Machinability of Metals.docx

Machinability of Metals.docx

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MachinabilityofMetals

MachinabilityofMetals

3.1Introduction

Theconditionandphysicalpropertiesoftheworkmaterialhaveadirectinfluenceonthemachinabilityofaworkmaterial.Thevariousconditionsandcharacteristicsdescribedasconditionofworkmaterial’,individuallyandincombinations,directlyinfluenceanddeterminethemachinability.Operatingconditions,toolmaterialandgeometry,andworkpiecerequirementsexerciseindirecteffectsonmachinabilityandcanoftenbeusedtoovercomedifficultconditionspresentedbytheworkmaterial.Ontheotherhand,theycancreatesituationsthatincreasemachiningdifficultyiftheyareignored.Athoroughunderstandingofallofthefactorsaffectingmachinabilityandmachiningwillhelpinselectingmaterialandworkpiecedesignstoachievetheoptimummachiningcombinationscriticaltomaximumproductivity.

3.2ConditionofWorkMaterial

Thefollowingeightfactorsdeterminetheconditionoftheworkmaterial:

microstructure,grainsize,heattreatment,chemicalcomposition,fabrication,hardness,yieldstrength,andtensilestrength.360毕业设计网

Microstructure:

Themicrostructureofametalreferstoitscrystalorgrainstructure

asshownthroughexaminationofetchedandpolishedsurfacesunderamicroscope.

Metalswhosemicrostructuresaresimilarhavelikemachiningproperties.Buttherecan

bevariationsinthemicrostructureofthesameworkpiece,thatwillaffectmachinability.

GrainSize:

Grainsizeandstructureofametalserveasgeneralindicatorsofits

machinability.Ametalwithsmallundistortedgrainstendstocuteasilyandfinisheasily.

Suchametalisductile,butitisalso‘gummy’.Metalsofanintermediategrainsize

representacompromisethatpermitsbothcuttingandfinishingmachinability.Hardness

ofametalmustbecorrelatedwithgrainsizeanditisgenerallyusedasanindicatorof

machinability.

HeatTreatment:

Toprovidedesiredpropertiesinmetals,theyaresometimesput

throughaseriesofheatingandcoolingoperationswheninthesolidstate.Amaterialmay

betreatedtoreducebrittleness,removestress,toobtainductilityortoughness,toincrease

strength,toobtainadefinitemicrostructure,tochangehardness,ortomakeotherchanges

thataffectmachinability.

ChemicalComposition:

Chemicalcompositionofametalisamajorfactorindetermining

itsmachinability.Theeffectsofcompositionthough,arenotalwaysclear,

becausetheelementsthatmakeupanalloymetal,workbothsinglyandcollectively.

Certaingeneralizationsaboutchemicalcompositionofsteelsinrelationtomachinability

canbemade,butnon-ferrousalloysaretoonumerousandvariedtopermitsuchgeneralizations.

Fabrication:

Whetherametalhasbeenhotrolled,coldrolled,colddrawn,cast,or

forgedwillaffectitsgrainsize,ductility,strength,hardness,structure-andtherefore-its

machinability.Theterm‘wrought’referstothehammeringorformingofmaterialsintopremanfacturedshapeswhicharereadilyalteredintocomponentsorproductsusingtraditionalmanufacturingtechniques.Wroughtmetalsaredefinedasthatgroupofmaterialswhicharemechanicallyshapedintobars,billets,rolls,sheets,platesortubing.

Castinginvolvespouringmoltenmetalintoamoldtoarriveatanearcomponentshapewhichrequiresminimal,orinsomecasesnomachining.Moldsfortheseoperationsaremadefromsand,plaster,metalsandavarietyofothermaterials.

Hardness:

Thetextbookdefinitionofhardnessisthetendencyforamaterialtoresistdeformation.HardnessisoftenmeasuredusingeithertheBrinellorRockwellscale.Themethodusedtomeasurehardnessinvolvesembeddingaspecificsizeandshapedindentorintothesurfaceofthetestmaterial,usingapredeterminedloadorweight.ThedistancetheindentorpenetratesthematerialsurfacewillcorrespondtoaspecificBrinellorRockwellhardnessreading.Thegreatertheindentorsurfacepenetration,thelowertheultimateBrinellorRockwellnumber,andthusthelowerthecorrespondinghardnesslevel.Therefore,highBrinellorRockwellnumbersorreadingsrepresentaminimal

amountofindentorpenetrationintotheworkpieceandthus,bydefinition,areanindicationofanextremelyhardpart.

TheBrinellhardnesstestinvolvesembeddingasteelballofaspecificdiameter,usingakilogramload,inthesurfaceofatestpiece.TheBrinellHardnessNumber（BHN）isdeterminedbydividingthekilogramloadbythearea（insquaremillimeters）ofthecirclecreatedattherimofthedimpleorimpressionleftintheworkpiecesurface.Thisstandardizedapproachprovidesaconsistentmethodtomakeparativetestsbetweenavarietyofworkpiecematerialsorasinglematerialwhichhasundergonevarioushardeningprocesses.TheRockwelltestcanbeperformedwithvariousindentorsizesandloads.SeveraldifferentscalesexistfortheRockwellmethodorhardnesstesting.Thethreemostpopularareoutlinedbelowintermsoftheactualapplicationthetestisdesignedtoaddress:

RockwellTesting

ScaleApplication

AFortungstencarbideandotherextremelyhardmaterials&thin,hardsheets.

