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Lecture No 12 1
Metal Removal Processes
Dr. Ramon E. GoforthAdjunct Professor of Mechanical
EngineeringSouthern Methodist University
Lecture No 12 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 12 3
Basic Cutting Processes
• Rotating part - turning– Creates round shapes
• Stationary part - milling, drilling, sawing, etc
Lecture No 12 4
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
Lecture No 12 5
Turning Process Parameters
N
d
f
Lecture No 12 6
Turning Parameters•Tool Geometry
– Rake angles– Side rake angle - more important than– Back rake angle– Cutting edge angles
Lecture No 12 7
Turning Parameters
• Tool Geometry• Tool Materials• Feeds and speeds, N,d,f
– (see table 22.4 for recommendations)• Cutting fluids • Material Removal rates
– = 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)
Lecture No 12 8
Power used
• Power used is the material removal rate, MRR, times the specific energy
Lecture No 12 9
Feed Marks in Turning
• Scallops created• The depth depends on the feed rate, surface
velocity and tool shape
Scallops
Lecture No 12 10Machining 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
Lecture No 12 11Machining Processes for Round Shapes
Kalpakjian p 663
Lecture No 12 12
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
Lecture No 12 13
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
Lecture No 12 14
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
Lecture No 12 15Non Round Machining - Slab Milling
• 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
Lecture No 12 16 Non Round Machining - Face milling
– Axis of rotation perpendicular to workpiece surface
– Large multi-insert cutter
Lecture No 12 17Non Round Machining - Face Milling
• Difference between climb and conventional face milling
Action of an insert in face milling
Climb Milling Conventional milling
Parameters in face milling
Lecture No 12 18
Non Round Machining
Lecture No 12 19
Generic Milling formula• Cutting (peripheral) speed,
– V = D N– 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
Lecture No 12 20
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
Lecture No 12 21
Feed Marks from Milling
Lecture No 12 22
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
Lecture No 12 23
Other Non Round Machining Processes
• Drilling• Straddle milling• Planing• Broaching• Sawing
– Generally used for cutting off pieces to be worked on by other processes
• Filing and finishing• Gear machining
Lecture No 12 24
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 (D2fN/4)
Lecture No 12 25
Drilling Operations and Drill bits
Lecture No 12 26
Drilling Guidelines
• Design holes perpendicular to the surface• Do not design interrupted/overlapping
holes• Design bottoms to match standard drill-
point 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
Lecture No 12 27
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
Lecture No 12 28
Machining Economics
Lecture No 12 29
Metal Removal Machines
Lecture No 12 30
Basic Lathe
Lecture No 12 31
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
Lecture No 12 32
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
Lecture No 12 33
A Manual Lathe
Lecture No 12 34
Turning Machines
• Lathes – Tracer– Automatic– Automatic bar machines– Turret – Vertical
• For very large diameters– Boring
• Vertical• Horizontal (like a milling machine)
– Computer controlled
Lecture No 12 35
Turret Lathe
Lecture No 12 36MORI SEIKI SL-3 SLANT BED CNC LATHE
Lecture No 12 37
Vertical Boring Mill
Lecture No 12 38
Milling Machines
• Column and Knee type– Horizontal spindle– Vertical spindle
• Bed type– Skin mills
• Other types– Planer type– Rotary tables– Duplicating machines– Profiling milling– More than three axes
Lecture No 12 39#4 VERTICAL MILLING MACHINE W/SLIDING HEAD
Lecture No 12 40
Machining and Turning Centers
• Combines turning with milling• Computer control essential• Multiaxis capabilities• Replacing simple lathes or milling machines
Lecture No 12 41
NC Turning Center
Lecture No 12 42
Giddings & Lewis dv15-l smart turn twin-spindle vertical production center
Lecture No 12 43
Drilling Machines
• Drill presses• Radial machines• CNC Three axis drilling machine
Lecture No 12 44
Trends
• High speed machining• Dry machining• Combining milling, drilling and turning
operations• New, stiffer and highly damped machine tools
– Graphite epoxy, ceramics (high modulus)• Modular machines• Multiple loading stations• More sensors• More and more automation
– Automated program generation
Lecture No 12 45
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