機(jī)床加工外文翻譯參考文獻(xiàn)

機(jī)床加工外文翻譯參考文獻(xiàn)BasicMachiningOperationsandCuttingTechnologyMachinetoolshaveevolvedfromtheearlyfoot-poweredlathesoftheEgyptiansandJohnWilkinson'sboringmill.Theyaredesignedtoproviderigidsupportforboththepieceworkerandthecuttingtoolandcanpreciselycontroltheirrelativepositionsandthevelocityofthetoolwithrespecttothepieceworker.Basically,inmetalcutting,asharpenedwedge-shapedtoolremovesarathernarrowstripofmetalfromthesurfaceofaductilepieceworkerintheformofaseverelydeformedchip.Thechipisawasteproductthatisconsiderablyshorterthanthepieceworkerfromwhichitcamebutwithacorrespondingincreaseinthicknessoftheuncutchip.Thegeometricalshapeofpieceworkerdependsontheshapeofthetoolanditspathduringthemachiningoperation.Mostmachiningoperationsproducepartsofdifferinggeometry.Ifaroughcylindricalpieceworkerrevolvesaboutacentralaxisandthetoolpenetratesbeneathitssurfaceandtravelsparalleltothecenterofrotation,asurfaceofrevolutionisproduced,andtheoperationiscalledturning.Ifahollowtubeismachinedontheinsideinasimilarmanner,theoperationiscalledboring.Producinganexternalconicalsurfaceuniformlyvaryingdiameteriscalledtaperturning,ifthetoolpointtravelsinapathofvaryingradius,acontouredsurfacelikethatofabowlingpincanbeproduced;or,ifthepieceisshortenoughandthesupportissufficientlyrigid,acontouredsurfacecouldbeproducedbyfeedingashapedtoolnormaltotheaxisofrotation.Shorttaperedorcylindricalsurfacescouldalsobecontourformed.Flatorplanesurfacesarefrequentlyrequired.Theycanbegeneratedbyradialturningorfacing,inwhichthetoolpointmovesnormaltotheaxisofrotation.Inothercases,itismoreconvenienttoholdthepieceworkersteadyandreciprocatethetoolacrossitinaseriesofstraight-linecutswithacrosswisefeedincrementbeforeeachcuttingstroke.ThisoperationiscalledplanningandiscarriedoutonaSharpe.Forlargerpiecesitiseasiertokeepthetoolstationaryanddrawthepieceworkerunderitasinplanning.Thetoolisfedateachreciprocation.Contouredsurfacescanbeproducedbyusingshapedtools.Multiple-edgedtoolscanalsobeused.Drillingusesatwin-edgedflutedtoolforholeswithdepthsupto5to10timesthedrilldiameter.Whetherthedrillturnsorthepieceworkerrotates,relativemotionbetweenthecuttingedgeandthepieceworkeristheimportantfactor.Inmillingoperationsarotarycutterwithanumberofcuttingedgesengagesthepieceworker.Whichmovesslowlywithrespecttothecutter.Planeorcontouredsurfacesmaybeproduced,dependingonthegeometryofthecutterandthetypeoffeed.Horizontalorverticalaxesofrotationmaybeused,andthefeedofthepieceworkermaybeinanyofthethreecoordinatedirections.MachinetoolsareusedtoproduceapartofaspecifiedgeometricalshapeandpreciseIsizebyremovingmetalfromaductilematerialintheformofchips.Thelatterareawasteproductandvaryfromlongcontinuousribbonsofaductilematerialsuchassteel,whichareundesirablefromadisposalpointofview,toeasilyhandledwell-brokenchipsresultingfromcastiron.Machinetoolsperformfivebasicmetal-removalprocesses:Iturning,planning,drilling,milling,andgrinding.Allothermetal-removalprocessesaremodificationsofthesefivebasicprocesses.Forexample,boringisinternalturning;reaming,tapping,andcounterboringmodifydrilledholesandarerelatedtodrilling;bobbingandgearcuttingarefundamentallymillingoperations;hacksawingandbroachingareaformofplanningandhoning;lapping,superfinishing.Polishingandbuffingarevariantsofgrindingorabrasiveremovaloperations.Therefore,thereareonlyfourtypesofbasicmachinetools,whichusecuttingtoolsofspecificcontrollablegeometry:1.lathes,2.planers,3.drillingmachines,and4.millingmachines.Thegrindingprocessformschips,butthegeometryoftheabrasivegrainisuncontrollable.TheamountandrateofmaterialremovedbythevariousmachiningprocessesmaybeIlarge,asinheavyturningoperations,orextremelysmall,asinlappingorsuperfinishingoperationswhereonlythehighspotsofasurfaceareremoved.Amachinetoolperformsthreemajorfunctions:1.itrigidlysupportsthepieceworkeroritsholderandthecuttingtool;2.itprovidesrelativemotionbetweenthepieceworkerandthecuttingtool;3.itprovidesarangeoffeedsandspeedsusuallyrangingfrom4to32choicesineachcase.Speeds,feeds,anddepthofcutarethethreemajorvariablesforeconomicalmachining.Othervariablesaretheworkandtoolmaterials,coolantandgeometryofthecuttingtool.Therateofmetalremovalandpowerrequiredformachiningdependuponthesevariables.Thedepthofcut,feed,andcuttingspeedaremachinesettingsthatmustbeestablishedinanymetal-cuttingoperation.Theyallaffecttheforces,thepower,andtherateofmetalremoval.Theycanbedefinedbycomparingthemtotheneedleandrecordofaphonograph.Thecuttingspeed(V)isrepresentedbythevelocityof-therecordsurfacerelativetotheneedleinthetonearmatanyinstant.Feedisrepresentedbytheadvanceoftheneedleradiallyinwardperrevolution,oristhedifferenceinpositionbetweentwoadjacentgrooves.Thedepthofcutisthepenetrationoftheneedleintotherecordorthedepthofthegrooves.Thebasicoperationsperformedonanenginelatheareillustrated.Thoseoperationsperformedonexternalsurfaceswithasinglepointcuttingtoolarecalledturning.Exceptfordrilling,reaming,andlapping,theoperationsoninternalsurfacesarealsoperformedbyasinglepointcuttingtool.Allmachiningoperations,includingturningandboring,canbeclassifiedasroughing,finishing,orsemi-finishing.Theobjectiveofaroughingoperationistoremovethebulkofthematerialasrapidlyandasefficientlyaspossible,whileleavingasmallamountofmaterialonthework-pieceforthefinishingoperation.Finishingoperationsareperformedtoobtainthefinalsize,shape,andsurfacefinishonthepieceworker.Sometimesasemi-finishingoperationwillprecedethefinishingoperationtoleaveasmallpredeterminedanduniformamountofstockonthework-piecetoberemovedbythefinishingoperation.Generally,longerpieceworkareturnedwhilesupportedononeortwolathecenters.Coneshapedholes,calledcenterholes,whichfitthelathecentersaredrilledintheendsofthepieceworker-usuallyalongtheaxisofthecylindricalpart.TheendofthepieceworkeradjacenttotheStockholmisalwayssupportedbyaStockholmcenter,whiletheendneartheheadstockmaybesupportedbyaheadstockcenterorheldinachuck.Theheadstockendofthepieceworkermaybeheldinafour-jawchuck,orinatypechuck.Thismethodholdsthepieceworkerfirmlyandtransfersthepowertothepieceworkersmoothly;theadditionalsupporttothepieceworkerprovidedbythechucklessensthetendencyforchattertooccurwhencutting.Preciseresultscanbeobtainedwiththismethodifcareistakentoholdthepieceworkeraccuratelyinthechuck.Verypreciseresultscanbeobtainedbysupportingthepieceworkerbetweentwocenters.Alathedogisclampedtothepieceworker;togethertheyaredrivenbyadriverplatemountedonthespindlenose.OneendofthePieceworkerismedicine;thenthepieceworkercanbeturnedaroundinthelathetomachinetheotherend.Thecenterholesinthepieceworkerserveaspreciselocatingsurfacesaswellasbearingsurfacestocarrytheweightofthepieceworker

andtoresistthecuttingforces.Afterthepieceworkerhasbeenremovedfromthelatheforanyreason,thecenterholeswillaccuratelyalignthepieceworkerbackinthelatheorinanotherlathe,orinacylindricalgrindingmachine.Thepieceworkermustneverbeheldattheheadstockendbybothachuckandalathecenter.Whileatfirstthoughtthisseemslikeaquickmethodofaligningthepieceworkerinthechuck,thismustnotbedonebecauseitisnotpossibletopressevenlywiththejawsagainstthepieceworkerwhileitisalsosupportedbythecenter.Thealignmentprovidedbythecenterwillnotbemaintainedandthepressureofthejawsmaydamagethecenterhole,thelathecenter,andperhapseventhelathespindle.Compensatingorfloatingjawchucksusedalmostexclusivelyonhighproductionworkprovideanexceptiontothestatementsmadeabove.Thesechucksarereallyworkdriversandcannotbeusedforthesamepurposeasordinarythreeorfour-jawchucks.Whileverylargediameterpieceworkaresometimesmountedontwocenters,theyarepreferablyheldattheheadstockendbyfaceplatejawstoobtainthesmoothpowertransmission;moreover,largelathedogsthatareadequatetotransmitthepowernotgenerallyavailable,althoughtheycanbemadeasaspecial.Faceplatejawsarelikechuckjawsexceptthattheyaremountedonafaceplate,whichhaslessoverhangfromthespindlebearingsthanalargechuckwouldhave.Machiningasashape-producingmethodisthemostuniversallyusedandthemostimportantofallmanufacturingprocesses.Machiningisashape-producingprocessinwhichapower-drivendevicecausesmaterialtoberemovedinchipform.Mostmachiningisdonewithequipmentthatsupportsboththeworkpieceandcuttingtoolalthoughinsomecasesportableequipmentisusedwithunsupportedpieceworker.LowsetupcostforsmallQuantities.Machininghastwoapplicationsinmanufacturing.Forcasting,forging,andpressworking,eachspecificshapetobeproduced,evenonepart,nearlyalwayshasahightoolingcost.Theshapesthatmayheproducedbyweldingdependtoalargedegreeontheshapesofrawmaterialthatareavailable.Bymakinguseofgenerallyhighcostequipmentbutwithoutspecialtooling,itispossible,bymachining;tostartwithnearlyanyformofrawmaterial,sotongastheexteriordimensionsaregreatenough,andproduceanydesiredshapefromanymaterial.Therefore.machiningisusuallythepreferredmethodforproducingoneorafewparts,evenwhenthedesignofthepartwouldlogicallyleadtocasting,forgingorpressworkingifahighquantityweretobeproduced.Closeaccuracies,goodfinishes.Thesecondapplicationformachiningisbasedonthehighaccuraciesandsurfacefinishespossible.Manyofthepartsmachinedinlowquantitieswouldbeproducedwithlowerbutacceptabletolerancesifproducedinhighquantitiesbysomeotherprocess.Ontheotherhand,manypartsaregiventheirgeneralshapesbysomehighquantitydeformationprocessandmachinedonlyonselectedsurfaceswherehighaccuraciesareneeded.Internalthreads,forexample,areseldomproducedbyanymeansotherthanmachiningandsmallholesinpressworkedpartsmaybemachinedfollowingthepressworkingoperations.Thebasictool-workrelationshipincuttingisadequatelydescribedbymeansoffourfactors:toolgeometry,cuttingspeed,feed,anddepthofcut.Thecuttingtoolmustbemadeofanappropriatematerial;itmustbestrong,tough,hard,andwearresistant.Thetoolsgeometrycharacterizedbyplanesandangles,mustbecorrectforeachcuttingoperation.Cuttingspeedistherateatwhichtheworksurfacepassesbythecuttingedge.Itmaybeexpressedinfeetperminute.Forefficientmachiningthecuttingspeedmustbeofamagnitudeappropriatetotheparticularwork-toolcombination.Ingeneral,thehardertheworkmaterial,theslowerthespeed.Feedistherateatwhichthecuttingtooladvancesintothepieceworker."Wherethepieceworkerorthetoolrotates,feedismeasuredininchesperrevolution.Whenthetoolortheworkreciprocates,feedismeasuredininchesperstroke,Generally,feedvariesinverselywithcuttingspeedforotherwisesimilarconditions.Thedepthofcut,measuredinchesisthedistancethetoolissetintothework.Itisthewidthofthechipinturningorthethicknessofthechipinarectilinearcut.Inroughingoperations,thedepthofcutcanbelargerthanforfinishingoperations.Inmetalcuttingoperationsheatisgeneratedintheprimaryandsecondarydeformationzonesandtheseresultsinacomplextemperaturedistributionthroughoutthetool,pieceworkerandchip.Atypicalsetofisothermsisshowninfigurewhereitcanbeseenthat,ascouldbeexpected,thereisaverylargetemperaturegradientthroughoutthewidthofthechipasthepieceworkermaterialisshearedinprimarydeformationandthereisafurtherlargetemperatureinthechipadjacenttothefaceasthechipisshearedinsecondarydeformation.Thisleadstoamaximumcuttingtemperatureashortdistanceupthefacefromthecuttingedgeandasmalldistanceintothechip.Sincevirtuallyalltheworkdoneinmetalcuttingisconvertedintoheat,itcouldbeexpectedthatfactorswhichincreasethepowerconsumedperunitvolumeofmetalremovedwillincreasethecuttingtemperature.Thusanincreaseintherakeangle,allotherparametersremainingconstant,willreducethepowerperunitvolumeofmetalremovedandthecuttingtemperatureswillreduce.Whenconsideringincreaseinunreformedchipthicknessandcuttingspeedthesituationismorecomplex.Anincreaseininformedchipthicknesstendstobeascaleeffectwheretheamountsofheatwhichpasstothepieceworker,thetoolandchipremaininfixedproportionsandthechangesincuttingtemperaturetendtobesmall.Increaseincuttingspeed;however,reducetheamountofheatwhichpassesintothepieceworkerandthisincreasethetemperatureriseofthechipmprimarydeformation.Further,thesecondarydeformationzonetendstobesmallerandthishastheeffectofincreasingthetemperaturesinthiszone.Otherchangesincuttingparametershavevirtuallynoeffectonthepowerconsumedperunitvolumeofmetalremovedandconsequentlyhavevirtuallynoeffectonthecuttingtemperatures.Sinceithasbeenshownthatevensmallchangesincuttingtemperaturehaveasignificanteffectontoolwearrateitisappropriatetoindicatehowcuttingtemperaturescanbeassessedfromcuttingdata.Themostdirectandaccuratemethodformeasuringtemperaturesinhigh-speed-steelcuttingtoolsisthatofWright&.Trentwhichalsoyieldsdetailedinformationontemperaturedistributionsinhigh-speed-steelcuttingtools.Thetechniqueisbasedonthecrystallographyexaminationofsectionedhigh-speed-steeltoolswhichrelatesmicrostructurechangestothermalhistory.Trenthasdescribedmeasurementsofcuttingtemperaturesandtemperature

distributionsforhigh-speed-steeltoolswhenmachiningawiderangeofpieceworkermaterials.Thistechniquehasbeenfurtherdevelopedbyusingscanningelectron

