Carbide Cutting Tools Unit 31

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

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

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

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

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 McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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

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

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

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

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

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

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

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

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

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

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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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 McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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

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

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

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 McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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

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

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

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

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

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

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

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 McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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