Microshotblastingofmachinetoolsforimprovingsurfacefinishandreducingcutting sinmanufacturingD.M.Kennedy*,J.Vahey,D.FacultyofEngineering,DublinInstituteofTechnology,BoltonStreet,Dublin1,Received5January2004;accepted3FebruaryAvailableonline13AprilMicroblastingofcuttingtipsandtoolsisaveryeffectiveandreliablemethodofadvancingthelifeoftoolsundertheactionofturning,milling,drilling,punchingandcutting.Thispaperoutlinesthewaysinwhichmicroblastedtools,bothcoatedanduncoatedhavebenefitedfromshotblastingandresultedingreaterproductivity,lowercuttings,improvedsurfacefinishoftheworkpiecesandlessmachinedowntime.Theprocessofmicroblastingisdiscussedinthepaper.Itseffectivenessdependsonmanyparametersincludingtheshotmediaandsize,themechanicsofimpactandtheapplicationoftheshotviathemicroshotblastingunit.Controloftheprocesstoproviderepeatabilityandreliabilityintheshotblastingunitisdiscussed.Comparisonsbetweentreatedanduntreatedcuttingtoolsaremadeandresultsoftoollifeforthesecuttingtipsoutlined.Theprocesshasshowntobeofmajorbenefittotoollifeimprovement.2004Elsevier.Allrights :Microshotblasting;Surfacefinish;MachineManymoderntechniqueshavebeendevelopedtoenhancethelifeofcomponentsinservice,suchasalloyingadditions,heattreatment,surfaceengineering,surfacecoating,imntationprocesses,lasertreatmentandsurfaceshapedesign.Processessuchasthinfilmtechnology,smaspraying,vacuumtechniquesdepositingarangeofmulti-layeredcoatingshavegreatlyenhancedthelife,useandapplicationsofengineeringcomponentsandmachinetools.Bombardmentwithmillionsofmicroshotranginginsizefrom4to50lmwithacontrolledprocesscanleadtodramaticoperatinglifeimprovementsofcomponents.StandardshotpeeningwasfirstusedinaproductionprocesstoextendthelifeofvalvespringsforBuickandCadillacenginesintheearly1930s[1,2]butpriortothisitwasawellknownprocessusedbyblacksmithsandswordmakersovertimetoimprovethetoughnessofthecuttingedgesoftheirtoolsandweapons.Today,cuttingtipsandtoolscanbegreatlyimprovedbytheprocessofmicroshotblastingtheirsurfacestoinducecompressiveresidualstresses.Theoperatinglifeoftoolssuchasdrills,turningtips,millingtips,punches,knifeedges,slicers,blades,andarangeofotherworkingpartscanalsobenefitfromthisprocess.Standardcomponents,suchassprings,dies,shafts,cams,anddynamiccomponentsinmachinesandenginescanbeenhancedbythisprocess.Thefatiguelifeofcompressorcomponentsforexample,treatedbyshotpeeninghaveincreaseddramaticallyasreportedbyEckersleyandFerrelli[3].Otherfactorssuchasimprovedfatigue,microcrackclosure,reducedcorrosionandanimprovedsurfacefinishcanalsobedesignedintocomponentsasaresultofthisthepeeningprocess.Notonlycanimprovementsbemadetothesurfacefinishofthecuttingtipsandbutalsothesurfacefinishoftheworkpiecesmachinedwiththesetoolshaveimprovedasaresultofthistechnique.Engineeringmaterialssuchastoolssteels,carbides,ceramics,coatedcarbides,throughtopolymersandevenrubbers(elastomers)canbenefit.Thekeyrequirementforthisprocessistodevelopanautomatedmicroblastingprocesstofitinsideasprayboothorstandardshotblastingbooth.Shotmaterial,sizeandmass,operatingpressures,operatingvelocities,kineticenergy,densityandcoveragetimewillneedtobeperfectedandoptimisedforarangeofmaterials.Theprocessisalineofsightmethodbutcanbeappliedtocomplexsurfaceshapessuchasthetipsofdrillbits.MethodofOneoftheprimarywaysthatcomponentsfailinerviceisthroughfatigue.Thisiscloselyassociatedwithcyclicstressesandacceleratedbytensilestresses,microcrackpropagationandstresscorrosioncracking.Cracksreducethecrosssectionofamaterialandeventuallyitwillfailtosupporttheappliedloads.Onesimplemethodofreducingfailurebyfatigueistoarrestthesetensilestressesbyinducingcompressivestressesintoasurface.Thebenefitsobtainedwithshotpeeningareadirectresultoftheresidualcompressivestressesproducedinacomponent.AtypicalshotstrikingasurfaceisshowninFig.1.Anyappliedtensileloadswouldhaveto