microscopytostudyfine-scalemicrostructurechangesarisingfromovertemperingofthetemperedmartensticmatrixofvarioushigh-speed-steels.Thistechniquehasalsobeenusedtostudytemperaturedistributionsinbothhigh-speed-steelsinglepointturningtoolsandtwistdrills.Discountingbrittlefractureandedgechipping,whichhavealreadybeendealtwith,toolwearisbasicallyofthreetypes.Flankwear,craterwear,andnotchwear.Flankwearoccursonboththemajorandtheminorcuttingedges.Onthemajorcuttingedge,whichisresponsibleforbulkmetalremoval,theseresultsinincreasedcuttingforcesandhighertemperatureswhichifleftuncheckedcanleadtovibrationofthetoolandpieceworkerandaconditionwhereefficientcuttingcannolongertakeplace.Ontheminorcuttingedge,whichdeterminespieceworkersizeandsurfacefinish,flankwearcanresultinanoversizedproductwhichhaspoorsurfacefinish.Undermostpracticalcuttingconditions,thetoolwillfailduetomajorflankwearbeforetheminorflankwearissufficientlylargetoresultinthemanufactureofanunacceptablecomponent.Becauseofthestressdistributiononthetoolface,thefrictionalstressintheregionofslidingcontactbetweenthechipandthefaceisatamaximumatthestartoftheslidingcontactregionandiszeroattheend.Thusabrasiveweartakesplaceinthisregionwithmoreweartakingplaceadjacenttotheseizureregionthanadjacenttothepointatwhichthechiplosescontactwiththeface.Thisresultinlocalizedpittingofthetoolfacesomedistanceupthefacewhichisusuallyreferredtoascateringandwhichnormallyhasasectionintheformofacirculararc.Inmanyrespectsandforpracticalcuttingconditions,craterwearisalesssevereformofwearthanflankwearandconsequentlyflankwearisamorecommontoolfailurecriterion.However,sincevariousauthorshaveshownthatthetemperatureonthefaceincreasesmorerapidlywithincreasingcuttingspeedthanthetemperatureontheflank,andsincetherateofwearofanytypeissignificantlyaffectedbychangesintemperature,craterwearusuallyoccursathighcuttingspeeds.Attheendofthemajorflankwearlandwherethetoolisincontactwiththeuncutpieceworkersurfaceitiscommonfortheflankweartobemorepronouncedthanalongtherestofthewearland.Thisisbecauseoflocalizedeffectssuchasahardenedlayerontheuncutsurfacecausedbyworkhardeningintroducedbyapreviouscut,anoxidescale,andlocalizedhightemperaturesresultingfromtheedgeeffect.Thislocalizedwearisusuallyreferredtoasnotchwearandoccasionallyisverysevere.Althoughthepresenceofthenotchwillnotsignificantlyaffectthecuttingpropertiesofthetool,thenotchisoftenrelativelydeepandifcuttingweretocontinuetherewouldbeagoodchancethatthetoolwouldfracture.Ifanyformofprogressivewearallowedtocontinue,dramaticallyandthetoolwouldfailcatastrophically,i.e.thetoolwouldbenolongercapableofcuttingand,atbest,thepieceworkerwouldbescrappedwhilst,atworst,damagecouldbecausedtothemachinetool.Forcarbidecuttingtoolsandforalltypesofwear,thetoolissaidtohavereachedtheendofitsusefullifelongbeforetheonsetofcatastrophicfailure.Forhigh-speed-steelcuttingtools,however,wheretheweartendstobenon-uniformithasbeenfoundthatthemostmeaningfulandreproducibleresultscanbeobtainedwhenthewearisallowedtocontinuetotheonsetofcatastrophicfailureeventhough,ofcourse,inpracticeacuttingtimefarlessthanthattofailurewouldbeused.Theonsetofcatastrophicfailureischaracterizedbyoneofseveralphenomena,themostcommonbeingasuddenincreaseincuttingforce,thepresenceofburnishedringsonthepieceworker,andasignificantincreaseinthenoiselevel.Therearebasicallyfivemechanismswhichcontributetotheproductionofasurfacewhichhavebeenmachined.Theseare:(l)Thebasicgeometryofthecuttingprocess.In,forexample,singlepointturningthetoolwilladvanceaconstantdistanceaxiallyperrevolutionofthepieceworkandtheresultantsurfacewillhaveonit,whenviewedperpendicularlytothedirectionoftoolfeedmotion,aseriesofcuspswhichwillhaveabasicformwhichreplicatestheshapeofthetoolincut.