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TheLatheandItsConstruction

Alatheisamachinetoolusedprimarilyforproducingsurfacesofrevolutionflatedges.Basedontheirpurpose,construction,numberoftoolsthatcansimultaneouslybemounted,anddegreeofautomation,lathesor,moreaccurately,lathe-typemachinetoolscanbeclassifiedasfollows:

（1）Enginelathes

（2）Toolroomlathes

（3）Turretlathes

（4）Verticalturningandboringmills

（5）Automaticlathes

（6）Special-purposelathes

Inspiteofthatdiversityoflathe-typemachinetools,theyallhaveallhavecommonfeatureswithrespecttoconstructionandprincipleofoperation.Thesefeaturescanbestbeillustratedbyconsideringthecommonlyusedrepresentativetype,theenginelathe.Followingisadescriptionofeachofthemainelementsofanenginelathe,whichisshowninFig.11.1.

Lathebed.Thelathebedisthemainframe,involvingahorizontalbeamontwoverticalsupporis.Itisusuallymadeofgreyornodularcastirontodampvibrationsandismadebycasting.Ithasguidewaystoallowthecarriagetoslideeasilylengthwise.Theheightofthelathebedshouldbeappropriatetoenablethetechniciantodohisorherjibeasilyandcomfortably.

Headstock.Theheadstockisfixedatthelefthandsideofthelathebedandincludesthespindlewhoseaxisisparalleltotheguideways（thesildesurfaceofthebed）.Thespindleisdriventhroughthegearbox,whichishousedwithintheheadstock.Thefunctionofthegearboxistoprovideanumberofdifferentspindlespeeds（usually6upto18speeds）.Somemodernlatheshaveheadstockswithinfinitelyvariablespindlespeeds,whichemployfrictional,electrical,orhydraulicdrives.

Thespindleisalwayshollow,I.e,ithasathroughholeextendinglengthwise.Barstockscanbefedthrougththatholeifcontinousproductionisadopted.Also,thatholehasataperedsurfacetoallowmountingaplainlathecenter.Theoutersurfaceofthespindleisthreadedtoallowmountingofachuck,afaceplate,orthelike.

Tailstock.Thetailstockassemblyconsistsbasicallyofthreeparts,itslowerbase,anintermediatepart,andthequill.Thelowerbaseisacastingthatcanslideonthelathebedalongtheguidewayes,andithasaclampingdevicetoenablelockingtheentiretailstockatanydesiredlocation,dependinguponthelengthoftheworkpiece.Theintermediateparteisacastingthatcanbemovedtransverselytoenablealignmentoftheaxisofthethetailstockwiththatoftheheadstock.Thethirdpart,thequill,isahardenedsteeltube,whichcanbemovedlongitudinallyinandoutoftheintermediatepartasrequired.Thisisachievedthroughtheuseofahandwheelandascrew,aroundwhichanutfixedtothequilliscanbelockedatanypointalongitstravelpathbymeansofaclampingdevice.

Thecarriage.Themainfunctionofthecarriageismountingofthecuttingtoolsandgeneratinglongitudinaland/orcrossfeeds.ItisactuallyanH-shapedblockthatslidesonthelathebedbetweentheheadstockandtailstockwhilebeingguidedbytheV-shapedguidewaysofthebed.Thecarriagecanbemovedeithermanuallyormechanicallybymeansoftheapronandeitherthefeedrodortheleadscrew.

Whencuttingscrewthreads,powerisprovidedtothegearboxoftheapronbytheleadscrew.Inallotherturningoperations,itisthefeedrodthatdrivesthecarriage.Theleadscrewgoesthroughapairohalfnuts,whicharefixedtotherearoftheapron.Whenactuatingacertainlever,thehalfnutsareclampedtogetherandengagewiththerotatingleadscrewasasinglenut,whichisfed,togetherwithcarriage,alongthebed.whentheleverisdisengaged,thehalfnutsarereleasedandthecarriagestops.Ontheotherhand,whenthefeedrodisused,itsuppliespowertotheapronthroughawromgear.Thelatteriskeyedtofeedrodandtravelswiththeapronalongthefeedrod,whichhasakeywayextendingtocoveritswholelength.Amodernlatheusuallyhasaquick-changegearboxlocatedundertheheadstockanddrivenfromthespindlethroughatrainofgears.Itisconnectedtoboththefeedrodandtheleadscrewandenablesselectingavarietyoffeedseasilyandrapidlybysimplyshiftingtheappropriatelevers,thequick-changegearboxisemployedinplainturning,facingandthreadcuttingoperations.Sincethatgearboxislinkedtospindle,thedistancethattheapron（andthecuttingtool）travelsforeachrevolutionofthespindlecanbecontrolledandisreferredtoasthefeed.

LatheCuttingTools

Theshapeandgeometryofthelathetoolsdependuponthepurposeforwhichtheyareemployed.Turningtoolscanbeclassifiedintotowmaingroups,namely,externalcuttingtoolsandinternalcuttingtools,Eachofthesegroupsincludethefollowingtypesoftools:

Turningtools.Turingtoolscanbeeitherfinishingorroughturningtools.Roughturningtoolshavesmallnoseradiiandareusedforobtainingthefinalrequireddimensionswithgoodsurfacefinishbymarkingslightdepthofcut.Roughturningtoolscanberight–handorleft-handtypes,dependinguponthedirectionoffeed.Theycanhavestraight,bent,oroffsetshanks.