etheresidualcompressivestressesbeforeacrackcouldinitiateasdescribedbyAlmen[4].Poormachiningofmaterialscanresultinresidualstressesaccruingatthesurface.Roughsurfaceshavedeepernotches,wherecrackscaninitiateduetotensilestressconcentrationsatthesepoints.Manystandardmachiningprocessessuchasgrinding,milling,turning,andcoatingprocessessuchaselectrotinginduceresidualtensilestressesinsurfacesandthiscanleadtoearlyfailureofcomponents.Furthertensileloadinginservicewouldleadtoearlyfailureandthiscanbepreventedbyshotpeeningandmicroblastingofcomponentsurfaces.Microshotblastingwillchangethefollowinginamaterialssurface:(i)tofatigue(ii)tostress(iii)achangeinresidual(iv)modificationofsurfaceItisacoldworkingprocessinvolvingbombardingpowderssuchasceramics,glassandmetalsofmainlysphericalshapesagainstsurfacesandcanbeusedinconjunctionwithotherprocesses.Themainstagesinvolvedinthisdynamicprocessincludeelasticrecoveryofthesubstrateafterimpact,somesticdeformationofthesubstrateiftheimpactpressureexceedstheyieldstress,increasedsticdeformationduetoanincreaseinimpactpressureandfinallysomereboundoftheshotduetoareleaseofelasticenergy.Somecriticaldesigncharacteristicsofthemicroshotpeeningprocessincludetheshotsize,shape,hardness,density,durability,angleofimpact,velocityandintensity.Alloftheseparameterswillinfluencetheresidualcompressivestressesproducedinthesubstrate.ExperimentalToolmaterialssuchasTungstenCarbide,HighSpeedSteelsusedinmillingandturningtoolswere subjectedtothemicropeeningprocessusingdifferentshotmedia(ceramicandglassbead)andshotsize.Testspriortoandfollowingtheblastingprocesswereconductedtoascertainanyimprovementsresultingfromtheprocess.ThemicroshotpeeningunitisshowninPhoto1itincorporatesanairfilter,pressureregulatorandgauge,airflowregulator,pressurisedblastmediacontainerandaventuriblastnozzlefordirectingthestreamofmicroshot.TheunitisPLCcontrolledandasteppermotor,usedtodrivealeadscrew,isusedtomovetheblastnozzleacrossthesampleinordertocontrolmediashotcoverage.Theblastnozzlecanalsoberotatedtoallowshotmediatostrikethesamplesatdifferentangles.Testsundertakenincludesurfacefinishandroughnessmeasurement,machiningtestsonstandardlathesandmills,hardnesstests,cuttingsonturningoperations,toolwearandthedeterminationofsurfacefinishoftheworkpiecesmachined.Figs.2and3showatypicalhighspeedsteel(HSS)tippriortoandfollowingthemicroshotpeeningprocessusingceramicbeadatapressureof5.5bar.ExperimentalTestingoftreatedanduntreatedcuttingtipsandtoolswasconductedonHSSsforturningandmillingaswellascoatedanduncoatedcarbideinserts.Adynamometerwasusedtomeasurecutting