Cutting tools

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Cutting Tools

Session 5

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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

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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

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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

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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

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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• 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

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• 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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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 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

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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 edge2. Amount of machine downtime reduced

considerable thus production increased3. Time normally spent in regrinding eliminated4. Faster speeds and feeds can be used5. Cost of diamond wheels eliminated6. 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

• Determined by size of tungsten carbide particles and percentage of cobalt

1. Finer the grain particles, lower the tool toughness2. Finer the grain particles, higher tool hardness3. Higher the hardness, greater wear resistance4. Lower cobalt content, lower tool toughness5. 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. 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

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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


SIDE RELIEF

SIDE CLEARANCE

Terms adopted by ASME

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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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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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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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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

• 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 UsingCemented-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 UsingCemented-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 beeased into work

• Use largest nose radiusoperating conditions permit


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Tool Setup

1. Hold carbide tool in turret-type holder• Amount of tool overhang enough for chip clearance

2. Cutting tool set exactly on center3. Designed to operate while bottom of tool shank is in

horizontal position4. 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

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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 Operation1. Never bring tool point against work that is

stationary2. Always use heaviest depth of cut possible for

machine and size of cutting tool3. Never stop machine while feed engaged

• 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 tool5. 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

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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• 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

2. Use silicon carbide wheels for rough grinding• Use diamond wheels when high surface finishes

required3. 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

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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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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 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

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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 clearanceangles correct and proper speed and feed used, a continuouschip 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 rakeand clearanceangles on cuttingtool

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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


Face Milling

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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


Nomenclature of an End Mill

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Nomenclature of an End Mill


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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


Characteristics of a Drill Point

Cutting-point angles for standard drill

Chip formation of a drill

Engineering

Cutting tools