Facingtools.Facingtoolsareemployedinfacingoperationsformachiningplanesideorendsurfaces.Therearetoolsformachiningleft-hand-sidesurfacesandtoolsforright-hand-sidesurfaces.Thosesidesurfacesaregeneratedthroughtheuseofthecrossfeed,contrarytoturningoperations,wheretheusuallongitudinalfeedisused.

Cutofftools.Cutofftools,whicharesometimescalledpartingtools,servetoseparatetheworkpieceintopartsand/ormachineexternalannualgrooves.

Thread-cuttingtools.Thread-cuttingtoolshaveeithertriangular,square,ortranpezoidalcuttingedges,dependinguponthecrosssectionofthedesiredthread.Also,theplaneanglesofthesetoolsmustalwaysbeidenticaltothoseofthethreadforms.Thread-cuttingtoolshavestraightshanksforexternalthreadcuttingandareofthebent-shanktypewhencuttinginternalthreads.

Formtools.Formtoolshaveedgesespeciallymanufacturedtotakeacertainform,whichisoppositetothedesiredshapeofthemachinedworkpiece.AnHSStoolsisusuallymadeintheformofasinglepiece,contrarytocementedcarbidesorceramic,whicharemadeintheformoftipes.Thelatterarebrazedormechanicallyfastenedtosteelshanks.Fig.1indicatesanarrangementofthislattertype,whichincludesthecarbidetip,thechipbreaker,thepad,theclampingscrew（withawasherandanut）,andtheshank..Asthenamesuggests,thefunctionofthechipbreakeristobreaklongchipseverynowandthen,thuspreventingtheformationofverylongtwistedribbonsthatmaycauseproblemsduringthemachiningoperations.Thecarbidetips（orceramictips）canhavedifferentshapes,dependinguponthemachiningoperationsforwhichtheyaretobeemployed.Thetipscaneitherbesolidorwithacentralthroughhole,dependingonwhetherbrazingormechanicalclampingisemployedformountingthetipontheshank.

Fig.1

LatheOperations

Inthefollowingsection,wediscussthevariousmachiningoperationsthatcanbeperformedonaconventionalenginelathe.Itmustbeborneinmind,however,thatmoderncomputerizednumericallycontrolledlatheshavemorecapabiblitiesanddootheroperations,suchascontouring,forexample.Followingareconventionallatheoperations.

Cylindricalturning.Cylindricalturningisthethesimplestandthemostcommonofalllatheoperations.Asinglefullturnoftheworkpiecegenerateacirclewhosecenterfallsonthelatheaxis;

thismotionisthenreproducednumeroustimesasaresultoftheaxialfeedmotionofthetool.Theresultingmachiningmarksare,therefore,ahelixhavingaverysmallpitch,whichisequaltothefeed.Consequently,themachinedsurfaceisalwayscylindrical.

Theaxialfeedisprovidedbythecarriageorthecompoundrest,eithermanuallyorautomatically,whereasthedepthsofcutsiscontrolledbythecrossslide.Inroughingcuts,itisrecommendedthatlargedepthsofcuts（upto0.25in.or6mm,dependingupontheworkpiecematerial）andsmallerfeedswouldbeused.Ontheotherhand,veryfinefeeds,smallerdepthofcut（lessthan0.05in.,or0.4mm）,andhighcuttingspeedsarepreferredforfinishingcuts.

Facing.Theresultofafacingoperationisaflatsurfacethatiseitherthewholeendsurfaceoftheworkpieceoranannularintermediatesurfacelikeashoulder.Duringafacingoperation,feedisprovidedbythecrossslide,whereasthedepthofcutiscontrolledbythecarriageorcompoundrest.Facingcanbecarriedouteitherfromtheperipheryinwardorfromthecenteroftheworkpieceoutward.Itisobviousthatthemachiningmarksinbothcasestacktheformofaspiral.Usually,itispreferredtoclampthecarriageduringafacingoperation,sincethecuttingforcetendstopushthetool（and,ofcourse,thewholecarriage）awayfromtheworkpiece.Inmostfacingoperations,theworkpieceisheldinachuckoronafaceplate.

Groovecutting.Incut-offandgroove-cuttingoperations,onlycrossfeedofthetoolisemployed.Thecut-offandgroovingtools,whichwerepreviouslydiscussed,areemployed.

Boringandinternalturning.Boringandinternalareperformedontheinternalsurfacesbyaboringbarorsuitableinternalworkpieceissolid,adrillingoperationmustbeperformedfirst.Thedrillingtoolisheldinthetailstock,andlatteristhenfedagainsttheworkpiece.

Taperturning.Taperturningisachievedbydrivingthetoolinadirectionthatisnotparalledtothelatheaxisbutinclinedtoitwithananglethatisequaltothedesire