Chapter 21CUTTING TOOL MATERIALS& CUTTING FLUIDS

TOPICS :Introduction Carbon and medium alloy steelsHigh speed steelsCast-cobalt alloysCarbidesCoated toolsAlumna-based ceramicsCubic Boron NitrideSilicon Nitride based ceramicsDiamond Whisker-reinforced tool materials Cutting-Tool ReconditioningCutting fluids

Introduction: Characteristics of cutting tool :

Hardness (Elevated temperatures)Toughness (Impact forces on tool in interrupted operations)Wear resistance (tool life to be considered)Chemical stability or inertness (to avoid adverse reactions)

Cutting tool materials Carbon & medium alloy steels High speed steels Cast-cobalt alloysCarbidesCoated toolsAlumina-based ceramicsCubic boron nitrideSilicon-nitride-base ceramics DiamondWhisker-reinforced materials

Carbon and Medium alloy steels : Oldest of tool materialsUsed for drills taps,broaches ,reamersInexpensive ,easily shaped ,sharpenedNo sufficient hardness and wear resistanceLimited to low cutting speed operation

High speed steels (HSS)Hardened to various depths Good wear resistance RelativelySuitable for high positive rake angle tools

Two basic types of HSSMolybdenum ( M-series)Tungsten ( T-series)

M-series - Contains 10% molybdenum, chromium, vanadium, tungsten, cobaltHigher, abrasion resistanceH.S.S. are majorly made of M-series

T-series - 12 % - 18 % tungsten, chromium, vanadium & cobaltundergoes less distortion during heat treating

H.S.S. available in wrought ,cast & sintered (Powder metallurgy)

Coated for better performance

Subjected to surface treatments such as case-hardening for improved hardness and wear resistance or steam treatment at elevated temperatures

High speed steels account for largest tonnage

Cast-Cobalt alloysCommonly known as stellite toolsComposition ranges 38% - 53 % cobalt30%- 33% chromium10%-20%tungstenGood wear resistance ( higher hardness)Less tough than high-speed steels and sensitive to impact forcesLess suitable than high-speed steels for interrupted cutting operationsContinuous roughing cuts relatively high g=feeds & speedsFinishing cuts are at lower feed and depth of cut

Carbides :3-groups of materials Alloy steels High speed steelsCast alloys

These carbides are also known as cemented or sintered carbidesHigh elastic modulus,thermal conductivityLow thermal expansion

2-groups of carbides used for machining operationstungsten carbidetitanium carbide

Tungsten CarbideComposite material consisting of tungsten-carbide particles bonded together Alternate name is cemented carbides

Manufactured with powder metallurgy techniques

Particles 1-5 Mum in size are pressed & sintered to desired shape Amount of cobalt present affects properties of carbide toolsAs cobalt content increases strength hardness & wear resistance increases

Titanium carbide Titanium carbide has higher wear resistance than tungsten carbide

Nickel-Molybdenum alloy as matrix Tic suitable for machining hard materials

Steels & cast irons

Speeds higher than those for tungsten carbide

Inserts

InsertsIndividual cutting tool with severed cutting pointsClamped on tool shanks with locking mechanismsInserts also brazed to the tools Clamping is preferred method for securing an insertCarbide Inserts available in various shapes-Square, Triangle, Diamond and roundStrength depends on the shape Inserts honed, chamfered or produced with negative land to improve edge strength

Insert AttachmentFig : Methods of attaching inserts to toolholders : (a) Clamping and (b) Wing lockpins. (c) Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws.

Edge StrengthFig : Relative edge strength and tendency for chipping and breaking of inserts with various shapes. Strength refers to the cutting edge shown by the included angles.Fig : edge preparation of inserts to improve edge strength.

Chip breakers:Purpose :Eliminating long chipsControlling chip flow during machiningReducing vibration & heat generatedSelection depends on feed and depth of cutWork piece material,type of chip produced during cutting

Coated tools :High strength and toughness but generally abrasive and chemically reactive with tool materials

Unique Properties :Lower Friction High resistance to cracks and wear High Cutting speeds and low time & costsLonger tool life

Coating materialsTitanium nitride (TiN)Titanium carbide (Tic)Titanium Carbonitride (TicN)Aluminum oxide (Al2O3)thickness range 2-15 m (80-600Mu.in)

Techniques used :Chemical vapor deposition (CVD) Plasma assisted CVDPhysical-vapor deposition(PVD)Medium temperature chemical- vapor deposition(MTCVD)

Properties for Group of MaterialsFig : Ranges of properties for various groups of tool materials.

Cutting tool Characteristics for coating :High hardnessChemical stabilityLow thermal conductivityGood bondingLittle or no Porosity

Titanium nitride (TiN) coating :Low friction coefficientsHigh hardnessResistance to high temperaturesGood adhesion to substrateHigh life of high speed-steel tools

Titanium carbide (TiC) coating:Titanium carbide coatings on tungsten-carbide inserts have high flank wear resistance.

