Cutting Tools
Session 5
1
Cutting Tools
• One of most important components in machining
process
• Performance will determine efficiency of operation
• Two basic types (excluding abrasives)
• Single point and multiple point
• Must have rake and clearance angles ground or
formed on them
2
Cutting-Tool Materials
• Toolbits generally made of seven materials
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High-speed steel
Cast alloys (such as stellite)
Cemented carbides
Ceramics
Cermets
Cubic Boron Nitride
Polycrystalline Diamond
3
Cutting Tool Properties
• Hardness
• Cutting tool material must be 1 1/2 times harder than
the material it is being used to machine.
• Capable of maintaining a red hardness
during machining operation
• Red hardness: ability of cutting tool to maintain
sharp cutting edge
• Also referred to as hot hardness or hot strength
4
Cutting Tool Properties
• Wear Resistance
• Able to maintain sharpened edge throughout the
cutting operation
• Same as abrasive resistance
• Shock Resistance
• Able to take the cutting loads and forces
5
Cutting Tool Properties
• Shape and Configuration
• Must be available for use in different sizes and
shapes.
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High-Speed Steel
• May contain combinations of tungsten,
chromium, vanadium, molybdenum, cobalt
• Can take heavy cuts, withstand shock and
maintain sharp cutting edge under red heat
• Generally two types (general purpose)
• Molybdenum-base (Group M)
• Tungsten-base (Group T)
• Cobalt added if more red hardness desired
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Cast Alloy
• Usually contain 25% to 35% chromium, 4% to 25%
tungsten and 1% to 3% carbon
• Remainder cobalt
• Qualities
• High hardness
• High resistance to wear
• Excellent red-hardness
• Operate 2 ½ times speed of high-speed steel
• Weaker and more brittle than high-speed steel
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Carbide Cutting Tools
• First used in Germany during WW II as
substitute for diamonds
• Various types of cemented (sintered) carbides
developed to suit different materials and
machining operations
• Good wear resistance
• Operate at speeds ranging 150 to 1200 sf/min
• Can machine metals at speeds that cause
cutting edge to become red hot without loosing
harness
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Manufacture of Cemented
Carbides
• Products of powder metallurgy process
• Tantalum, titanium, niobium
• Operations
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Blending
Compaction
Presintering
Sintering
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Blending
• Five types of powders
• Tungsten carbide, titanium carbide, cobalt,
tantalum carbide, niobium carbide
• One or combination blended in different
proportions depending on grade desired
• Powder mixed in alcohol (24 to 190 h)
• Alcohol drained off
• Paraffin added to simplify pressing operation
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Compaction
• Must be molded to shape and size
• Five different methods to
compact powder
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Extrusion process
Hot press
Isostatic press
Ingot press
Pill press
• Green (pressed) compacts soft, must be
presintered to dissolve paraffin
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12
Presintering
• Green compacts heated to about 1500º F in
furnace under protective atmosphere of
hydrogen
• Carbide blanks have consistency of chalk
• May be machined to required shape
• 40% oversize to allow for shrinkage that occurs
during final sintering
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Sintering
• Last step in process
• Converts presintered machine blanks into
cemented carbide
• Carried out in either hydrogen atmosphere or
vacuum
• Temperatures between 2550º and 2730º F
• Binder (cobalt) unites and cements carbide
powders into dense structure of extremely hard
carbide crystals
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Cemented-Carbide
Applications
• Used extensively in manufacture of metalcutting tools
• Extreme hardness and good wear-resistance
• First used in machining operations as lathe
cutting tools
• Majority are single-point cutting tools used on
lathes and milling machines
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Types of Carbide Lathe
Cutting Tools
• Blazed-tip type
• Cemented-carbide tips brazed to steel shanks
• Wide variety of styles and sizes
• Indexable insert type
• Throwaway inserts
• Wide variety of shapes: triangular, square,
diamond, and round
• Triangular: has three cutting edges
• Inserts held mechanically in special holder
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Reasons Indexable Inserts More
Popular than Brazed-Tip Tools
1. Less time required to change cutting edge
2. Amount of machine downtime reduced
considerable thus production increased
3. Time normally spent in regrinding eliminated
4. Faster speeds and feeds can be used
5. Cost of diamond wheels eliminated
6. Indexable inserts cheaper than brazed-tip
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Cemented-Carbide Insert
Identification
• American Standards Association has
developed system by which indexable inserts
can be identified quickly and accurately
• Adopted by manufacturers
• Table 31.1 in text
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Grades of Cemented Carbides
• Two main groups of carbides
• Straight tungsten carbide
• Contains only tungsten carbide and cobalt
• Strongest and most wear-resistant
• Used for machining cast iron and nonmetals
• Crater-resistant
• Contain titanium carbide and tantalum carbide in
addition to tungsten carbide and cobalt
• Used for machining most steels
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Qualities of Tungsten
Carbide Tools
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Determined by size of tungsten carbide
particles and percentage of cobalt
1.
