Cutting Tools Session 5 1 Cutting Tools One

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 • • 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. 6

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 7

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 8

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 9

Manufacture of Cemented Carbides • Products of powder metallurgy process • Tantalum, titanium, niobium • Operations • • Blending Compaction Presintering Sintering 10

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 11

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 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 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 13

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 14

Cemented-Carbide Applications • Used extensively in manufacture of metal-cutting tools • Extreme hardness and good wearresistance • First used in machining operations as lathe cutting tools • Majority are single-point cutting tools used on lathes and milling machines 15

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 16

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 17 6. Indexable inserts cheaper than

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 18

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 19

Qualities of Tungsten Carbide Tools • Determined by size of tungsten carbide particles and percentage of cobalt 1. 2. 3. 4. 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 20

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 21

General Rules for Selection of Proper Cemented-Carbide Grade 1. Use grade with lowest cobalt content and finest grain size 2. Use straight tungsten carbide grades to combat abrasive wear 3. To combat cratering, seizing, welding, and galling, use titanium carbide grades 4. For crater and abrasive wear resistance, use tantalum carbide grades 5. Use tantalum carbide grades for heavy cuts in steel, when heat and pressure might deform cutting edge 22

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 23

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 24

Tool Geometry Terms adopted by ASME SIDE RELIEF SIDE CLEARANCE Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 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 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 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 • 27 Too large – chatter; too small – weakens point

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 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 28

Back Rake • Angle formed between top face of tool and top of tool shank • Positive • • Negative • • Top face slopes downward away from point Top face slopes upward away from point Neutral Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 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 30

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 work and machine Power rating of machine 31

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 32

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 33

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 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 34

Tool Setup 1. Hold carbide tool in turret-type holder • Amount of tool overhang enough for chip clearance 2. Cutting tool set exactly on center 3. Designed to operate while bottom of tool shank is in horizontal position 4. If rocker-type toolpost: remove rocker, invert rocker base, shim tool to correct height, Use special carbide toolholder (having no rake) 5. Always keep it from touching work and machine parts to avoid damaging tool point 35

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 36

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 • • Will break cutting edge Stop feed and allow tool to clear before 37 stopping machine

4. Never continue to use dull cutting tool 5. Dull cutting tool recognized by • • • Work produced oversize with glazed finish Rough and ragged finish Change in shape or color of chips 6. Apply cutting fluid only if • • Can be applied under pressure Can be directed at point of cutting and kept there at all times 38

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 39

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 40

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 and protractor so necessary tool angles and clearances may be ground accurately 41

Tool Grinding 1. Regrind cutting tool to angles and clearances recommend by manufacturer 2. Use silicon carbide wheels for rough grinding • Use diamond wheels when high surface finishes required 3. Move carbide tool back and forth over grinding wheel face to keep amount of head generated to minimum 4. Never quench carbide tools that become hot during grinding – allow them to cool gradually 42

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 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 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 44

Cemented-Carbide • Capable of cutting speeds 3 to 4 times high-speed 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 45

Metal-Cutting 46

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 47

Turning Assume cutting machine steel: If rake and relief clearance angles correct and proper speed and feed used, a continuous chip should be formed. 48

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 49

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 50

Plain Milling 51 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

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 52

Face Milling 53 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

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 54

Nomenclature of an End Mill 55 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Nomenclature of an End Mill 56 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

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 57

Characteristics of a Drill Point Chip formation of a drill Cutting-point angles for standard drill Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 58