Ceramics :Low thermal conductivity ,resistance ,high temperature Resistance to flank wear and crater wearCeramics are suitable materials for toolsAl2O3 (most commonly used)

Multi Phase Coatings :First layer Should bond well with substrateOuter layer Resist wear and have low thermal conductivityIntermediate layer Bond well & compatible with both layersCoatings of alternating multipurpose layers are also formed.

Multiphase CoatingsFig : Multiphase coatings on a tungsten-carbide substrate. Three alternating layers of aluminum oxide are separated by very thin layers of titanium nitride. Inserts with as many as thirteen layers of coatings have been made. Coating thick nesses are typically in the range of 2 to 10 m.

Diamond Coated tools :Use of Polycrystalline diamond as a coatingDifficult to adhere diamond film to substrateThin-film diamond coated inserts now commercially availableThin films deposited on substrate with PVD & CVD techniquesThick films obtained by growing large sheet of pure diamondDiamond coated tools particularly effective in machining non-ferrous and abrasive materials

New Coating materials :Titanium carbo nitride (TiCN)Titanium Aluminum Nitride(TiAlN)Chromium Based coatingsChromium carbideZirconium Nitride (ZrN)Hafnium nitride (HfN)Recent developments gives nano coating & composite coating

Ion Implementation :Ions placed into the surface of cutting toolNo change in the dimensions of toolNitrogen-ion Implanted carbide tools used for alloy steels & stainless steelsXeon ion implantation of tools as under development

Alumina-Based ceramics:Cold-Pressed Into insert shapes under high pressure and sintered at high temperatureHigh Abrasion resistance and hot hardnessChemically stable than high speed steels & carbidesSo less tendency to adhere to metalsGood surface finish obtained in cutting cast iron and steels Negative rake-angle preferred to avoid chipping due to poor tensile strength

Cermets, Black or Hot- Pressed :70% aluminum oxide & 30 % titanium carbidecermets(ceramics & metal)Cermets contain molybdenum carbide, niobium carbide and tantalum carbide.

Cubic boron Nitride ( CBN ) :Made by bonding ( 0.5-1.0 mm ( 0.02-0.04-in)Layer of poly crystalline cubic boron nitride to a carbide substrate by sintering under pressureWhile carbide provides shock resistance CBN layer provides high resistance and cutting edge strengthCubic boron nitride tools are made in small sizes without substrate

Fig : (a) Construction of a polycrystalline cubic boron nitride or a diamond layer on a tungsten-carbide insert. (b) Inserts with polycrystalline cubic boron nitride tips (top row) and solid polycrystalline CBN inserts (bottom row).

Silicon-Nitride based ceramics (SiN)They consists various addition of Aluminum Oxide ythrium oxide, titanium carbide

SiN have toughness, hot hardened & good thermal shock resistance

SiN base material is Silicon

High thermal & shock resistance

Recommended for machining cast iron and nickel based super alloys at intermediate cutting speeds

Diamond :Hardest known substance Low friction, high wear resistanceAbility to maintain sharp cutting edgeSingle crystal diamond of various carats used for special applicationsMachining copperfront precision optical mirrors for ( SDI)Diamond is brittle , tool shape & sharpened is importantLow rake angle used for string cutting edge

Polycrystalline-Diamond ( PCD ) Tools: Used for wire drawing of fine wiresSmall synthesis crystal fused by high pressure and temperatureBonded to a carbide substrateDiamond tools can be used fir any speed Suitable for light un-interrupted finishing cutsTo avoid tool fracture single crystal diamond is to be re-sharpened as it becomes dullAlso used as an abrasive in grinding and polishing operations

Whisker reinforced & Nanocrystalline tool materials New tool materials with enhanced properties :

High fracture toughness Resistance to thermal shock Cutting edge strengthHot hardness

Whiskers used as reinforcing fibers :Examples: Silicon-nitride base tools reinforced with silicon-carbide( Sic)

Aluminum oxide based tools reinforced with silicon-carbide with ferrous metals makes Sic-reinforced tools

Progress in nanomaterial has lead to the development of cutting tools

Made of fine grained structures as (micro grain) carbides

Cutting-Tool ReconditioningWhen tools get worned, they are reconditioned for further use Reconditioning also involves recoating used tools with titanium nitride

Cutting Fluids: (Lubricants + Coolants)Used in machining as well as abrasive machining processesReduces friction wearReduce forces and energy consumptionCools the cutting zoneWash away the chipsProtect Machined surfaces from environmental corrosion

Application of Cutting FluidsFig : Schematic illustration of proper methods of applying cutting fluids in various machining operations: (a)turning, (b)milling, (c)thread grinding, and (d)drilling

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