机械工程英语

Conventionalmachiningisthegroupofmachiningoperationsthatusesingle-ormulti-pointtoolstoremovematerialintheformofchips.Metalcuttinginvolvesremovingmetalthroughmachiningoperations.Machiningtraditionallytakesplaceonlathes,drillpresses,andmillingmachineswiththeuseofvariouscuttingtools.Mostmachininghasverylowset-upcostcomparedwithforming,molding,andcastingprocesses.However,machiningismuchmoreexpensiveforhighvolumes.Machiningisnecessarywheretighttolerancesondimensionsandfinishesarerequired.Turningisoneofthemostcommonofmetalcuttingoperations.Inturning,aworkpieceisrotatedaboutitsaxisassingle-pointcuttingtoolsarefedintoit,shearingawayexcessmaterialandcreatingthedesiredcylindricalsurface.Turningcanoccuronbothexternalandinternalsurfacestoproduceanaxially-symmetricalcontouredpart.Partsrangingfrompocketwatchcomponentstolargediametermarinepropellershaftscanbeturnedonalathe.Apartfromturning,severalotheroperationscanalsobeperformedonlathe.Boringandinternalturning.Boringandinternalturningareperformedontheinternalsurfacesbyaboringbarorsuitableinternalcuttingtools.Iftheinitialworkpieceissolid,adrillingoperationmustbeperformedfirst.Thedrillingtoolisheldinthetailstock,andthelatteristhenfedagainsttheworkpiece.Whenboringisdoneinalathe,theworkusuallyisheldinachuckoronafaceplate.Holesmaybeboredstraight,tapered,ortoirregularcontours.Boringisessentiallyinternalturningwhilefeedingthetoolparalleltotherotationaxisoftheworkpiece.Facing.Facingistheproducingofaflatsurfaceastheresultofatool’sbeingfedacrosstheendoftherotatingworkpiece.Unlesstheworkisheldonamandrel,ifbothendsoftheworkaretobefaced,itmustbeturnedaroundafterthefirstendiscompletedandthenthefacingoperationrepeated.Thecuttingspeedshouldbedeterminedfromthelargestdiameterofthesurfacetobefaced.Facingmaybedoneeitherfromtheoutsideinwardorfromthecenteroutward.Ineithercase,thepointofthetoolmustbesetexactlyattheheightofcenterofrotation.Becausethecuttingforcetendstopushthetoolawayfromthework,itisusuallydesirabletoclampthecarriagetothelathebedduringeachfacingcuttopreventitfrommovingslightlyandthusproducingasurfacethatisnotflat.Inthefacingofcastingorothermaterialsthathaveahardsurface,thedepthofthefirstcutshouldbesufficienttopenetratethehardmaterialtoavoidexcessivetoolwear.Parting.Partingistheoperationbywhichonesectionofaworkpieceisseveredfromtheremainderbymeansofcutofftool.Becausecuttingtoolsarequitethinandmusthaveconsiderableoverhang,thisprocessislessaccurateandmoredifficult.Thetoolshouldbesetexactlyattheheightofaxisofrotation,bekeptsharp,haveproperclearanceangles,andbefedintotheworkpieceataproperanduniformfeedrate.Threading.Threadingcanbeconsideredasturningsincethepathtobetravelledbythecuttingtoolishelical.However,therearesomemajordifferencesbetweenturningandthreading.Whileinturning,theinterestisingeneratingasmoothcylindricalsurface,inthreadingtheinterestisincuttingahelicalthreadofagivenformanddepthwhichcanbecalculatedfromtheformulae.Anaccuratelyshapedandproperlymountedtoolisneededbecausethreadcuttingisaform-cuttingoperation.Theresultingthreadprofileisdeterminedbytheshapeofthetoolanditspositionrelativetotheworkpiece.Thesecondbyrequirementisthatthetoolmustmovelongitudinallyinaspecificrelationshiptotherotationofworkpiece,becausethisdeterminestheleadofthethread.Thisrequirementismetthroughtheuseoftheleadscrewandthesplitunit,whichprovidepositivemotionofcarriagerelativetotherotationofspindleLathebedisfoundationoftheenginelathe,whichheavy,ruggedcastingismadetosupporttheworkingpartsofthelathe.Thesizeandmassofthebedgivestherigiditynecessaryforaccurateengineeringtolerancesrequiredinmanufacturing.Ontopofthebedaremachinedslidewaysthatguideandalignthecarriageandtailstock,astheyaremovefromoneendofthelathetotheother.Headstockisclampedatopthebedatleft-handendofthelatheandcontainsthemotorthatdrivesthespindlewhoseaxisisparalleltotheguidewaysthroughaseriesofgearshousedwithinthegearbox.Thefunctionofgearboxistogenerateanumberofdifferentspindlespeeds.Aspindlegearismountedontherearofthespindletotransmitpowerthroughthechangegearstothefeedingboxthatdistributesthepowertotheleadscrewforthreadingortothefeedrodforturning.Thespindlehasathroughholeextendinglengthwisethroughwhichbarstockscanbefedifcontinuousproductionisused.Theholecanholdaplainlathecenterbyitstaperedinnersurfaceandmountachuck,afaceplateorcolletbyitsthreadedoutersurface.CarriageassemblyisactuallyanH-shapedblockthatsitsacrosstheguidewaysandinfrontoflathebed.Thefunctionofthecarriageistocarryandmovethecuttingtoollongitudinally.Itcanbemovedbyhandorbypowerandcanbeclampedintopositionwithalockingnut.Thecarriageiscomposedofthecrossslide,compoundrest,toolsaddle,andapron.Thecrossslideismountedonthedovetailguidewaysonthetopofthesaddleanditmovedbackandforthat90°totheaxisofthelathebythecrossslideleadscrew.Theleadscrewcanbehandorpoweractivated.Thecompoundrestismountedonthecrossslideandcanbeswiveledandclampedatanyangleinahorizontalplane.Thecompoundistypicallyusedforcuttingchamfersortapers,butmustalsobeusedwhencuttingthread.Thecompoundrestcanonlybefedbyhand.Thereisnopowert