Grinding-SomeobservationsFortheproductionoffinishedcomponentsofdesiredshape,sizeandaccuracy,machiningisthecommonlyusedmanufacturingprocess.Machiningprocessinvolvestheusageofsingleormultiplepointcuttingtoolstoremovetheunwantedmaterialsformthestockintheformofchips(Komandurai,1993).Amongthevariousmetalcuttingprocessavailable,Grindingisoneoftheimportantmetalcuttingprocessusedextensivelyinthefinishingoperationofdiscretecomponents.Itisaversatileandalsofinishmachiningprocessintheproductionofcomponentsrequiringclosedimensionaltolerances,geometricalaccuraciesandrequiredsurfacefinish(Rajmohanetal.,1994).Mostoftheproductionprocessesareincompletewithoutgrindingprocess.AccordingtoSubramanian(1999),itisamajormanufacturingprocess,whichaccountsforabout25%ofthetotalexpenditureonmachiningoperationsinindustrializedcountries.Almostalltheengineeringcomponentsareprocessedingrindingmachiningmachinesatsomestagesofitsproduction.Grindingisaslowprocessintermsofunitremovalofthestock.Hence,othermethodsareusedfirsttobingtheworkclosetoitsrequireddimensionsandthenitisgroundtoachievethedesiredfinish.Insomeapplications,grindingisalsoemployedforhighermetalremovalrate.Insuchheavydutygrindingoperationsmoreabrasiveisconsumed.Inthesecases,themainobjectiveistoremovemoreamountofmaterialthattooasquicklyandeffectivelyaspossible.Thus,thegrindingprocesscanbeappliedsuccessfullytoalmostanycomponentrequiringprecisionorhardmachininganditisalsooneofthewidelyusedmethodsofremovingmaterialfromtheworkpieceafterhardening.Inordertodecreasethecostandincreasetheproductionrate,thegrindingmachinemustbesettooperatewithintheshortestpossiblegrindingcycletime.Hence,itisoftenimportanttosetthecorrectgrindingmachineparameterssoastoproducepartsofrequiredquality.Theselectionofgrindingparametersifitisdoneonhitandmisstechniquenotonlywastestimebutalsoleadstoaninefficientprocess.Toovercomethisdifficulty,Guptaetal.(2001)intheirworkoptimizedthegrindingprocessparametersusingtheenumerationmethod.Theparametersshouldbeselectedsoastoresultinanoptimalsolution.Selectionofgrindingprocessparametersismadeeasyemployingthe“Expertsystem”.ShajiandRadhakrishnan(2002)analyzedtheprocessparameterssuchasspeed,feed,unfeedandmodeofdressingasinfluentialfactorsontheforcecomponentsandsurfacefinishdevelopedbasedonTaguchi’sExperimentaldesignmethods.FengguoCaoetal.(2003)developedtheconceptofintegratingneuralnetwork,greyrelationalanalysisandgeneticalgorithmfortheoptimizationofprocessparametersinincreased.ExplosiveElectricalDischargeGrindingProcessliesintheproperselectionandintroductionofsuitabledesignofexperimentattheearlieststageoftheprocessandproductdevelopmentcyclessoasobtainqualityandproductivityimprovement.Amongtheexistingtypesofgrindingprocesses,cylindricalgrindingprocessistheone,whichisverywidelyusedinthefinishmachiningofnumberofautomobilecomponentswithsurfacesofrevolution.Incylindricalgrindingprocess,thefrictionalresistanceencounteredbetweentheworkmaterialandthetool,chiptoolinterfaceandtheresistancetodeformationduringshearingofthechipscontributestoriseintemperatureatthecontactzone(Triggeretal.1951).Thetemperaturegeneratedisnotonlyveryhighbutthetemperaturegradientsarealsosevere.Suchtemperaturesofsufficientmagnitudecancauseadversechangesinworkpiecemetallurgicalstructure,lossindimensionalaccuracyandacceleratedwear[or]dullingofthetool(DesRuisseauxandZerkle,1970;TakashiUedaetal.,1985).Inadditiontocausingsurfacedamage,grindingheatmaycausethermalexpansion/distortioninthecomponentgroundandthusadverselyaffecttheattainableaccuracy.MasudaandShiozaki(1974)demonstratedhowgrindingheatinplungesurfacegrindingresultsinout-of-flatnessofthefinishedpart.Betterflatnesswasobtainedwithsmallerdepthsofcutandhigherworkpiecevelocities.Bothofthemcauselessergrindingheatandwithincreasedcoolantflowratethecoolingoftheworkpieceisenhancedandthethermaldistortionisminimized.Chandrsekaretal.(1996)studiedthethermalaspectsofsurfacefinishingprocess.Ingrinding,thelocalizedabrasiveworkpiececontactpressuresandhighslidingspeedproducehightemperaturesattheinterfacebetweenanabrasiveparticleandtheworksurface,aswellasintheworksub-surfacesduetofrictionalheating.Hightemperaturesaretheimportantsourceofdamageonthemachinedsurface.First,thetransienttemperatureandthetemperaturegradientaretheprinciplesourcesforresidualstressesandmicrocrackingongroundsurfaces.Secondly,thelocalizedtemperaturescancausewarpingofthecomponentsbeingmachined,especially,whenitisofsmallsizeandhasarelativelylargesurfaceareatovolumeratio.Thisisaseriousprobleminthefinishingofsmallelectronicdevicessuchasrecordingheads.Thirdly,thishightemperaturecanalsoleadtophasetransformationsinthematerialsbeingmachined.ThenatureofgrindingdamagewassurveyedbyTarasov(1950),whoidentifiedthreemainkindsofgrindingdamage,namelycracking,rehardeningburnandtemperingburn.Duringgrindingofhardenedsteel,ifthesurfacetemperatureoftheworkpieceissufficientlyhigh,thesurfacereaustenizesandisrapidlyquenched.Consequently,thereisaformationofbrittle,untemperedmartensiteatthesurface.Thistypeofthermaldamageisalsocommonlyreferredtoasworkpieceburnandishighlyundesirable(Tarasov,1950;Torrance,1978).Amartensitictypeofphasetransformationalsooccursduringthegrindingoftoughenedzirconia.Here,thetransientmechanicalandthermalstressesgeneratedduringgrindingdrivesthetransformation.Theseformsofthermaldamagechangethemechanical,magneticandelectricalpropertiesoftheworkmaterials.Thelocaltemperaturesplayanimportantroleinthedegradationoftheabrasiveparticlesandthebondingpropertyofthematerial.Theheatgeneratedduringgrindingischaracterizedby,i)Instantaneousconcentratedsource,ii)Highrateofliberation,andiii)Verysmallcontactperiod.Heatassociatedwiththeenergyexpendedbygrindingistransportedawayfromthegrindingzonebytheworkpiece,grindingfluid,grindingchipsandgrindingwheel.Ofparticularinterestisthefractionofthetotalgrindingenergytransportedtotheworkpieceatthegrindingzone,whichcausestheriseinworkpiecetemperatureandpossiblethermaldamage.ForregulargrindingwithconventionalAluminumoxidewheels,theenergypartitiontotheworkpiecetypicallyrangesfrom60-80%dependingontheactualgrindingsituation(MalkinandAnderson,1974;Roweetal.,1995and1997).Onlyafewisolatedattemptshavebeenreportedsofaronexperimentalanalysisofthetemperaturedevelopedatthewheelworkcontactzone,energypartitionratio,graincontacttimeandthermaldamages.Atthispoint,itappearsthatpracticaloptimizationstrategyandreliablemathematicalmodelsarestillrequiredtoanalyzethethermaldamageingrinding.FieldandKahles(1971)investigatedthedissipationofheatingrindingandtheresultinginfluenceonthesurfaceintegrityoftheworkpiece.GuoandMalkin(1992)describedthatdependingonthegrindingconditiontheheatfluxtakespartmainlyviatheworkpieceandleadstoalargethermalloadinginthesurface.Thisthermalloadissuperimposedbymechanicalloadcausingahightemperatureinthesurface.Thisthermo-mechanicalloadcausessomeundesiredalterationsinthesurfacelayer,likecracks,temperedzoneorwhiteetchingareas(WEA).ShawandVyas(1994)gaveanimpressivetheoreticaldescriptionofmetallurgicalchangesingroundsurfaces.Underabusivegrindingconditions,theformationofheat-affectedzonewasobserved.DesRuisseauxandZerkle(1970)analyzedthattheheat-affectedzoneunderabusivegrindingconditionsdamagesthegroundsurfaceofthehardenedsteelveryfrequently.Athermallydamagedcomponentmaythereforeincurasignificantcosttothemanufacturerinfailingqualitystandard.Thus,thethermalphenomenaplayakeyroleintheeconomicsandmechanicsofabrasivemachiningprocesses.Anestimationoftheamountofenergygenerated,worksurfacetemperatureandanunderstandingoftheirroleinmetallurgicalchangesongroundsurfacesarestillchallengingtotheproductionengineers(SoyesandMaris1978).MalkinandFedoseev(1991)analyzedthemethodtopredicttheundesiredalterationstoavoidthermaldamagesingrindinggardenedsteel.Inanycase,thegeneratedheatquantitiesingrindingareconsideredasarestrictingfactor.Theinventionofadvancedgrindingprocesses,whichenabledthesurfacehardeningofsteelparts,wasdescribedforthefirsttimein1994.Insuchoperations,namedgrindhardening,thedissipatedheatingrindingisutilizedtoinducedmartensiticphasetransformationinthesurfacelayerofcomponents(BrinksmeierandBrockhoff,1997).Bettersurfacefinishwithincreasedhardnessatthesurfacebyutilizingtheheatgeneratedduringgrindingispossibleunderoptimumoperatingconditions.Thus,oneoftheareafortheresearcherstoconcernabouttheuniqueoptimalsettingsofgrindingprocessparameters-Depthofcut,Numberofpasses,WheelspeedandworkspeedformaximizingthesurfacehardnessandminimizingthesurfaceroughnesswhilegrindingAISIsteelmaterialswithAl2O3grindingwheels.