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ORIGINALARTICLEPerformanceimprovementofhardturningwithsolidlubricantsSinghDilbag&P.V.RaoReceived:10January2007/Accepted:30April2007/Publishedonline:7July2007#Springer-VerlagLondonLimited2007AbstractProductqualityisoneofthemostimportantcriteriafortheassessmentofhardturningprocess.However,inviewofthehightemperaturesdevelopedinhardturningprocess,thesurfacequalitydeterioratesduetothetoolwear.Becauseofthestrictenvironmentalrestric-tionsontheuseofcuttingfluids,newcuttingtechniquesarerequiredtobeinvestigatedtoreducethetoolwear.Inthepresentwork,theuseofsolidlubricantsduringhardturninghasbeenexploredwhilemachiningbearingsteelwithmixedceramicinsertsatdifferentcuttingconditionsandtoolgeometry.Resultsshowconsiderableimprovementinthesurfacefinishwiththeuseofsolidlubricants.Duetothepresenceofsolidlubricants,thereisadecreaseofsurfaceroughnessvaluesfrom8to15%ascomparedtodryhardturning.KeywordsHardturning.Effectiverakeangle.Noseradius.Solidlubricants1IntroductionHardenedsteelsarewidelyusedinautomobile,bearing,toolanddieindustries.Thetraditionalmethodofmachin-inghardenedmaterialsincludesroughturning,heattreatment,andthengrindingprocess.However,hardturningeliminatessomeoftheunnecessarystepsinvolvedinthemachiningofhardmaterialsandhenceresultsintheincreaseofproductivityrate.Thevariousadvantagesofhardtuningarehigherproductivity,reducedsetuptimes,surfacefinishclosertogrindingandabilitytomachinethecomplexparts.Nowadays,hardturningisbeingemployedinindustriesasasubstituteforgrindingprocess.ThishasbecomepossibleduetothedevelopmentofnewcuttingtoolmaterialssuchasCBNandmixedceramics1.Variousresearchershavestudiedthemechanismofchipformation,residualstresses,surfacefinish,andtoolwearinthehardturningprocess.Lotofattentionisbeinggiventothequalityofthesurfacefinishproducedinthehardturningprocess.Therearealargenumberofparameterswhichaffectthecuttingforcesandsurfaceroughness.Theseincludecuttingtoolvariables,workpiecematerialvariables,andcuttingconditionsandthetypeoflubrication.Toolvariablesincludetoolmaterial,noseradius,rakeangle,cuttingedgegeometry,toolvibration,tooloverhang,toolpointangle.Workpiecevariablesincludematerial,hardnessandothermechanicalproperties.Cuttingcondi-tionsincludespeed,feed,anddepthofcut.Typeoflubricationincludesdryhardturning,hardturningwithfloodedorwithminimumquantityoflubricantandsolidlubrication.Lowsurfaceroughness,minimalmicrostruc-turalalterationsandhighdimensionalaccuracycanonlybeattainedwithnewcuttingtools.So,theproperselectionofallthesevariablesfacilitatestheobjectivestobeachievedinhardturning.Duetothestrictregulationsandtheirenforcementregardingtheuseofthecuttingfluidsinindustries,theresearchershavestartedexploringthealterativemethodol-ogies.Oneofthesolutionsistousethesolidlubricantsinordertoreducethetoolwearandtoimprovetheoverallperformanceofthemachiningprocess.Inthepresentstudy,theeffectoftoolgeometryandthecuttingconditionsonthesurfacefinishbyusingsolidlubricantsgraphiteandIntJAdvManufTechnol(2008)38:529535DOI10.1007/s00170-007-1079-8S.Dilbag:P.V.Rao(*)MechanicalEngineeringDepartment,IndianInstituteofTechnology,Delhi,NewDelhi110016,Indiae-mail:pvraomech.iitd.ernet.inmolybdenumdisulphideinthehardturningofthebearingsteelwiththemixedceramictoolswasstudiedandthecomparisonhasbeenmadewithdryhardturning.2LiteraturereviewHardturningprocessisassociatedwiththehightoolwearduetothehightemperaturedeveloped.Cuttingfluidsaregenerallyusedinmachiningprocessestoreducefrictionandwear,thusimprovingthetoollifeandsurfacefinish.Thesearealsousedtoreducetheforcesandenergyconsumption,tocool/lubricatethecuttingzone,washawaythechips,andtoprotectthemachinedsurfacesfromenvironmentalcorrosion.Tonshoffetal.2investigatedthetribologicalaspectsofhardturningwithceramictools.Accordingtothisstudy,coolingeffectreducesthethermalloadofthecuttingedge,andthusincreasestoollifeascomparedtodrycutting.Inaddition,surfacefinishoftheworkpiecescanbeimprovedeitherbychemicalinteractionbetweentheworkpiecesurfaceandextreme-pressureadditivesofthecoolantorbymineraloilapplication.But,rehardenedlayerswerefoundbothindryandwetcutting.vilaandAbrao3investigatedtheeffectofcuttingfluidsonthemachiningofhardenedAISI4340steel.Inthiswork,theperformanceofthreetypesofcuttingfluids(twoemulsionsandonesyntheticfluid)hasbeencomparedtodrymachiningusingmixedaluminainserts.Resultsshowthattheapplicationofacuttingfluidbasedonanemulsionwithoutmineraloilresultsinlongertoollifecomparedtodrycuttingandtheuseofcuttingfluidisresponsibleforreducingthescatterinthesurfacefinishvaluesathighcuttingspeeds.Dinizetal.4studiedtheinfluenceofrefrigeration/lubricationconditiononSAE52100hardenedsteelturningatseveralcuttingspeeds.Dryandminimumvolumeofoil(MVO)showedthesimilarvaluesofflankwear,whichisalwayssmallerthanthevaluesforwetmachining.AlsowetmachiningdidnotshowbettervaluesofsurfaceroughnesscomparedtoMVOanddrymachin-ing.Varadarajanetal.5investigatedthehardturningwithminimalfluidapplication(HTMF),anditsapplicationwithdryandwetturning.Resultsshowthatoverallperformanceduringminimumcuttingfluidapplicationisfoundtobesuperiortothatduringdryturningandconventionalwetturningonthebasisofcuttingforces,toollife,surfacefinish,cuttingratio,cuttingtemperatureandtool-chipcontactlength.However,drymachiningisanewtrendbecauseofincreasedconcernaboutenvironmentalissuesandstrictregulationsregardingtheuseofcuttingfluids.Drycuttingisbeneficialbecauseofpollutionfreeenvironment.Alsothemachiningcostisreducedduetotheeliminationofthecuttingfluids.KlockeandEisenbltter6havereportedondrycutting.Accordingtothisstudy,cuttingfluidshelptoachieveaspecifiedresultintermsoftoollife,surfacefinishanddimensionalaccuracy,andfacilitatechipbreakingandtransport.However,thesealsoimposeproblemsrelatedtowastedisposalandenvironmentalproblems.So,withdrycutting,alltheproblemsrelatedwithwetmachiningcanbeeliminated.SreejithandNgoi7havereportedthedrymachiningasthemachiningofthefuture.Thispaperconcludesthatthedrymachiningcaneliminatecuttingfluidsandthisispossibleduetotheadvancementofthecuttingtoolmaterials.DinizandMicaroni8havestudiedthecuttingconditionsforfinishturningprocessindrycutting.Accordingtothisstudy,drymachiningrequireslesspowerandproducessmoothersurfacethanwetturning.Itisclearfromtheabove-mentionedliteraturethatdryturningimprovesthesurfacefinish,butthetoollifeandwearproblemsareassociatedwithit.Andhardturningwithcuttingfluids(whetherfloodedorminimumquantity)isnotenvironmentalfriendlyandtheiruseisbeingrestricted.So,analternativemethodofincreasingthelifeofthecuttingtoolisessentialinhardturning.Thiscanbedonebyprovidingthenegativechamferangleonthecuttingtoolinsertsandbyreducingthefrictionbetweenthecuttingtoolandtheworkpiece.Recentlytheeffectsofthesolidlubricantsonthemachiningprocesseshavebeeninvestigated.Thecurrentliquidlubricantsappeartobeineffectiveforapplicationsinvolvinghightemperatures9.Lubricantsandadditivesofnewtypesareurgentlyneeded.Solidlubricantsaretheonlyoptionavailableforcontrollingwearandfrictioninalltypesoftribosystemsinvolvingseveretribologicalconditions(e.g.,hightemperature,corrosivemedia,vacuumenvironment,highloadandspeed).Strongadhesionisessentialforlongserviceofsolidlubricantfilms.Ion-beamprocessesarecapableofimpartingstrongadhesionbetweensolidlubricantfilmsandceramicsub-strates.Ion-beammixingofceramicswithconventionalsolidlubricants,suchasMoS2,isfeasibleandappearspromisingfordemandingaerospaceapplications.Auniquesolidlubricant,boricacid,whichformsnaturallyonthesurfacesofceramicscontainingboricoxideandboron,hasbeenrecentlybeendiscovered.Ithasbeenestablishedthatthislubricantcanimpartremarkablylowfrictioncoeffi-cientstoslidingceramicinterfacesinhumidenvironments,whereMoS2isknowntobeineffective.Erdemiretal.10havereportedthesolid/liquidlubricationofceramicsatelevatedtemperatures.Accordingtothisstudy,thesimul-taneoususeofliquidandsolid-filmlubricantsatslidinginterfacesofceramic/ceramicandgraycastiron/ceramicpairscansignificantlyreducefrictionandwear.Thesolidlubricantwasthesilverfilmandtheliquidwaspolyolester-basesyntheticoil.530IntJAdvManufTechnol(2008)38:529535Someresearchershavereportedtheuseofsolidlubricantsinthemachiningprocess.ShajiandRadhakrishnan11haveinvestigatedtheeffectofsolidlubricant(graphite)onthesurfacegrindingprocess.Resultsshowtheimprove-mentofsurfacefinishincaseofhardermaterialswiththeapplicationofsolidlubricant.ShajiandRadhakrishnan12havealsoreportedtheapplicationofsolidlubricantsingrindingasanalternativefortheconventionalcoolants.Thesolidlubricantsusedinthisinvestigationweregraphite,calciumfluoride,bariumfluorideandmolybdenumtriox-ide.Improvedprocessresultsrelatedtofrictionhavebeenreportedinthisstudy.Nakamuraetal.13havestudiedthelubricationbehaviorofsolidlubricantsintheupsettingprocess.Fourkindsofsolidlubricantsweretestedinordertoexaminethefrictionalcharacteristicsandtheyieldshearstressbythefrictiontestingapparatus.ThesolidlubricantsusedwerePTFE,UHMWPE,MoS2andgraphite.ItwasconfirmedintheFEMsimulationandalsobytheexperimentthatsolidlubricantscouldlubricatesuccessfullywithmetaltometalcontact.GopalandRao14haveinvestigatedtheuseofthesolidlubricantinthegrindingoftheSiChasbeeninvestigated.Ithasbeenestablishedbytheseauthorsthatthesurfacefinishimproveswiththegraphiteassistedmachining.ReddyandRao15havereportedtheperformanceoftheendmillingprocessbytheuseofthesolidlubricantsgraphiteandMoS2.Recently,Jianxinetal.16havereportedthetribologicalbehaviorsofhot-pressedAl2O3/TiCceramiccompositeswiththeadditionsofCaF2solidlubricants.ThesestudieshaveconfirmedthatfrictioncoefficientofAl2O3/TiC/CaF2ceramiccompositesde-creasedwhenslidingagainstcementedcarbideandhard-enedsteelwithanincreaseinCaF2content.ThereasonwasthattheCaF2releasedandsmearedonthewearsurfaceandactedasasolidlubricantbetweentheslidingcouple.AlsothewearrateofthesecompositedecreasedwiththeadditionofCaF2.Theabove-mentionedstudiesindicatethatthesurfacefinishcanbeimprovedbyreducingthetoolwear.Drymachiningandmachiningwiththeuseofcuttingfluids(floodedorminimumquantity)havenotrespondedprop-erlyfortherequirementofimprovedtoollifeandsurfacefinish.Theuseofthesolidlubricantsinmachiningmaybetheviablealternativeofcuttingfluidsashasbeenreportedinsomeoftheabovementionedstudies.Soanattempthasbeenmadeinthisworktoinvestigatetheeffectoftoolgeometryandthecuttingconditionsonthesurfacefinishbyusingsolidlubricantsgraphiteandmolybdenumdisulphideinthehardturningofthebearingsteelwiththemixedceramictoolsandthecomparisonhasbeenmadebetweendryhardturningandthesolidlubricantsassistedhardturning.3ExperimentationTheperformanceofhardturningismeasuredintermsofcuttingforces,surfacefinish,andtoolwear.Therearealargenumberofvariablesaffectingtheperformance.Themainimportantparametersarethecuttingconditions,toolgeometryandthetypeoflubricant.Thatiswhy,thefourparametersnamelycuttingspeed,feed,effectiverakeangleandthenoseradiusofthecuttingtoolwereselectedfortheexperimentationinthisstudy.Fivelevelsofeachfactorwereselectedinordertoseetheeffectofeachparameter.Designofexperimentplaysaveryimportantroleinperformingtheexperimentswiththeavailableresources.Accordingtothedesignofexperiments,acentralcompositedesignwasselectedforexperimentationtoreducethenumberofexperiments.Thecuttingforcesandsurfacefinishwereselectedastheresponsevariables.Thecuttingspeed,feed,effectiverakeangle,andthenoseradiusaretheindependentvariablesinthisstudy.ThevariousprocessvariablesandtheirlevelsareshownintheTable1asunder:Accordingtocentralcompositedesign,atotalof31experimentswerecarriedoutwithandwithoutthesolidlubricantsasperthedesignmatrixshownintheTable2.Alltheexperimentswerecarriedoutataconstantdepthofcutof0.2mm.AhighprecisionNH-22HMTlathewasusedforexperimentation.Ithashighdegreeofaccuracyandrigidity,whicharerequiredforthehardturningprocess.Inthisinvestigation,theworkpiecematerialwastheAISI52100steelofdiameter70mm.Theworkpiecematerialwasheattreated(through-hardened)toget5802HRC.Thismaterialisbeingusedforthemanufacturingoftheballandrollerbearingsandautomotivecomponents.ThechemicalcompositionofthematerialisshownintheTable3.Mixedceramicinsertsofdifferentgeometrywereused.ISOdesignationofceramicinsertsisSNGNwithdifferentnoseradiiandchamferangles.ISOfesignationofthetoolholderisCSBNR2525M12.Theapproachangleofthistoolholderwas75.ThesurfaceroughnesswasmeasuredwithaTalysurf-6at0.8mmcut-offvalue.Anaverageofthreemeasurementswasusedasaresponsevalue.ThetoolwearwascheckedwithaMitutoyoopticalTable1ProcessvariablesandtheirlevelsFactorsLevel1Level2Level3Level4Level5v(m/min)5075100125150f(mm/rev)0.040.00()1621263136r(mm)1.62.0IntJAdvManufTechnol(2008)38:529535531microscope(1mresolution)at30magnificationtomeasurethewearafterexperimentation.Foreachexperi-mentalset,newcuttinginsertshavebeenused.Thesolidlubricantsselectedforthisstudyweregraphiteandmolybdenumdisulphide.Thefinepowderof2maverageparticlesizehasbeenused.Anexperimentalsetupforsupplyingthesolidlubricantontothecuttingzonehasbeendesignedanddevelopedforthisstudy.Theapparatushasbeensodesignedthatitcansupplythesolidlubricantfrom0.5gm/minto15gm/min.Aprovisionhasbeenprovidedontheapparatusforitsproperpositioningtosupplytheexactquantityofsolidlubricantsontothecuttingzone.Investigationwascarriedouttodeterminetheoptimumflowrateofsolidlubricantpowderusingthedesignedapparatus.Atthisstageitisalsorequiredtoseetheeffectofincreasingtheflowrateonthecuttingforceforthemachiningofhardenedbearingsteels.Figure1showsthevariationofcuttingforcewithflowrateatacuttingspeedof100m/min,feedof0.12mm/rev,26effectiverakeangleand1.2mmnoseradiusingraphiteandmolybdenumdisulphideassistedmachining,respectively.Ithasbeenobservedfromgraphiteassistedmachiningthatthecuttingforcedecreaseastheflowrateincreasesfrom1gm/minto2gm/min.Afterthatthereisnosubstantialreductionofthecuttingforceeveniftheflowratehasbeenincreasedfrom2gm/minto10gm/min.Thesimilartrendhasbeenobservedfortheothercuttingconditions.Thesameisthecaseformolybdenumdisulphideassistedmachiningalso.Itcanbeconcludedthatflowrateof2gm/minissufficienttoprovidetherequiredlubrication.Hence,inthepresentinvestigation,flowrateofgraphiteandmolybdenumdisulphidepowdershasbeenkeptat2gm/minduringthemachiningofhardenedbearingsteel(Fig.1).4ResultsanddiscussionDuringhardturning,muchheatisgeneratedattheprimarydeformationzone,secondarydeformationzoneandmaxi-mumtemperatureisdevelopedatthetool/chipinterfacewhichmayresultintheearlycuttingtoolfailureleadingtopoorqualityofthesurfaceproduced.So,the