(2)Theefficiencyofthecuttingoperation.Ithasalreadybeenmentionedthatcuttingwithunstablebuilt-up-edgeswillproduceasurfacewhichcontainshardbuilt-up-edgefragmentswhichwillresultinadegradationofthesurfacefinish.Itcanalsobedemonstratedthatcuttingunderadverseconditionssuchasapplywhenusinglargefeedssmallrakeanglesandlowcuttingspeeds,besidesproducingconditionswhichleadtounstablebuilt-up-edgeproduction,thecuttingprocessitselfcanbecomeunstableandinsteadofcontinuousshearoccurringintheshearzone,tearingtakesplace,discontinuouschipsofuneventhicknessareproduced,andtheresultantsurfaceispoor.Thissituationisparticularlynoticeablewhenmachiningveryductilematerialssuchascopperandaluminum.(3)Thestabilityofthemachinetool.Undersomecombinationsofcuttingconditions;pieceworkersize,methodofclamping,andcuttingtoolrigidityrelativetothemachinetoolstructure,instabilitycanbesetupinthetoolwhichcausesittovibrate.Undersomeconditionsthisvibrationwillreachandmaintainsteadyamplitudewhilstunderotherconditionsthevibrationwillbuiltupandunlesscuttingisstoppedconsiderabledamagetoboththecuttingtoolandpieceworkermayoccur.Thisphenomenonisknownaschatterandinaxialturningischaracterizedbylongpitchhelicalbandsonthepieceworkersurfaceandshortpitchundulationsonthetransientmachinedsurface.(4)Theeffectivenessofremovingdwarf.Indiscontinuouschipproductionmachining,suchasmillingorturningofbrittlematerials,itisexpectedthatthechip(sward)willleavethecuttingzoneeitherundergravityorwiththeassistanceofajetofcuttingfluidandthattheywillnotinfluencethecutsurfaceinanyway.However,whencontinuouschipproductionisevident,unlessstepsaretakentocontrolthedwarfitislikelythatitwillimpingeonthecutsurfaceandmarkit.Inevitably,thismarkingbesideslooking.(5)Theeffectiveclearanceangleonthecuttingtool.Forcertaingeometriesofminorcuttingedgereliefandclearanceanglesitispossibletocutonthemajorcuttingedgeandburnishontheminorcuttingedge.Thiscanproduceagoodsurfacefinishbut,ofcourse,itisstrictlyacombinationofmetalcuttingandmetalformingandisnottoberecommendedasapracticalcuttingmethod.However,duetocuttingtoolwear,theseconditionsoccasionallyariseandleadtoamarkedchangeinthesurfacecharacteristics.Machinepartsaremanufacturedsotheyareinterchangeable.Inotherwords,eachpartofamachineormechanismismadetoacertainsizeandshapesowillfitintoanyothermachineormechanismofthesametype.Tomakethepartinterchangeable,eachindividualpartmustbemadetoasizethatwillfitthematingpartinthecorrectway.Itisnotonlyimpossible,butalsoimpracticaltomakemanypartstoanexactsize.Thisisbecausemachinesarenotperfect,andthetoolsbecomeworn.Aslightvariationfromtheexactsizeisalwaysallowed.Theamountofthisvariationdependsonthekindofpartbeingmanufactured.Forexamplespartmightbemade6in.longwithavariationallowedof0.003(three-thousandths)in.aboveandbelowthissize.Therefore,thepartcouldbe5.997to6.003in.andstillbethecorrectsize.Theseareknownasthelimits.Thedifferencebetweenupperandlowerlimitsiscalledthetolerance.Atoleranceisthetotalpermissiblevariationinthesizeofapart.Thebasicsizeisthatsizefromwhichlimitsofsizearcderivedbytheapplicationofallowancesandtolerances.Sometimesthelimitisallowedinonlyonedirection.Thisisknownasunilateraltolerance.Unilateraltoleranceisasystemofdissensionwherethetolerance(thatisvariation)isshowninonlyonedirectionfromthenominalsize.Unilateraltoleranceallowthechangingoftoleranceonaholeorshaftwithoutseriouslyaffectingthefit.Whenthetoleranceisinbothdirectionsfromthebasicsizeitisknownasabilateraltolerance(plusandminus).Bilateraltoleranceisasystemofdissensionwherethetolerance(thatisvariation)issplitandisshownoneithersideofthenominalsize.Limitdissensionisasystemofdissensionwhereonlythemaximumandminimumdimensionsarcshown.Thus,thetoleranceisthedifferencebetweenthesetwodimensions.Productsthathavebeencompletedtotheirpropershapeandsizefrequentlyrequiresometypeofsurfacefinishingtoenablethemtosatisfactorilyfulfilltheirfunction.Insomecases,itisnecessarytoimprovethephysicalpropertiesofthesurfacematerialforresistancetopenetrationorabrasion.Inmanymanufacturingprocesses,theproductsurfaceisleftwithdirt.chips,grease,orotherharmfulmaterialuponit.Assembliesthataremadeofdifferentmaterials,orfromthesamematerialsprocessedindifferentmanners,mayrequiresomespecialsurfacetreatmenttoprovideuniformityofappearance.Surfacefinishingmaysometimesbecomeanintermediatestepprocessing.Forinstance,cleaningandpolishingareusuallyessentialbeforeanykindofplatingprocess.Someofthecleaningproceduresarealsousedforimprovingsurfacesmoothnessonmatingpartsandforremovingburrsandsharpcorners,whichmightbeharmfulinlateruse.Anotherimportantneedforsurfacefinishingisforcorrosionprotectioninavarietyof:environments.Thetypeofprotectionprocedurewilldependlargelyupontheanticipatedexposure,withdueconsiderationtothematerialbeingprotectedandtheeconomicfactorsinvolved.Satisfyingtheaboveobjectivesnecessitatestheuseofmainsurface-finishingmethodsthatinvolvechemicalchangeofthesurfacemechanicalworkaffectingsurfaceproperties,cleaningbyavarietyofmethods,andtheapplicationofprotectivecoatings,organicandmetallic.Intheearlydaysofengineering,thematingofpartswasachievedbymachiningonepartasnearlyaspossibletotherequiredsize,machiningthematingpartnearlytosize,andthencompletingitsmachining,continuallyofferingtheotherparttoit,untilthedesiredrelationshipwasobtained.Ifitwasinconvenienttoofferoneparttotheotherpartduringmachining,thefinalworkwasdoneatthebenchbyafitter,whoscrapedthematingpartsuntilthedesiredfitwasobtained,thefitterthereforebeinga'fitter'intheliteralsense.JItisobviousthatthetwopartswouldhavetoremaintogether,andmtheeventofonehavingtobereplaced,thefittingwouldhavetobedonealloveragain.Inthesedays,weexpecttobeabletopurchaseareplacementforabrokenpart,andforittofunctioncorrectlywithouttheneedforscrapingandotherfittingoperations.Whenonepartcanbeused'offtheshelf'toreplaceanotherofthesamedimensionandmaterialspecification,thepartsaresaidtobeinterchangeable.Asystemofinterchangeabilityusuallylowerstheproductioncostsasthereisnoneedforanexpensive,'fiddling'operation,anditbenefitsthecustomerintheeventoftheneedtoreplacewornparts.Traditionalsynchronousgrippersforassemblyequipmentmovepartstothegrippercenter-line,assuringthatthepartswillbeinaknownpositionaftertheyarcpickedfromaconveyorornest.However,insomeapplications,forcingtheparttothec