HP used = Spec. Energy times MRR (pD2fN/4)
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Drilling Operations and Drill bits
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Drilling Guidelines
• Design holes perpendicular to the surface
• Do not design interrupted/overlapping
holes
• Design bottoms to match standard drillpoint angles
• Through holes are preferred over blind
holes
• If need large diameter holes design in
smaller hole for casting
• Design to minimize fixturing
• Avoid reaming blind or intersecting holes
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Machining Economics
• Cost per piece decreases with cutting
speed
• Tool cost increases with cutting speed
• Tool change time increases with cutting
speed
• Total cost goes through a minimum
• Time spent removing material usually
small fraction (<5%) of total time on
machine
Kalpakjian p 775/698
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Machining Economics
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Metal Removal Machines
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Basic Lathe
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Turning Machine Components
• Bed
– Supports all other major components
– Top part has two ways
• Carriage
– Slides along the ways
– Consists of the cross-slide, tool post and
apron
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Turning Machine Components
• Headstock
– Fixed
– Contains the motors, pulley and belts to drive
the spindle
– Spindle has fixtures for attaching the
workpiece
• Tailstock
– Can slide along the ways
– Supports the other end of the workpiece
• Feed rod and lead screw
– Provides motion to the carriage and cross
slide
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A Manual Lathe
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Turning Machines
• Lathes
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Tracer
Automatic
Automatic bar machines
Turret
Vertical
• For very large diameters
– Boring
• Vertical
• Horizontal (like a milling machine)
– Computer controlled
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Turret Lathe
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MORI SEIKI SL-3 SLANT BED CNC
LATHE
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Vertical Boring Mill
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Milling Machines
• Column and Knee type
– Horizontal spindle
– Vertical spindle
• Bed type
– Skin mills
• Other types
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Planer type
Rotary tables
Duplicating machines
Profiling milling
More than three axes
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#4 VERTICAL MILLING MACHINE
W/SLIDING HEAD
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Machining and Turning Centers
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Combines turning with milling
Computer control essential
Multiaxis capabilities
Replacing simple lathes or milling machines
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NC Turning Center
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Giddings & Lewis dv15-l smart turn twinspindle vertical production center
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Drilling Machines
• Drill presses
• Radial machines
• CNC Three axis drilling machine
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Trends
• High speed machining
• Dry machining
• Combining milling, drilling and turning
operations
• New, stiffer and highly damped machine tools
– Graphite epoxy, ceramics (high modulus)
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Modular machines
Multiple loading stations
More sensors
More and more automation
– Automated program generation
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Summary
• There are many different types of
machining operations
• That is what makes it so versatile and
attractive to industry
• The basic cutting process is the same in
all
• Must consider the cutting operation as a
system
• Actual cutting time is a small fraction of
the total time to create a part by
machining