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外文翻译--制造工程与技术(机加工)英文版.pdf

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外文翻译--制造工程与技术(机加工)英文版.pdf

ManufacturingEngineeringandTechnologyMachiningSeropekalpakjianStevenR.Schmid20.9MACHINABILITYHHHThemachinabilityofamaterialusuallydefinedintermsoffourfactors1、Surfacefinishandintegrityofthemachinedpart2、Toollifeobtained3、Forceandpowerrequirements4、Chipcontrol.Thus,goodmachinabilitygoodsurfacefinishandintegrity,longtoollife,andlowforceandpowerrequirements.Asforchipcontrol,longandthinstringycuredchips,ifnotbrokenup,canseverelyinterferewiththecuttingoperationbybecomingentangledinthecuttingzone.Becauseofthecomplexnatureofcuttingoperations,itisdifficulttoestablishrelationshipsthatquantitativelydefinethemachinabilityofamaterial.Inmanufacturingplants,toollifeandsurfaceroughnessaregenerallyconsideredtobethemostimportantfactorsinmachinability.Althoughnotusedmuchanymore,approximatemachinabilityratingsareavailableintheexamplebelow.20.9.1MachinabilityOfOOOSteelsBecausesteelsareamongthemostimportantengineeringmaterialsasnotedinChapter5,theirmachinabilityhasbeenstudiedextensively.Themachinabilityofsteelshasbeenmainlyimprovedbyaddingleadandsulfurtoobtainsocalledfreemachiningsteels.ResulfurizedandRephosphorizedsteels.Sulfurinsteelsformsmanganesesulfideinclusionssecondphaseparticles,whichactasstressraisersintheprimaryshearzone.Asaresult,thechipsproducedbreakupeasilyandaresmallthisimprovesmachinability.Thesize,shape,distribution,andconcentrationoftheseinclusionssignificantlyinfluencemachinability.Elementssuchastelluriumandselenium,whicharebothchemicallysimilartosulfur,actasinclusionmodifiersinresulfurizedsteels.Phosphorusinsteelshastwomajoreffects.Itstrengthenstheferrite,causingincreasedhardness.HarderHHHsteelsresultinbetterchipformationandsurfacefinish.Notethatsoftsteelscanbedifficulttomachine,withbuiltupedgeformationandpoorsurfacefinish.Thesecondeffectisthatincreasedhardnesscausestheformationofshortchipsinsteadofcontinuousstringyones,therebyimprovingmachinability.LeadedSteels.Ahighpercentageofleadinsteelssolidifiesatthetipofmanganesesulfideinclusions.Innonresulfurizedgradesofsteel,leadtakestheformofdispersedfineparticles.Leadisinsolubleiniron,copper,andaluminumandtheiralloys.Becauseofitslowshearstrength,therefore,leadactsasasolidlubricantSection32.11andissmearedoverthetoolchipinterfaceduringcutting.Thisbehaviorhasbeenverifiedbythepresenceofhighconcentrationsofleadonthetoolsidefaceofchipswhenmachiningleadedsteels.Whenthetemperatureissufficientlyhighforinstance,athighcuttingspeedsandfeedsSection20.6theleadmeltsdirectlyinfrontofthetool,actingasaliquidlubricant.Inadditiontothiseffect,leadlowerstheshearstressintheprimaryshearzone,reducingcuttingforcesandpowerconsumption.Leadcanbeusedineverygradeofsteel,suchas10xx,11xx,12xx,41xx,etc.LeadedsteelsareidentifiedbytheletterLbetweenthesecondandthirdnumeralsforexample,10L45.Notethatinstainlesssteels,similaruseoftheletterLmeanslowcarbon,aconditionthatimprovestheircorrosionresistance.However,HHHbecauseleadisawellknowntoxinandapollutant,thereareseriousenvironmentalconcernsaboutitsuseinsteelsestimatedat4500tonsofleadconsumptioneveryyearintheproductionofsteels.Consequently,thereisacontinuingtrendtowardeliminatingtheuseofleadinsteelsleadfreesteels.Bismuthandtinarenowbeinginvestigatedaspossiblesubstitutesforleadinsteels.CalciumDeoxidizedSteels.Animportantdevelopmentiscalciumdeoxidizedsteels,inwhichoxideflakesofcalciumsilicatesCaSoareformed.Theseflakes,inturn,reducethestrengthofthesecondaryshearzone,decreasingtoolchipinterfaceandwear.Temperatureiscorrespondinglyreduced.Consequently,thesesteelsproducelesscraterwear,especiallyathighcuttingspeeds.StainlessSteels.Austenitic300seriessteelsaregenerallydifficulttomachine.Chattercanbeaproblem,necessitatingmachinetoolswithhighstiffness.However,HHHferriticstainlesssteelsalso300serieshavegoodmachinability.Martensitic400seriessteelsareabrasive,tendtoformabuiltupedge,andrequiretoolmaterialswithhighhothardnessandcraterwearresistance.Precipitationhardeningstainlesssteelsarestrongandabrasive,requiringhardandabrasionresistanttoolmaterials.TheEffectsofOtherOOOElementsinSteelsonMachinability.Thepresenceofaluminumandsiliconinsteelsisalwaysharmfulbecausetheseelementscombinewithoxygentoformaluminumoxideandsilicates,whicharehardandabrasive.Thesecompoundsincreasetoolwearandreducemachinability.Itisessentialtoproduceandusecleansteels.Carbonandmanganesehavevariouseffectsonthemachinabilityofsteels,dependingontheircomposition.Plainlowcarbonsteelslessthan0.15Ccanproducepoorsurfacefinishbyformingabuiltupedge.Caststeelsaremoreabrasive,althoughtheirmachinabilityissimilartothatofwroughtsteels.Toolanddiesteelsareverydifficulttomachineandusuallyrequireannealingpriortomachining.Machinabilityofmoststeelsisimprovedbycoldworking,whichhardensthematerialandreducesthetendencyforbuiltupedgeformation.OtherOOOalloyingelements,suchasnickel,chromium,molybdenum,andvanadium,whichimprovethepropertiesofsteels,generallyreducemachinability.Theeffectofboronisnegligible.GaseousGGGelementssuchashydrogenandnitrogencanhaveparticularlydetrimentaleffectsonthepropertiesofsteel.OxygenOOOhasbeenshowntohaveastrongeffectontheaspectratioofthemanganesesulfideinclusionsthehighertheoxygencontent,thelowertheaspectratioandthehigherthemachinability.Inselectingvariouselementstoimprovemachinability,weshouldconsiderthepossibledetrimentaleffectsoftheseelementsonthepropertiesandstrengthofthemachinedpartinservice.Atelevatedtemperatures,forexample,leadcausesembrittlementofsteelsliquidmetalembrittlement,hotshortnessseeSection1.4.3,althoughatroomtemperatureithasnoeffectonmechanicalproperties.Sulfurcanseverelyreducethehotworkabilityofsteels,becauseoftheformationofironsulfide,unlesssufficientmanganeseispresenttopreventsuchformation.Atroomtemperature,themechanicalpropertiesofresulfurizedsteelsdependontheorientationofthedeformedmanganesesulfideinclusionsanisotropy.Rephosphorizedsteelsaresignificantlylessductile,andareproducedsolelytoimprovemachinability.20.9.2MachinabilityofVariousOtherOOOMetalsAluminumisgenerallyveryeasytomachine,althoughthesoftergradestendtoformabuiltupedge,resultinginpoorsurfacefinish.HighHHHcuttingspeeds,highrakeangles,andhighreliefanglesarerecommended.Wroughtaluminumalloyswithhighsiliconcontentandcastaluminumalloysmaybeabrasivetheyrequirehardertoolmaterials.Dimensionaltolerancecontrolmaybeaprobleminmachiningaluminum,sinceithasahighthermalcoefficientofexpansionandarelativelylowelasticmodulus.Berylliumissimilartocastirons.Becauseitismoreabrasiveandtoxic,though,itrequiresmachininginacontrolledenvironment.Castgrayironsaregenerallymachinablebutare.Freecarbidesincastingsreducetheirmachinabilityandcausetoolchippingorfracture,necessitatingtoolswithhightoughness.Nodularandmalleableironsaremachinablewithhardtoolmaterials.Cobaltbasedalloysareabrasiveandhighlyworkhardening.Theyrequiresharp,abrasionresistanttoolmaterialsandlowfeedsandspeeds.Wroughtcoppercanbedifficulttomachinebecauseofbuiltupedgeformation,althoughcastcopperalloysareeasytomachine.Brassesareeasytomachine,especiallywiththeadditionpfleadleadedfreemachiningbrass.Bronzesaremoredifficulttomachinethanbrass.Magnesiumisveryeasytomachine,withgoodsurfacefinishandprolongedtoollife.HoweverHHHcareshouldbeexercisedbecauseofitshighrateofoxidationandthedangeroffiretheelementispyrophoric.Molybdenumisductileandworkhardening,soitcanproducepoorsurfacefinish.Sharptoolsarenecessary.

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