sontheturningtool(Lathe).Thecuttingprocessconsistedofadepthofcutof2mmonastandardbrightmildsteelspecimenoveralengthof750mmwhilemillingtestsconsistedofmachininga25_25_150mmpieceofmildsteelusingadepthofcutof1mmwithaslotmillingcutterof18mmdiameter.Surfaceroughnessmeasurementswereconductedonthemachinedcomponentspriortoandaftermachiningtoestablishwhetherthetreatedcuttingtipshadsuperiorperformancetotheuntreatedtips.MicroHardnesstestingwasalsocarriedouttoestablishiftherewasanyincreaseinsurfacehardnessduetothemicroshotpeeningprocess.Theimpactangleoftheshotwassetat90_asthisprovidestheoptimumcompressivelayer[5].Theshotvelocityonimpactwithasurfaceislargelydependentonthenozzlesize,theairpressureandthedistancefromthesubstrate.TheexposuretimewasadequatetogivesufficientcoverageofthesubstrateandthiswasdeterminedbytheAlmenstripsaturationtime,workpieceindentationtimeandvisualappearance.Hardermaterialssuchascarbideswillobviouslyrequirelongerexposuretimeorhardershotmedia.Themicropeeningmediausedwasaceramicbeadofapproximay40lmdiameterprovidinghighimpactstrengthandhardness(NFL06-824,approximay60HRc).MicrohardnessCombinedVickersmicrohardnesstestsgavetheresultsinTable1.forbothtreatedanduntreatedHSScuttingtips.SurfaceroughnessInallsurfaceroughnesstestsconducted,themicroblastedsurfacegaveanimprovedsurfaceroughnessvalue.SurfaceroughnessandprofiletestswerecarriedoutonbothaTalyorHobsonTallysurfinstrumentandanoncontactsurfaceprofileometer.SurfaceroughnessdetailsofatypicaluntreatedHSScuttingtipandatreatedoneareshowninFigs.4and5andTable2showstheresultsofsurfacemeasurementvaluesforothercuttingtipsandtoolsandworkpieces.Fig.6showsanuncoatedcarbidecuttingtipwhichwasnotsubjectedtomicroblasting.Theflankwearwasmeasuredusinganopticalmicroscopeandthevaluerecordedwas150lmafter676sofmachining.Fig.7showsanuncoatedcarbidetipsubjectedtomicroblasting.Theflankwearinthiscaseisonly90lmforthesamemachiningtime.and5andTable2showstheresultsofsurfacemeasurementvaluesforothercuttingtipsandtoolsandworkpieces.Fig.6showsanuncoatedcarbidecuttingtipwhichwasnotsubjectedtomicroblasting.Theflankwearwasmeasuredusinganopticalmicroscopeandthevaluerecordedwas150lmafter676sofmachining.Fig.7showsanuncoatedcarbidetipsubjectedtomicroblasting.Theflankwearinthiscaseisonly90lmforthesamemachiningtime.DynamometerFigs.8and9showthecomparisonforDynamometerresultsforHSSinthetreated(microblasted)anduntreatedstateswithrelevantcomments.Similarprofilesareshownforcoatedanduncoatedturningtipsinboththetreated(microblasted)anduntreatedconditionsinFigs.10–13.Inallcases,themicroblastedtipsprovidedanincreaseincuttingtiplifewithlowercuttingsrecorded.Thisresearchworkhasshownthatmicroshotblastingofcuttingtipsandtoolshasaverypositiveeffectoncomponentsurfacesbyincreasingtoughness,operatinglife,improvinghardnessandsurfacefinish.Fromthetestsconducted,itisobviousthattheprocessaffectstheresidualstressesatornearthesurfaceinabeneficialwaybyinducingcompressivestressesonthesubstratestested.Themicroblastingprocessisverysimpletoapplyandeconomicaltouse.Themechanicalpropertiesofthesubstrateswilldeterminethetypeoftreatment,i.e.shothardness,velocityanddurationofapplicationinordertoobtainumbenefitsfromthisprocess.Insomecases,authorshavereporteda4–10foldimprovementinfatiguelifeinarangeofdynamicmachinepartssubjectedtostandardshotblasting.Furthertestingwillneedtobeconductedatthemicroshotblastingstagetoobtainsimilarbenefits.Otherapplicationsforthemicroblastingprocessarecurrentlybeinginvestigatedandrubberbasedproductsthataresubjectedtofatigueandweararebeingtestedinordertoremovethesurfacevoidsthatactasstressconcentrationsinthesematerials.Impact.Bloomfield,CT:MetalImprovement;FallZimmerliFP.Heattreating,settingandshot-peeningofmechanicalsprings.Metalprocess;June1952.EckersleyJS,FerrelliB.Usingshot-peeningtomultiplythelifecompressorcomponents.In:Theshotpeener,Internation