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ELSEVIERcomparisonBetweenConstantForceandConstantRateofFeedinMaterialographicCut-OffMachines:SurfaceQualityinRelationtoCuttingSpeed,Force,andRateofFeedK.GeelsandM.RiickertStruersAIS,Copenhagen,DenmarkWiththeprecisioncuttingofmaterialographicsamples,itisoftheutmostimportancethatthecutsurfaceexhibitsaslittledeformationaspossible.Anumberoffactorsinfluencethequalityofthecutsurface:typeofcut-offwheel,rateoffeed(mm/s),force(N),andspecificpressureinthecut(N/mm*).Inthisstudy,cuttingtestswerecarriedoutandtwocuttingprinciples,constantforceandconstantrateoffeed,werecompared.Theresultantcutsur-faceswereexaminedusingbothlightopticalmicroscopyandatomicforcemicroscopy,withthecorrespondingsurfaceroughnessmeasuredonaprofilemeter.Thisstudyshowsthatwhencuttingworkpiecesofvaryingshape,themostuniformsurfaceisobtainedbyusingaconstantrateoffeed.Also,aconstantrateoffeed,combinedwithahighcuttingspeed,willproducesurfaceswiththeleastandmostuniformdeformation.0ElsmierScienceInc.,2996INTRODUCTIONDESCRIPTIONOFPRINCIPLESAbrasivecutting,usingeitherabradablecut-offwheels(mostlymadefromphenolicresin(Bakelite)withA1203asthecuttingmaterial)ordiamondwheels,isanimpor-tantprocessinthepreparationofmateria-lographicsamples.Thesurfaceobtainedfromcuttingisthebasisforfurtherprepa-rationandshould,therefore,exhibitmini-maldeformationwithasregularapatternaspossible.Withprecisioncutting,twomainprinciplesareused:constantforceandconstantrateoffeed.CONSTANTFORCE(FIG.1)Inthisarticle,theresultsofexperimentsperformedtoshowthedifferencesinthecutsurface,usingeitherconstantforceorconstantrateoffeed,arediscussed,togetherwithtestsconductedtoshowtheeffectontheprocessofvaryingtheparameters.Withmostmachinesbuilttouseaconstantforce,theworkpieceisguidedintothero-tatingcut-offwheel.Theforce,G(N),isap-pliedtothearmholdingtheworkpiece,andthemovementofthisarmconstitutestherateoffeed,F(mm/s).Whencuttingintotheworkpiece,thereisacontactarea,A(mm2),betweentheworkpieceandthecut-offwheel.Knowingtheforce,G,andthecontactarea,A,thespecificpressure,p,inthecutcanbecalculatedinN/mm2.CONSTANTRATEOFFEED(FIG.1)Withmostmachinesdesignedtousecon-stantrateoffeed,theworkpieceisguidedintotherotatingcut-offwheelinalinearMATERIALSCHARACTERIZATION36:225-233(1996)0ElsevierScienceInc.,1996655AvenueoftheAmericas,NewYork,NY1001022510445803/96/$15.00PIISlO44-5803(96)00054-X226K.GeelsandM.RiickerfConstantRateolFwdFIG.1movement.Thismovementistherateoffeed,F(mm/s),actedonbyaforce,G(N),andissettoafixedvaluepriortothestartoftheprocess.Thespecificpressure,p,inthecutiscalculatedinthesamemannerasforconstantforce.Comparisonofthetwoprinciples,asjustdescribed,showsthatwhencuttingacylin-dricalworkpiece,thecontactarea(A)var-iesfromaminimumvalueatentryandexittoamaximumvalueatthediameter.Thisisdescribedinmoredetaillater.Ifasquareworkpieceiscut,thisvariationwillofcoursenottakeplace.Constantforceindicatedbycurvesillus-tratingG,A,andpforacylindricalwork-piecearepresentedinFig.2.Hereweseetheworkpiecepassingthroughthecut-offwheelatthreedifferentpositions.TheFIG.2force,G,isconstant,whereasthecontactarea,A,variesfromaminimumvalueattheentryandexittoamaximumvalueatthediameteroftheworkpiece.Asaresultofthisvariation,thespecificpressure,p,variesfromhighatthebeginningandtheendtolowatthediameterofthework-piece.Therateoffeed,F,canbeexpressedbythesamecurveasthatforthespecificpressure,p.ConstantrateoffeedindicatedbycurvesillustratingG,A,andpforacylindricalworkpiecearepresentedinFig.3.Toobtainthepresetrateoffeed,theforce,G,willberelativelylowattheentrybecauseofthesmallcontactarea.Theforcewillincreaseaccordingtotheincreaseincontactarea,A,andthespecificpressure,p,willremainconstantduringtheentireprocess.Alsohere,thecurveforthespecificpressure,p,canbeexpressedbythesamecurveasfortherateoffeed,F.Itcanbeconcludedthatwhencuttingwithconstantforce,therateoffeedvariesconsiderably,comparedtocuttingwithaconstantrateoffeed.However,themostimportantpointishoweachofthesecondi-tionsinfluencesthequalityofthecutsurface.EXPERIMENTALPROGRAMToanalyzethedifferencebetweenthetwocuttingprinciples,aT-shapedworkpiecewasfabricated(Fig.4).Cuttingwasiniti-FIG.3ConstantForceVersusConstantFeedRateofFeedalTestWorkpieceFIG.4atedfromthelargecrosssection,thetwosectionshavingtheproportion3:1,andmeasurementsweretakenatthefourmea-suringpoints,Ml’14:constantforce,MT’1and3;andconstantrateoffeed,Ml’2and4.Thesurfaceroughnesswasexaminedatthefourpointsusinglightopticalmicros-copyandmeasuredusinganatomicforcemicroscope.Thefollowingparameterswerekeptconstant:cut-offwheel,357CA(Bakelite-bondedA1203cut-offwheel,125X0.5mm);cut-offwheelrotation,1000rpm;andlubricatingcondition.Anumberofmaterialsweretestedandallshowedthesametendency.Taking0.9%carbonsteelasanexample,withbothworkpieces,thelargesectionsMl’1and227Ml’2,hadnearlythesamesurfacecondi-tionscorrespondingtoMI’4,constantrateoffeed(Figs.5and6).Withthesmallsec-tions,Ml’3andMl’4,therewasaremark-abledifferencebetweenthesurfaces.Usingconstantrateoffeedcausednochangeinroughness,butusingconstantforcepro-ducedasurfaceofconsiderablyhigherrou