外文翻译--基本加工工序和切削技术.doc

英文原文BasicMachiningOperationsandCuttingTechnologyMachinetoolshaveevolvedfromtheearlyfoot-poweredlathesoftheEgyptiansandJohnWilkinsonsboringmill.Theyaredesignedtoproviderigidsupportforboththeworkpieceandthecuttingtoolandcanpreciselycontroltheirrelativepositionsandthevelocityofthetoolwithrespecttotheworkpiece.Basically,inmetalcutting,asharpenedwedge-shapedtoolremovesarathernarrowstripofmetalfromthesurfaceofaductileworkpieceintheformofaseverelydeformedchip.Thechipisawasteproductthatisconsiderablyshorterthantheworkpiecefromwhichitcamebutwithacorrespondingincreaseinthicknessoftheuncutchip.Thegeometricalshapeofworkpiecedependsontheshapeofthetoolanditspathduringthemachiningoperation.Mostmachiningoperationsproducepartsofdifferinggeometry.Ifaroughcylindricalworkpiecerevolvesaboutacentralaxisandthetoolpenetratesbeneathitssurfaceandtravelsparalleltothecenterofrotation,asurfaceofrevolutionisproduced,andtheoperationiscalledturning.Ifahollowtubeismachinedontheinsideinasimilarmanner,theoperationiscalledboring.Producinganexternalconicalsurfaceuniformlyvaryingdiameteriscalledtaperturning.Ifthetoolpointtravelsinapathofvaryingradius,acontouredsurfacelikethatofabowlingpincanbeproduced；or,ifthepieceisshortenoughandthesupportissufficientlyrigid,acontouredsurfacecouldbeproducedbyfeedingashapedtoolnormaltotheaxisofrotation.Shorttaperedorcylindricalsurfacescouldalsobecontourformed.Multiple-edgedtoolscanalsobeused.Drillingusesatwin-edgedflutedtoolforholeswithdepthsupto5to10timesthedrilldiameter.Whetherthedrillturnsortheworkpiecerotates,relativemotionbetweenthecuttingedgeandtheworkpieceistheimportantfactor.Inmillingoperationsarotarycutterwithanumberofcuttingedgesengagestheworkpiece,whichmovesslowlywithrespecttothecutter.Planeorcontouredsurfacesmaybeproduced,dependingonthegeometryofthecutterandthetypeoffeed.Horizontalorverticalaxesofrotationmaybeused,andthefeedoftheworkpiecemaybeinanyofthethreecoordinatedirections.IntroductionofMachiningMachiningasashape-producingmethodisthemostuniversallyusedandthemostimportantofallmanufacturingprocesses.Machiningisashape-producingprocessinwhichapower-drivendevicecausesmaterialtoberemovedinchipform.Mostmachiningisdonewithequipmentthatsupportsboththeworkpieceandcuttingtoolalthoughinsomecasesportableequipmentisusedwithunsupportedworkpiece.LowsetupcostforsmallQuantities.Machininghastwoapplicationsinmanufacturing.Forcasting,forging,andpressworking,eachspecificshapetobeproduced,evenonepart,nearlyalwayshasahightoolingcost.Theshapesthatmaybeproducedbyweldingdependtoalargedegreeontheshapesofrawmaterialthatareavailable.Bymakinguseofgenerallyhighcostequipmentbutwithoutspecialtooling,itispossible,bymachining；tostartwithnearlyanyformofrawmaterial,solongastheexteriordimensionsaregreatenough,andproduceanydesiredshapefromanymaterial.Therefore.machiningisusuallythepreferredmethodforproducingoneorafewparts,evenwhenthedesignofthepartwouldlogicallyleadtocasting,forgingorpressworkingifahighquantityweretobeproduced.Closeaccuracies,goodfinishes.Thesecondapplicationformachiningisbasedonthehighaccuraciesandsurfacefinishespossible.Manyofthepartsmachinedinlowquantitieswouldbeproducedwithlowerbutacceptabletolerancesifproducedinhighquantitiesbysomeotherprocess.Ontheotherhand,manypartsaregiventheirgeneralshapesbysomehighquantitydeformationprocessandmachinedonlyonselectedsurfaceswherehighaccuraciesareneeded.Internalthreads,forexample,areseldomproducedbyanymeansotherthanmachiningandsmallholesinpressworkedpartsmaybemachinedfollowingthepressworkingoperations.PrimaryCuttingParametersThebasictool-workrelationshipincuttingisadequatelydescribedbymeansoffourfactors:toolgeometry,cuttingspeed,feed,anddepthofcut.Thecuttingtoolmustbemadeofanappropriatematerial；itmustbestrong,tough,hard,andwearresistant.Thetoolsgeometrycharacterizedbyplanesandangles,mustbecorrectforeachcuttingoperation.Cuttingspeedistherateatwhichtheworksurfacepassesbythecuttingedge.Itmaybeexpressedinfeetperminute.Forefficientmachiningthecuttingspeedmustbeofamagnitudeappropriatetotheparticularwork-toolcombination.Ingeneral,thehardertheworkmaterial,theslowerthespeed.Feedistherateatwhichthecuttingtooladvancesintotheworkpiece.Wheretheworkpieceorthetoolrotates,feedismeasuredininchesperrevolution.Whenthetoolortheworkreciprocates,feedismeasuredininchesperstroke,Generally,feedvariesinverselywithcuttingspeedforotherwisesimilarconditions.Thedepthofcut,measuredinchesisthedistancethetoolissetintothework.Itisthewidthofthechipinturningorthethicknessofthechipinarectilinearcut.Inroughingoperations,thedepthofcutcanbelargerthanforfinishingoperations.TheEffectofChangesinCuttingParametersonCuttingTemperaturesInmetalcuttingoperationsheatisgeneratedintheprimaryandsecondarydeformationzonesandtheseresultsinacomplextemperaturedistributionthroughoutthetool,workpieceandchip.Atypicalsetofisothermsisshowninfigurewhereitcanbeseenthat,ascouldbeexpected,thereisaverylargetemperaturegradientthroughoutthewidthofthechipastheworkpiecematerialisshearedinprimarydeformationandthereisafurtherlargetemperatureinthechipadjacenttothefaceasthechipisshearedinsecondarydeformation.Thisleadstoamaximumcuttingtemperatureashortdistanceupthefacefromthecuttingedgeandasmalldistanceintothechip.Sincevirtuallyalltheworkdoneinmetalcuttingisconvertedintoheat,itcouldbeexpectedthatfactorswhichincreasethepowerconsumedperunitvolumeofmetalremovedwillincreasethecuttingtemperature.Thusanincreaseintherakeangle,allotherparametersremainingconstant,willreducethepowerperunitvolumeofmetalremovedandthecuttingtemperatureswillreduce.Whenconsideringincreaseinunreformedchipthicknessandcuttingspeedthesituationismorecomplex.Anincreaseinundeformedchipthicknesstendstobeascaleeffectwheretheamountsofheatwhichpasstotheworkpiece,thetoolandchipremaininfixedproportionsandthechangesincuttingtemperaturetendtobesmall.Increaseincuttingspeed；however,reducetheamountofheatwhichpassesintotheworkpieceandthisincreasethetemperatureriseofthechipmprimarydeformation.Further,thesecondarydeformationzonetendstobesmallerandthishastheeffectofincreasingthetemperaturesinthiszone.Otherchangesincuttingparametershavevirtuallynoeffectonthepowerconsumedperunitvolumeofmetalremovedandconsequentlyhavevirtuallynoeffectonthecuttingtemperatures.Sinceithasbeenshownthatevensmallchangesincuttingtemperaturehaveasignificanteffectontoolwearrateitisappropriatetoindicatehowcuttingtemperaturescanbeassessedfromcuttingdata.Themostdirectandaccuratemethodformeasuringtemperaturesinhigh-speed-steelcuttingtoolsisthatofWright&Trent,whichalsoyieldsdetailedinformationontemperaturedistributionsinhigh-speed-steelcuttingtools.Thetechniqueisbasedonthemetallographicexaminationofsectionedhigh-speed-steeltoolswhichrelatesmicrostructurechangestothermalhistory.Trenthasdescribedmeasurementsofcuttingtemperaturesandtemperaturedistributionsforhigh-speed-steeltoolswhenmachiningawiderangeofworkpiecematerials.Thistechniquehasbeenfurtherdevelopedbyusingscanningelectronmicroscopytostudyfine-scalemicrostructurechangesarisingfromovertemperingofthetemperedmartensticmatrixofvarioushigh-speed-steels.Thistechniquehasalsobeenusedtostudytemperaturedistributionsinbothhigh-speed-steelsinglepointturningtoolsandtwistdrills.WearsofCuttingToolDiscountingbrittlefractureandedgechipping,whichhavealreadybeendealtwith,toolwearisbasicallyofthreetypes.Flankwear,craterwear,andnotchwear.Flankwearoccursonboththemajorandtheminorcuttingedges.Onthemajorcuttingedge,whichisresponsibleforbulkmetalremoval,theseresultsinincreasedcuttingforcesandhighertemperatureswhichifleftuncheckedcanleadtovibrationofthetoolandworkpieceandaconditionwhereefficientcuttingcannolongertakeplace.Ontheminorcuttingedge,whichdeterminesworkpiecesizeandsurfacefinish,flankwearcanresultinanoversizedproductwhichhaspoorsurfacefinish.Undermostpracticalcuttingconditions,thetoolwillfailduetomajorflankwearbeforetheminorflankwearissufficientlylargetoresultinthemanufactureofanunacceptablecomponent.Becauseofthestressdistributiononthetoolface,thefrictionalstressintheregionofslidingcontactbetweenthechipandthefaceisatamaximumatthestartoftheslidingcontactregionandiszeroattheend.Thusabrasiveweartakesplaceinthisregionwithmoreweartakingplaceadjacenttotheseizureregionthanadjacenttothepointatwhichthechiplosescontactwiththeface.Thisresultinlocalizedpittingofthetoolfacesomedistanceupthefacewhichisusuallyreferredtoascateringandwhichnormallyhasasectionintheformofacirculararc.Inmanyrespectsandforpracticalcuttingconditions,craterwearisalesssevereformofwearthanflankwearandconsequentlyflankwearisamorecommontoolfailurecriterion.However,sincevariousauthorshaveshownthatthetemperatureonthefaceincreasesmorerapidlywithincreasingcuttingspeedthanthetemperatureontheflank,andsincetherateofwearofanytypeissignificantlyaffectedbychangesintemperature,craterwearusuallyoccursathighcuttingspeeds.Attheendofthemajorflankwearlandwherethetoolisincontactwiththeuncutworkpiecesurfaceitiscommonfortheflankweartobemorepronouncedthanalongtherestofthewearland.Thisisbecauseoflocalisedeffectssuchasahardenedlayerontheuncutsurfacecausedbyworkhardeningintroducedbyapreviouscut,anoxidescale,andlocalisedhightemperaturesresultingfromtheedgeeffect.Thislocalisedwearisusuallyreferredtoasnotchwearandoccasionallyisverysevere.Althoughthepresenceofthenotchwillnotsignificantlyaffectthecuttingpropertiesofthetool,thenotchisoftenrelativelydeepandifcuttingweretocontinuetherewouldbeagoodchancethatthetoolwouldfracture.Ifanyformofprogressivewearallowedtocontinue,dramaticallyandthetoolwouldfailcatastrophically,i.e.thetoolwouldbenolongercapableofcuttingand,atbest,theworkpiecewouldbescrappedwhilst,atworst,damagecouldbecausedtothemachinetool.Forcarbidecuttingtoolsandforalltypesofwear,thetoolissaidtohavereachedtheendofitsusefullifelongbeforetheonsetofcatastrophicfailure.Forhigh-speed-steelcuttingtools,however,wheretheweartendstobenon-uniformithasbeenfoundthatthemostmeaningfulandreproducibleresultscanbeobtainedwhenthewearisallowedtocontinuetotheonsetofcatastrophicfailureeventhough,ofcourse,inpracticeacuttingtimefarlessthanthattofailurewouldbeused.Theonsetofcatastrophicfailureischaracterizedbyoneofseveralphenomena,themostcommonbeingasuddenincreaseincuttingforce,thepresenceofburnishedringsontheworkpiece,andasignificantincreaseinthenoiselevel.MechanismofSurfaceFinishProductionTherearebasicallyfivemechanismswhichcontributetotheproductionofasurfacewhichhavebeenmachined.Theseare:(l)Thebasicgeometryofthecuttingprocess.In,forexample,singlepointturningth