机械毕业外文翻译Word格式.docx

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《ManufacturingEngineeringandTechnology—Machining》

机械工业出版社2004年3月第1版

美s.卡尔帕基安（Seropekalpakjian）

s.r施密德（StevenR.Schmid）著

原文：

20.9MACHINABILITY

Themachinabilityofamaterialusuallydefinedintermsoffourfactors:

1、Surfacefinishandintegrityofthemachinedpart;

2、Toollifeobtained;

3、Forceandpowerrequirements;

4、Chipcontrol.

Thus,goodmachinabilitygoodsurfacefinishandintegrity,longtoollife,andlowforceAndpowerrequirements.Asforchipcontrol,longandthin（stringy）curedchips,ifnotbrokenup,canseverelyinterferewiththecuttingoperationbybecomingentangledinthecuttingzone.

Becauseofthecomplexnatureofcuttingoperations,itisdifficulttoestablishrelationshipsthatquantitativelydefinethemachinabilityofamaterial.Inmanufacturingplants,toollifeandsurfaceroughnessaregenerallyconsideredtobethemostimportantfactorsinmachinability.Althoughnotusedmuchanymore,approximatemachinabilityratingsareavailableintheexamplebelow.

20.9.1MachinabilityOfSteels

Becausesteelsareamongthemostimportantengineeringmaterials（asnotedinChapter5）,theirmachinabilityhasbeenstudiedextensively.Themachinabilityofsteelshasbeenmainlyimprovedbyaddingleadandsulfurtoobtainso-calledfree-machiningsteels.

ResulfurizedandRephosphorizedsteels.Sulfurinsteelsformsmanganesesulfideinclusions（second-phaseparticles）,whichactasstressraisersintheprimaryshearzone.Asaresult,thechipsproducedbreakupeasilyandaresmall;

thisimprovesmachinability.Thesize,shape,distribution,andconcentrationoftheseinclusionssignificantlyinfluencemachinability.Elementssuchastelluriumandselenium,whicharebothchemicallysimilartosulfur,actasinclusionmodifiersinresulfurizedsteels.

Phosphorusinsteelshastwomajoreffects.Itstrengthenstheferrite,causingincreasedhardness.Hardersteelsresultinbetterchipformationandsurfacefinish.Notethatsoftsteelscanbedifficulttomachine,withbuilt-upedgeformationandpoorsurfacefinish.Thesecondeffectisthatincreasedhardnesscausestheformationofshortchipsinsteadofcontinuousstringyones,therebyimprovingmachinability.

LeadedSteels.Ahighpercentageofleadinsteelssolidifiesatthetipofmanganesesulfideinclusions.Innon-resulfurizedgradesofsteel,leadtakestheformofdispersedfineparticles.Leadisinsolubleiniron,copper,andaluminumandtheiralloys.Becauseofitslowshearstrength,therefore,leadactsasasolidlubricant（Section32.11）andissmearedoverthetool-chipinterfaceduringcutting.Thisbehaviorhasbeenverifiedbythepresenceofhighconcentrationsofleadonthetool-sidefaceofchipswhenmachiningleadedsteels.

Whenthetemperatureissufficientlyhigh-forinstance,athighcuttingspeedsandfeeds（Section20.6）—theleadmeltsdirectlyinfrontofthetool,actingasaliquidlubricant.Inadditiontothiseffect,leadlowerstheshearstressintheprimaryshearzone,reducingcuttingforcesandpowerconsumption.Leadcanbeusedineverygradeofsteel,suchas10xx,11xx,12xx,41xx,etc.LeadedsteelsareidentifiedbytheletterLbetweenthesecondandthirdnumerals（forexample,10L45）.（Notethatinstainlesssteels,similaruseoftheletterLmeans“lowcarbon,”aconditionthatimprovestheircorrosionresistance.）

However,becauseleadisawell-knowntoxinandapollutant,thereareseriousenvironmentalconcernsaboutitsuseinsteels（estimatedat4500tonsofleadconsumptioneveryyearintheproductionofsteels）.Consequently,thereisacontinuingtrendtowardeliminatingtheuseofleadinsteels（lead-freesteels）.Bismuthandtinarenowbeinginvestigatedaspossiblesubstitutesforleadinsteels.

Calcium-DeoxidizedSteels.Animportantdevelopmentiscalcium-deoxidizedsteels,inwhichoxideflakesofcalciumsilicates（CaSo）areformed.Theseflakes,inturn,reducethestrengthofthesecondaryshearzone,decreasingtool-chipinterfaceandwear.Temperatureiscorrespondinglyreduced.Consequently,thesesteelsproducelesscraterwear,especiallyathighcuttingspeeds.

StainlessSteels.Austenitic（300series）steelsaregenerallydifficulttomachine.Chattercanbesproblem,necessitatingmachinetoolswithhighstiffness.However,ferriticstainlesssteels（also300series）havegoodmachinability.Martensitic（400series）steelsareabrasive,tendtoformabuilt-upedge,andrequiretoolmaterialswithhighhothardnessandcrater-wearresistance.Precipitation-hardeningstainlesssteelsarestrongandabrasive,requiringhardandabrasion-resistanttoolmaterials.

TheEffectsofOtherElementsinSteelsonMachinability.Thepresenceofaluminumandsiliconinsteelsisalwaysharmfulbecausetheseelementscombinewithoxygentoformaluminumoxideandsilicates,whicharehardandabrasive.Thesecompoundsincreasetoolwearandreducemachinability.Itisessentialtoproduceandusecleansteels.

Carbonandmanganesehavevariouseffectsonthemachinabilityofsteels,dependingontheircomposition.Plainlow-carbonsteels（lessthan0.15%C）canproducepoorsurfacefinishbyformingabuilt-upedge.Caststeelsaremoreabrasive,althoughtheirmachinabilityissimilartothatofwroughtsteels.Toolanddiesteelsareverydifficulttomachineandusuallyrequireannealingpriortomachining.Machinabilityofmoststeelsisimprovedbycoldworking,whichhardensthematerialandreducesthetendencyforbuilt-upedgeformation.

Otheralloyingelements,suchasnickel,chromium,molybdenum,andvanadium,whichimprovethepropertiesofsteels,generallyreducemachinability.Theeffectofboronisnegligible.Gaseouselementssuchashydrogenandnitrogencanhaveparticularlydetrimentaleffectsonthepropertiesofsteel.Oxygenhasbeenshowntohaveastrongeffectontheaspectratioofthemanganesesulfideinclusions;

thehighertheoxygencontent,thelowertheaspectratioandthehigherthemachinability.

Inselectingvariouselementstoimprovemachinability,weshouldconsiderthepossibledetrimentaleffectsoftheseelementsonthepropertiesandstrengthofthemachinedpartinservice.Atelevatedtemperatures,forexample,leadcausesembrittlementofsteels（liquid-metalembrittlement,hotshortness;

seeSection1.4.3）,althoughatroomtemperatureithasnoeffectonmechanicalproperties.

Sulfurcanseverelyreducethehotworkabilityofsteels,becauseoftheformationofironsulfide,unlesssufficientmanganeseispresenttopreventsuchformation.Atroomtemperature,themechanicalpropertiesofresulfurizedsteelsdependontheorientationofthedeformedmanganesesulfideinclusions（anisotropy）.Rephosphorizedsteelsaresignificantlylessductile,andareproducedsolelytoimprovemachinability.

20.9.2MachinabilityofVariousOtherMetals

Aluminumisgenerallyveryeasytomachine,althoughthesoftergradestendtoformabuilt-upedge,resultinginpoorsurfacefinish.Highcuttingspeeds,highrakeangles,andhighreliefanglesarerecommended.Wroughtaluminumalloyswithhighsiliconcontentandcastaluminumalloysmaybeabrasive;

theyrequirehardertoolmaterials.Dimensionaltolerancecontrolmaybeaprobleminmachiningaluminum,sinceithasahighthermalcoefficientofexpansionandarelativelylowelasticmodulus.

Berylliumissimilartocastirons.Becauseitismoreabrasiveandtoxic,though,itrequiresmachininginacontrolledenvironment.

Castgrayironsaregenerallymachinablebutare.Freecarbidesincastingsreducetheirmachinabilityandcausetoolchippingorfracture,necessitatingtoolswithhightoughness.Nodularandmalleableironsaremachinablewithhardtoolmaterials.

Cobalt-basedalloysareabrasiveandhighlywork-hardening.Theyrequiresharp,abrasion-resistanttoolmaterialsandlowfeedsandspeeds.

Wroughtcoppercanbedifficulttomachinebecauseofbuilt-upedgeformation,althoughcastcopperalloysareeasytomachine.Brassesareeasytomachine,especiallywiththeadditionpflead（leadedfree-machiningbrass）.Bronzesaremoredifficulttomachinethanbrass.

Magnesiumisveryeasytomachine,withgoodsurfacefinishandprolongedtoollife.Howevercareshouldbeexercisedbecauseofitshighrateofoxidationandthedangeroffire（theelementispyrophoric）.

Molybdenumisductileandwork-hardening,soitcanproducepoorsurfacefinish.Sharptoolsarenecessary.

Nickel-basedalloysarework-hardening,abrasive,andstrongathightemperatures.Theirmachinabilityissimilartothatofstainlesssteels.

Tantalumisverywork-hardening,ductile,andsoft.Itproducesapoorsurfacefinish;

toolwearishigh.

Titaniumanditsalloyshavepoorthermalconductivity（indeed,thelowestofallmetals）,causingsignificanttemperatureriseandbuilt-upedge;

theycanbedifficulttomachine.

Tungstenisbrittle,strong,andveryabrasive,soitsmachinabilityislow,althoughitgreatlyimprovesatelevatedtemperatures.

Zirconiumhasgoodmachinability.Itrequiresacoolant-typecuttingfluid,however,becauseoftheexplosionandfire.

20.9.3MachinabilityofVariousMaterials

Graphiteisabrasive;

itrequireshard,abrasion-resistant,sharptools.

Thermoplasticsgenerallyhavelowthermalconductivity,lowelasticmodulus,andlowsofteningtemperature.Consequently,machiningthemrequirestoolswithpositiverakeangles（toreducecuttingforces）,largereliefangles,smalldepthsofcutandfeed,relativelyhighspeeds,and

propersupportoftheworkpiece.Toolsshouldbesharp.

Externalcoolingofthecuttingzonemaybenecessarytokeepthechipsfrombecoming“gummy”andstickingtothetools.Coolingcanusuallybeachieved