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Unit5ConventionalMachiningProcesses5.1IntroductionConventionalmachiningisthegroupofmachiningoperationsthatusesingle-ormulti-pointtoolstoremovematerialintheformofchips.Metalcuttinginvolvesremovingmetalthroughmachiningoperations.Machiningtraditionallytakesplaceonlathes,drillpresses,andmillingmachineswiththeuseofvariouscuttingtools.Mostmachininghasverylowset-upcostcomparedwithforming,molding,andcastingprocesses.However,machiningismuchmoreexpensiveforhighvolumes.Machiningisnecessarywheretighttolerancesondimensionsandfinishesarerequired.5.2TurningandLatheTurningisoneofthemostcommonofmetalcuttingoperations.Inturning,aworkpieceisrotatedaboutitsaxisassingle-pointcuttingtoolsarefedintoit,cuttingawayexcessmaterialandcreatingthedesiredcylindricalsurface.Turningcanoccuronbothexternalandinternalsurfacestoproduceanaxially-symmetricalcontouredpart.Partsrangingfrompocketwatchcomponentstolargediametermarinepropellershaftscanbeturnedonalathe.Apartfromturning,severalotheroperationscanalsobeperformedonalathe.Boringandinternalturning.Boringandinternalturningareperformedontheinternalsurfacesbyaboringbarorsuitableinternalcuttingtools.Iftheinitialworkpieceissolidsadrillingoperationmustbeperformedfirst.Thedrillingtoolisheldinthetailstock,andthelatteristhenfedagainsttheworkpiece.Whenboringisdoneinalathe,theworkusuallyisheldinachuckoronafaceplate.Holesmaybeboredstraighttapered,ortoirregularcontours.Boringisessentiallyinternalturningwhilefeedingthetoolparalleltotherotationaxisoftheworkpiece.Facing.Facingistheproducingofaflatsurfaceastheresultofatool'sbeingfedacrosstheendoftherotatingworkpiece.Unlesstheworkisheldonamandrel,ifbothendsoftheworkaretobefaced,itmustbeturnedaroundafterthefirstendiscompletedandthenthefeeingoperationrepeated.[2]Thecuttingspeedshouldbedeterminedfromthelargestdiameterofthesurfacetobefaced.Facingmaybedoneeitherfromtheoutsideinwardorfromthecenteroutward.Ineithercase，thepointofthetoolmustbesetexactlyattheheightofthecenterofrotation.Becausethecuttingforcetendstopushthetoolawayfromthework,itisusuallydesirabletoclampthecarriagetothelathebedduringeachfacingcuttopreventitfrommovingslightlyandthusproducingasurfacethatisnotflat.Inthefacingofcastingorothermaterialsthathaveahardsurface,thedepthofthefirstcutshouldbesufficienttopenetratethehardmaterialtoavoidexcessivetoolwear.Parting.Partingistheoperationbywhichonesectionofaworkpieceisseveredfromtheremainderbymeansofacutofftool.Becausecuttingtoolsarequitethinandmusthaveconsiderableoverhang,thisprocessislessaccurateandmoredifficult.Thetoolshouldbesetexactlyattheheightoftheaxisofrotation,bekeptsharp,haveproperclearanceangles,andbefedintotheworkpieceataproperanduniformfeedrate.Threading.Threadingcanbeconsideredasturningsincethepathtobetravelledbythecuttingtoolishelical.However,therearesomemajordifferencesbetweenturningandthreading.Whileinturning,theinterestisingeneratingasmoothcylindricalsurface，inthreadingtheinterestisincuttingahelicalthreadofagivenformanddepthwhichcanbecalculatedfromtheformulae.Therearetwobasicrequirementsforthreadcutting.Anaccuratelyshapedandproperlymountedtoolisneededbecausethreadcuttingisaform-cuttingoperation.Theresultingthreadprofileisdeterminedbytheshapeofthetoolanditspositionrelativetotheworkpiece.Thesecondrequirementisthatthetoolmustmovelongitudinallyinaspecificrelationshiptotherotationoftheworkpiece,becausethisdeterminestheleadofthethread.Thisrequirementismetthroughtheuseoftheleadscrewandthesplitunit,whichprovidepositivemotionofthecarriagerelativetotherotationofthespindle.Manytypesoflathesareusedforproductionturning.Accordingtopurposesandconstruction,lathe-typemachinetoolscanbeclassifiedasfollows:1.Enginelathes;2.Verticallathes;3.Turretlathes;4.Single-ormultiple-spindleautomaticorsemi-automaticlathes;5.Contouringlathes;6.Universallathes;7.Special-purposelathessuchascrankshaftlathes,camshaftlathes,carwheellathesandbacking-offlathes,etc.Theenginelatheisthemostrepresentativememberofthelathefamilyandisthemostwidelyused,sothereisadescriptionofeachofthemainelementsofanenginelathe,whichisshowninFigs’.Lathebedisthefoundationoftheenginelathe,whichaheavy,ruggedcastingismadetosupporttheworkingpartsofthelathe.Thesizeandmassofthebedgivetherigiditynecessaryforaccurateengineeringtolerancesrequiredinmanufacturing.Ontopofthebedaremachinedsidewaysthatguideandalignthecarriageandtailstock,astheymovefromoneendofthelathetotheother.Headstockisclampedatopthebedattheleft-handendofthelatheandcontainsthemotorthatdrivesthespindlewhoseaxisisparalleltotheguidewaysthroughaseriesofgearshousedwithinthegearbox.Thefunctionofgearboxistogenerateanumberofdifferentspindlespeeds.Aspindlegearismountedontherearofthespindletotransmitpowerthroughthechangegearstothefeedingboxthatdistributesthepowertotheleadscrewforthreadingortothefeedrodforturning.Thespindlehasathroughholeextendinglengthwisethroughwhichbarstockscanbefedifcontinuousproductionisused.Theholecanholdaplainlathecenterbyitstaperedinnersurfaceandmountachuck,afaceplateorcolletbyitsthreadedoutersurface.CarriageassemblyisactuallyanH-shaped