2.
3.
4.
5.
Finer the grain particles, lower the tool toughness
Finer the grain particles, higher tool hardness
Higher the hardness, greater wear resistance
Lower cobalt content, lower tool toughness
Lower cobalt content, higher hardness
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Additive Characteristics
• Titanium carbide
• Addition provides resistance to tool cratering
• Content increased
• Toughness of tool decreased
• Abrasive wear resistance at cutting edge lowered
• Tantalum carbide
• Addition provides resistance to tool cratering
• Without affecting abrasive wear resistance
• Addition increases tool's resistance to deformation
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General Rules for Selection of Proper
Cemented-Carbide Grade
1.
2.
3.
4.
5.
Use grade with lowest cobalt content and finest
grain size
Use straight tungsten carbide grades to combat
abrasive wear
To combat cratering, seizing, welding, and galling,
use titanium carbide grades
For crater and abrasive wear resistance, use
tantalum carbide grades
Use tantalum carbide grades for heavy cuts in
steel, when heat and pressure might deform cutting
edge
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Coated Carbide Inserts
• Give longer tool life, greater productivity and
freer-flowing chips
• Coating acts as permanent lubricant
• Permits higher speed, reduced heat and stress
• Two or three materials in coating give tool special
qualities
• Innermost layer of titanium carbide
• Thick layer of aluminum oxide
• Third, very thin layer titanium nitride
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Coatings
• Titanium carbide
• High wear and abrasion resistance
(moderate speed)
• Used for roughing and finishing
• Titanium nitride
• Extremely hard, good crater resistance
• Excellent lubricating properties
• Aluminum oxide
• Provides chemical stability
• Maintains hardness at high temperatures
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Tool
Geometry
Terms adopted
by ASME
SIDE RELIEF
SIDE CLEARANCE
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25
Cutting-Tool Terms
• Front, End, Relief
(Clearance)
• Allows end of cutting tool to
enter work
• Side Relief (Side)
• Permits side of tool to
advance into work
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26
Cutting-Tool Terms
• Side Cutting Edge Angle
• Angle cutting edge meets work
• Positive
• Negative - protects point at start and end of cut
• Nose Radius
• Strengthens finishing point of tool
• Improves surface finish on work
• Should be twice amount of feed per revolution
• Too large – chatter; too small – weakens point
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Side Rake
• Large as possible to allow
chips to escape
• Amount determined
• Type and grade of cutting tool
• Type of material being cut
• Feed per revolution
• Angle of keenness
• Formed by side rake and side
clearance
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28
Back Rake
• Angle formed between top face of tool and top
of tool shank
• Positive
• Top face slopes downward
away from point
• Negative
• Top face slopes upward
away from point
• Neutral
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29
Cemented-Carbide Cutting-Tool
Angles and Clearances
• Vary greatly
• Depend on three factors
• Hardness of cutting tool
• Workpiece material
• Type of cutting operation
• May have to be altered slightly to suit various
conditions encountered
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Cutting Speeds and Feeds
• Important factors that influence speeds,
feeds, and depth of cut
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Type and hardness of work material
Grade and shape of cutting tool
Rigidity of cutting tool
Rigidity of work and machine
Power rating of machine
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Machining with Carbide Tools
• To obtain maximum efficiency
• Precautions in machine setup
• Rigid and free from vibrations
• Equipped with heat-treated gears
• Sufficient power to maintain constant cutting speed
• Cutting operation
• Cutting tool held as rigidly as possible to avoid chatter
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Suggestions for Using
Cemented-Carbide Cutting Tools
• Work Setup
• Mount work in chuck or holding device to prevent
slipping and chattering
• Revolving center used in tailstock for turning work
between centers
• Tailstock spindle extended minimum distance and
locked securely
• Tailstock should be clamped firmly to lathe bed
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Suggestions for Using
Cemented-Carbide Cutting Tools
• Tool Selection
• Use cutting tool with proper rake and clearances
• Hone cutting edge
• Use side cutting edge angle
large enough tool can be
eased into work
• Use largest nose radius
operating conditions permit
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34
Tool Setup
1.
Hold carbide tool in turret-type holder
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2.
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5.
