1. Carbide Cutting Tools
Unit 31

2. Objectives
Identify and state the purpose of the two main types of carbide grades
Select the proper grade of carbide for various workpiece materials
Select the proper speeds and feeds for carbide tools

3. 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

4. Manufacture of Cemented Carbides
Products of powder metallurgy process
Tantalum, titanium, niobium
Operations
Blending
Compaction
Presintering
Sintering

5. 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

6. Compaction Must be molded to shape and size
Five different methods to compact powder
Extrusion process
Hot press
Isostatic press
Ingot press
Pill press
Green (pressed) compacts soft, must be presintered to dissolve paraffin
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7. 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

8. 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

9. Cemented-Carbide Applications
Used extensively in manufacture of metal-cutting 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

10. 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
Indexable Inserts

11. Reasons Indexable Inserts More Popular than Brazed-Tip Tools
Less time required to change cutting edge
Amount of machine downtime reduced considerable thus production increased
Time normally spent in regrinding eliminated
Faster speeds and feeds can be used
Cost of diamond wheels eliminated
Indexable inserts cheaper than brazed-tip

12. 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

13. 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

14. Qualities of Tungsten Carbide Tools
Determined by size of tungsten carbide particles and percentage of cobalt
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

15. 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
Without affecting abrasive wear resistance
Addition increases tool's resistance to deformation

16. General Rules for Selection of Proper Cemented-Carbide Grade
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

17. 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

18. Coatings Titanium carbide Titanium nitride Aluminum oxide
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

19. Tool Geometry Terms adopted by ASME SIDE RELIEF SIDE CLEARANCE
AA Tool Geometry
The geometry of cutting tools refers to the various angles and clearances machined or ground on the tool faces. Although the terms and definitions relating to single-point cutting tools vary greatly, those adopted by the American Society of Mechanical Engineers (ASME) and currently in general use are illustrated in Fig. 31-6.
SIDE RELIEF
SIDE CLEARANCE
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20. Cutting-Tool Terms Front, End, Relief (Clearance) Side Relief (Side)
Allows end of cutting tool to enter work
Side Relief (Side)
Permits side of tool to advance into work
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21. Cutting-Tool Terms Side Cutting Edge Angle Nose Radius
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

22. 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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23. 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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24. 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

25. Cutting Speeds and Feeds
Important factors that influence speeds, feeds, and depth of cut
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

26. 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

27. 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

28. 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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29. Tool Setup Hold carbide tool in turret-type holder
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

30. 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

31. Cutting Operation
Never bring tool point against work that is stationary
Always use heaviest depth of cut possible for machine and size of cutting tool
Never stop machine while feed engaged
Will break cutting edge
Stop feed and allow tool to clear before stopping machine

32. Never continue to use dull cutting tool
Dull cutting tool recognized by
Work produced oversize with glazed finish
Rough and ragged finish
Change in shape or color of chips
Apply cutting fluid only if
Can be applied under pressure
Can be directed at point of cutting and kept there at all times

33. 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
Horsepower available on machine tool
Rigidity of machine tool and toolholders
Shape of workpiece and setup
Speed and feed rates used for machining operation

34. Grinding Wheels
80-grit silicon carbide wheel used for rough grinding carbides
100-grit silicon carbide wheel used for finish grinding carbides
Diamond grinding wheels (100-grit) excellent for finish grinding; high finishes use 220-grit diamond wheel

35. 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

36. Tool Grinding
Regrind cutting tool to angles and clearances recommend by manufacturer
Use silicon carbide wheels for rough grinding
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

37. 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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38. 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
AA Cemented-Carbide Tool Problems