BFormediumhardnesslowandmediumcarbonsteelsintheannealedcondition.

CFormaterials>thanRockwell‘B’100.

Intermsofgeneralmachiningpractice,lowmaterialhardnessenhancesproductivity,sincecuttingspeedisoftenselectedbasedonmaterialhardness（thelowerthehardness,thehigherthespeed）.Toollifeisadverselyaffectedbyanincreaseinworkpiecehardness,sincethecuttingloadsandtemperaturesriseforaspecificcuttingspeedwithparthardness,therebyreducingtoollife.Indrillingandturning,theaddedcuttingtemperatureisdetrimentaltotoollife,sinceitproducesexcessheatcausingacceleratededgewear.Inmilling,increasedmaterialhardnessproduceshigherimpactloadsasinsertsenterthecut,whichoftenleadstoaprematurebreakdownofthecuttingedge.

YieldStrength:

Tensiletestworkisusedasameansofcomparisonofmetalmaterialconditions.Thesetestscanestablishtheyieldstrength,tensilestrengthandmanyotherconditionsofamaterialbasedonitsheattreatment.Inaddition,thesetestsareusedtocomparedifferentworkpiecematerials.Thetensiletestinvolvestakingacylindricalrodorshaftandpullingitfromoppositeendswithaprogressivelylargerforceinahydraulicmachine.Priortothestartofthetest,twomarkseithertwooreightinchesapartaremadeontherodorshaft.Astherodissystematicallysubjectedtoincreasedloads,themarksbegintomovefartherapart.Amaterialisintheso-called‘elasticzone’whentheloadcanberemovedfromtherodandthemarksreturntotheirinitialdistanceapartofeithertwooreightinches.Ifthetestisallowedtoprogress,apointisreachedwhere,whentheloadisremovedthemarkswillnotreturntotheirinitialdistanceapart.Atthispoint,permanentsetordeformationofthetestspecimenhastakenplace.

Yieldstrengthismeasuredjustpriortothepointbeforepermanentdeformationtakesplace.Yieldstrengthisstatedinpoundspersquareinch（PSI）andisdeterminedbydividingtheloadjustpriortopermanentdeformationbythecrosssectionalareaofthetestspecimen.Thismaterialpropertyhasbeenreferredtoasacondition,sinceitcanbealteredduringheattreatment.Increasedparthardnessproducesanincreaseinyieldstrengthandtherefore,asapartbecomesharder,ittakesalargerforcetoproducepermanentdeformationofthepart.Yieldstrengthshouldnotbeconfusedwithfracturestrength,crackingortheactualbreakingofthematerialintopieces,sincethesepropertiesarequitedifferentandunrelatedtothecurrentsubject.

Bydefinition,amaterialwithhighyieldstrength（forcerequiredperunitofareatocreatepermanentdeformation）requiresahighlevelofforcetoinitiatechipformationinamachiningoperation.Thisimpliesthatasamaterial’syieldstrengthincreases,strongerinsertshapesaswellaslesspositivecuttinggeometriesarenecessarytocombattheadditionalloadencounteredinthecuttingzone.Materialhardnessandyieldstrengthincreasesimultaneouslyduringheattreatment.Therefore,materialswithrelativelyhighyieldstrengthswillbemoredifficulttomachineandwillreducetoollifewhencomparedtomaterialswithmoremoderatestrengths.

TensileStrength:

Thetensilestrengthofamaterialincreasesalongwithyieldstrengthasitisheattreatedtogreaterhardnesslevels.Thismaterialconditionisalsoestablishedusingatensiletest.Tensilestrength（orultimatestrength）isdefinedasthemaximumloadthatresultsduringthetensiletest,dividedbythecross-sectionalareaofthetestspecimen.Therefore,tensilestrength,likeyieldstrength,isexpressedinPSI.Thisvalueisreferredtoasamaterialconditionratherthanaproperty,sinceitsleveljustlikeyieldstrengthandhardness,canbealteredbyheattreatment.Therefore,basedonthematerialselected,distincttensileandyieldstrengthlevelsexistforeachhardnessreading.

Justasincreasedyieldstrengthimpliedhighercuttingforcesduringmachiningoperations,thesamecouldbesaidforincreasedtensilestrength.Again,astheworkpiecetensilestrengthiselevated,strongercuttingedgegeometriesarerequiredforproductivemachiningandacceptabletoollife.

3.3PhysicalPropertiesofWorkMaterials

Physicalpropertieswillincludethosecharacteristicsincludedintheindividualmaterialgroups,suchasthemodulusofelasticity,thermalconductivity,thermalexpansionandworkhardening.

ModulusofElasticity:

Themodulusofelasticitycanbedeterminedduringatensiletestinthesamemannerasthepreviouslymentionedconditions.However,un