“Ishikawacauseeffectdiagram”ofmachiningisstudiedtoidentifytheinfluentialprocessparametersthatmayaffectthesurfaceintegrityofgroundedpartsbyRamamoorthyetal.,2001and;Harisinghetal.,2004.Taguchi’sparameterdesignapproachhasbeenusedtoaccomplishtheobjective.Aspecialmathematicaltoolknownasgreyrelationalanalysiscanbeusedwithresponsegraphapproachandsignaltonoiseratioapproachfortheoptimization.Itiswellknownthatphysicalsurfacepropertiescandeterminethelifetimeandthefunctionofhighlyloadedworkpieceandcomponents.Forthisreason,manufacturingindustriesrequireinformationaboutthetechniquestoinfluencethesurfacestateofworkpieceandachieveconsistentproperties(Kegg,1982).Thisinteresthasitsimportanceduetothefactthatmagnitudeoftheresidualstressinterferesonthefatiguestrengthofthematerials(Novasakietal.,1996).Residualsrtessisthemostrepresentativeparametertodescribethequalityofthesurface(Brinksmeieretal.,1982)amongvarioussurfacealterationslikephasetransformations,hardnessvariations,microcracks,grindingburnetc.BanerjeeandChattopadhyay(1987)investigatedthecontrolofresidualstressingrindingbycryogeniccoolingwhichresultsinmuchlesstensileresidualstresses.Kruszynskietal.(1991)madeanattempttopredictresidualstressesingrindingofmetalswiththeaidofanewgrindingparameter.Hucker(1994)showedthattherewasaquantitativerelationbetweentheeffectivework-surfacetemperatureandtheresidualstressproducedongroundsurfacesofhardenedsteels.X-raydiffractiontechniqueswereusedtomeasuretheresidualstresses.ItwasreportedthatCBNgrindingisfoundtoproducecompressivestressatthesurfaceincontrasttoAl2O3grinding.However,manyoftheresearchesprovedthatundertheconditionsofmartensiticformation(roughgrinding)compressiveresidualstressesareformedwhengroundwithAl2O3wheel.BrockhoffandBrinksmeier(1997)intheircomprehensiveviewongrindhardeningfundoutthatcompressiveresidualstressesareexistingintheWhiteEtchingAreas,whichcontinueintotheareaofetchablemartensiteandwhicharecompensatedbylowtensileresidualstressesinagreaterdistancefromthesurface.LitmannandWulff(1955)foundthatforhardenedsteels,whichhavebeenburnedduringgrinding,theworkpiecesub-surfaceconsistsofarehardenedzonenearthesurfaceandasoftenedtemperedzonebeneathit.Thiswouldsuggestthattheonsetofburningischaracterizedbytheformationofausteniteoversomeportionoftheworkpiecesub-surface.Rehardeningatthesurfaceoccursbyacicularmartensite(thatappearsintheformofparallelneedleswithinformeraustenitegrains)formationasthecoolermaterialinthebulkoftheworkpiecequenchesthesurface.Thisreferstophasetransformationingrinding.Aftergrindingunderidealconditions,thegroundsurfacewillbecrackfreeandwillexhibitcompressiveresidualstressesfavorableforcorrosionresistanceandlonglifeundercyclicloadingconditions.Incontrast,manygrindingconditionsaresuchthatthesurfaceproducedsufferstensilestresses,sub-surfacecrackingandoxidationleadingtofailureinsurface.Inordertostrikeabalancebetweenqualityandstrengthingroundedpartsitisdesirabletohaveacontrolovertheresidualstress.Thisnecessitatesadetailedstudyofthefreework-surfacetemperature,amountofheatgeneratedandthemagnitudeofresidualstressformed.