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

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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

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

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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

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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– Planer type– Rotary tables– Duplicating machines– Profiling milling– More than three axes

Lecture No 12 39#4 VERTICAL MILLING MACHINE W/SLIDING HEAD

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Machining and Turning Centers

• Combines turning with milling• Computer control essential• Multiaxis capabilities• Replacing simple lathes or milling machines

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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

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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)• 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