Amount of tool overhang enough for chip clearance
Cutting tool set exactly on center
Designed to operate while bottom of tool shank is in
horizontal position
If rocker-type toolpost: remove rocker, invert rocker
base, shim tool to correct height, Use special carbide
toolholder (having no rake)
Always keep it from touching work and machine parts
to avoid damaging tool point
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Machine Setup
• Always make sure machine has adequate power rating
for machining operation and no slippage in clutch and
belts
• Set correct speed for material cut and operation
performed
• Too high cause rapid tool failure
• Too low result in inefficient cutting action
• Set machine feed for good metal-removal rate and good
surface finish
• Too light causes rubbing
• Too coarse slows down machine creates heat
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Cutting Operation
1. Never bring tool point against work that is
stationary
2. Always use heaviest depth of cut possible for
machine and size of cutting tool
3. Never stop machine while feed engaged
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Will break cutting edge
Stop feed and allow tool to clear before stopping
machine
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4. Never continue to use dull cutting tool
5. Dull cutting tool recognized by
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Work produced oversize with glazed finish
Rough and ragged finish
Change in shape or color of chips
6. Apply cutting fluid only if
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Can be applied under pressure
Can be directed at point of cutting and kept there
at all times
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Tool Selection and
Application Guide
• Table 31.7 in text lists points to follow to obtain
most efficient metal-removal rates
• Other factors affecting optimum life
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Horsepower available on machine tool
Rigidity of machine tool and toolholders
Shape of workpiece and setup
Speed and feed rates used for machining
operation
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Grinding Wheels
1. 80-grit silicon carbide wheel used for rough
grinding carbides
2. 100-grit silicon carbide wheel used for finish
grinding carbides
3. Diamond grinding wheels (100-grit) excellent
for finish grinding; high finishes use 220-grit
diamond wheel
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Type of Grinder
• Heavy-duty grinder used for grinding carbides
• Cutting pressures required to remove carbide are
5 to 10 times as great as high-speed steel tools
• Should be equipped with adjustable table and
protractor so necessary tool angles and
clearances may be ground accurately
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Tool Grinding
1.
Regrind cutting tool to angles and clearances
recommend by manufacturer
Use silicon carbide wheels for rough grinding
2.
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3.
4.
Use diamond wheels when high surface finishes
required
Move carbide tool back and forth over grinding
wheel face to keep amount of head generated to
minimum
Never quench carbide tools that become hot during
grinding – allow them to cool gradually
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Honing
• Remove fine, ragged edge left by grinding wheel
• Fine, nicked edge fragile
• Suggestions for successful honing
• 320-grit silicon carbide or diamond hone
• 45º chamfer .002 to .004 in. wide honed
on cutting edge when cutting steel
• No chamfer if used for aluminum,
magnesium and plastics
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43
Cemented-Carbide
Tool Problems
• Consult Table 31.8 in text for possible causes
and remedies
• Change only one thing at a time until problem
corrected
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Cemented-Carbide
• Capable of cutting speeds 3 to 4 times highspeed steel toolbits
• Low toughness but high hardness and excellent
red-hardness
• Consist of tungsten carbide sintered in cobalt
matrix
• Straight tungsten used to machine cast iron and
nonferrous materials (crater easily)
• Different grades for different work
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Metal-Cutting
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Turning
• High proportion of work machined in shop
turned on lathe
• Workpiece held securely in chuck or between lathe
centers
• Turning tool set to given depth of cut, fed parallel to
axis of work (reduces diameter of work)
• Chip forms and slides along cutting tool's upper surface
created by side rake
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Turning
Assume cutting machine steel: If rake and relief clearance
angles correct and proper speed and feed used, a continuous
chip should be formed.
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Planing or Shaping
• Workpiece moved back and forth under
cutting tool
• Fed sideways a set amount at end of each table
reversal
• Should have
proper rake
and clearance
angles on cutting
tool
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Plain Milling
• Multi-tooth tool having several equally spaced cutting
edges around periphery
• Each tooth considered single-point cutting tool (must
have proper rake and clearance angles)
• Workpiece held in vise or fastened to table
• Fed into horizontal revolving cutter
• Each tooth makes successive cuts
• Produces smooth, flat, or profiled surface depending on
shape of cutter
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Plain Milling
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Inserted Blade Face Mill
• Consists of body that holds several equally
spaced inserts
• Required rake angle
• Lower edge of each insert has relief or clearance
angle ground on it
• Cutting action occurs at lower corner of insert
• Corners chamfered to give strength
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Face Milling
53
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End Milling
• Multi-fluted cutters held vertically in vertical
milling machine spindle or attachment
• Used primarily for cutting slots or grooves
• Workpiece held in vise and fed into revolving
cutter
• End milling
• Cutting done by periphery of teeth
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Nomenclature of an
End Mill
55
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Nomenclature of an
End Mill
56
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Drilling
• Multi-edge cutting tool that cuts on the point
• Drill's cutting edges (lips) provided with lip clearance
to permit point to penetrate workpiece as drill
revolves
• Rake angle provided by helical-shaped flutes
• Slope away from cutting edge
• Angle of keeness
• Angle between rake angle and clearance angle
57
Characteristics of a
Drill Point
Chip formation
of a drill
Cutting-point angles for
standard drill
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58