對(duì)磨削的一些觀察為了使在零部件的生產(chǎn)中達(dá)到預(yù)期的形狀、尺寸和精度，機(jī)械加工被廣泛運(yùn)用于生產(chǎn)加工工藝中。機(jī)械加工過(guò)程中會(huì)運(yùn)用到一個(gè)或多個(gè)切削工具，來(lái)去除工件上不需要的部分，使之成為切屑。在眾多已應(yīng)用的金屬切削工藝中，磨削加工是金屬加工工藝常用于零件最終加工的重要加工工藝之一。它用途廣泛，也經(jīng)常用于尺寸公差、幾何精度和表面精度要求高的零件的機(jī)械加工工藝中。絕大多數(shù)產(chǎn)品的生產(chǎn)工藝中都少不了磨削加工。根據(jù)Subramanian的統(tǒng)計(jì)數(shù)據(jù)，在工業(yè)國(guó)家的生產(chǎn)支出中，磨削加工占了25%，處于主要地位。幾乎所有的工程零件在其生產(chǎn)的某些階段會(huì)在磨削機(jī)床上加工。在工件的單元切削中，磨削加工是一個(gè)緩慢的過(guò)程。因此，在工件開(kāi)始加工時(shí)，一般采用其他的加工方式使工件達(dá)到與要求相近的精度，然后采用磨削完成加工。在某些應(yīng)用中，磨削也具備更高的金屬切削效率。在如重載磨削中，更多的磨料會(huì)被消耗，在這些情況下，盡可能快而有效去除更多的金屬材料是主要的目標(biāo)。因而，磨削加工能成功地用于任何高精度或難加工零件的加工過(guò)程中，并且它也是可廣泛應(yīng)用于硬化表面材料去除的加工工藝之一。Shaw曾報(bào)告稱，磨削加工是存在很多相關(guān)變量的復(fù)雜工藝，而這些相互作用的變量是同磨削方式所決定的。在平面磨削中所產(chǎn)生的幾何形狀會(huì)受到如下因素的影響：砂輪因素：砂輪直徑、磨粒類型和尺寸、砂輪等級(jí)、砂輪構(gòu)造、粘結(jié)劑、敷料工藝、砂輪的平衡等級(jí)等。工件因素：加工表面硬度、構(gòu)造、化學(xué)特征等。機(jī)床因素：主軸和工作臺(tái)剛度、阻尼、動(dòng)力特性等。加工參數(shù)：砂輪轉(zhuǎn)速、進(jìn)給量、背吃刀量、磨削液等。為了減少消耗，提高生產(chǎn)效率，磨削機(jī)床必須設(shè)定加工時(shí)間處于最短的可能磨削周期內(nèi)。因此，設(shè)置正確磨削機(jī)床參數(shù)對(duì)獲得需要的精度往往非常重要。如果磨削參數(shù)選擇不符合技術(shù)要求，就會(huì)導(dǎo)致時(shí)間浪費(fèi)效率低下。為了解決這個(gè)問(wèn)題，Gupta在他們的研究中，采用列表的方法來(lái)使磨削參數(shù)最優(yōu)化。參數(shù)的選取應(yīng)使工作方案最優(yōu)化，當(dāng)采用“專家系統(tǒng)”時(shí)，磨削工藝參數(shù)的選取就變得容易了。Shaji和Radhakrishnan在Tagudhi的實(shí)驗(yàn)設(shè)計(jì)方法基礎(chǔ)上分析了砂輪轉(zhuǎn)速、進(jìn)給量、背吃刀量、敷料的方式對(duì)磨削力的構(gòu)成、表面加工的影響。FengguoCao提出了一體化神經(jīng)網(wǎng)絡(luò)、灰色相關(guān)分性分析、遺傳算法的概念，來(lái)對(duì)工藝參數(shù)進(jìn)行優(yōu)化提高。爆炸式電火花磨削工藝正是立足于對(duì)最早工藝階段和產(chǎn)品開(kāi)發(fā)周期的合理實(shí)驗(yàn)設(shè)計(jì)的選擇推廣上，來(lái)獲得品質(zhì)和效率的提升。在已有的磨削工藝形式中，外加磨削廣泛應(yīng)用于汽車回轉(zhuǎn)零件的表面加工中。在外圓，產(chǎn)生于工件材料與刀具之間的摩擦阻力，刀具表面的剪切變形抗力會(huì)使得接觸區(qū)域的溫度上升。產(chǎn)生的溫度很高并且分布很不均勻，這樣劇烈的高溫會(huì)使工件的金相結(jié)構(gòu)發(fā)生不利的改變，使其尺寸精度喪失，并且加速刀具鈍化。除了導(dǎo)致表面損壞，磨削熱也會(huì)使工件在磨削過(guò)程產(chǎn)生熱膨脹或熱變形，從而對(duì)工件精度產(chǎn)生不利的影響。Masuda和Shiozaki闡述了磨削熱如何尋到工件表面變得不平整。當(dāng)采用較小的切深和更高的切削速度時(shí)，會(huì)獲得更高的平面度。同時(shí)，也能減少磨削熱的產(chǎn)生。再加快冷卻液的流動(dòng)速率，使工件冷卻效果加強(qiáng)，能使熱變形減小。Chandrsekar研究了表面加工過(guò)程的熱效應(yīng)。在磨削過(guò)程中，局部的接觸應(yīng)力和高的滑動(dòng)速度會(huì)在工件和磨削刃接觸面產(chǎn)生高溫，同時(shí)在次層面產(chǎn)生摩擦熱。高溫是造成已加工表面損壞的重要因素。首先，分布不均勻的瞬態(tài)高溫是工件殘余應(yīng)力和表面微裂紋的主要來(lái)源。第二，局部高溫會(huì)使已加工的部分發(fā)生形變。尤其是對(duì)尺寸較小卻具有較大體積比率的工件，變形尤為嚴(yán)重，這對(duì)于某些小型電子設(shè)備如電磁記錄頭的加工，是一個(gè)很嚴(yán)峻的問(wèn)題。第三，高溫會(huì)導(dǎo)致已加工材料的物相發(fā)生改變。Tarasov對(duì)磨削操作的性質(zhì)做了調(diào)查研究，確定了三種主要的損傷類型，分別是開(kāi)裂，二次淬火燒傷和高溫?zé)齻Ｔ谀ハ饔操|(zhì)的鋼材時(shí)，如果表面溫度過(guò)高，就會(huì)發(fā)生表面再次奧氏體化，并急速冷卻，從而在工件表面會(huì)形成具有的回火馬氏體。這種形式的熱損傷也是覺(jué)的工件燒傷形式，需要避免。在更質(zhì)氧化鋯的磨削過(guò)程中，也會(huì)發(fā)生類似馬氏體類型的物相變化。這種變化是同磨削過(guò)程中產(chǎn)生的瞬態(tài)機(jī)械應(yīng)力和熱應(yīng)