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1、3.1.2Effectsofpre-strainandrakeangleinmachiningcopperIntheprevioussection,themachiningofannealedmetalsbya6°rakeangletoolwasconsidered.Bothpre-strainandanincreasedrakeangleresultinreducedspecificcuttingforcesandreducedcuttingtemperatures,buthavelittleeffectonthestressseson
2、thetool.Thesegeneralizationsmaybeillustratedbythecuttingofcopper,ametalsufficientlysoft(asalsoisaluminium)toallowmachiningbytoolsofrakeangleuptoaround40°.Figure3.6showsexamplesofspecificforcesandshearplaneanglesmeasuredinturningannealedandheavilycold-workedcopperatfeedsin
3、therange0.15to0.2mm,withhighspeedsteeltoolsofrakeanglefrom6°to35°.Specificforcesvaryoverasixfoldrangeatthelowestcuttingspeed,withshearplaneanglesfrom8°to32°.TheleftpanelofFigure3.7showsthattheestimatedtoolcontactstresseschangelittlewithrakeangle,althoughtheyareclearlylarg
4、erfortheannealedthanthepre-strainedmaterial.Theright-handpanelshowsthatthetemperaturerisesarehalvedonchangingfroma6°to35°rakeangletool.Theseobservations,thattoolstressesaredeterminedbythematerialbeingcutanddonotvarymuchwiththecuttingconditions,whiletemperaturesdependstron
5、glyonboththematerialbeingcutandthecuttingconditions,isacontinuingthemethatwillbedevelopedformetalalloysinthefollowingsections.3.1.3MachiningcopperandaluminiumalloysItisoftenfoundthatalloysofmetalsmachinewithlargershearplaneanglesandhencelowerspecificforcesthantheelemental
6、metalsthemselves.SometimesastrongreasonisalowervalueofthestrainhardeningparameterDk/kmax,atothertimesthechip/toolfriction(asindicatedbythefrictioncoefficient)isless;andatothersagainitisnotatallobviouswhythisshouldbeso.Butevenwhenthespecificforcesarelower,thetoolcontactstr
7、esscanbehigher.Inthissection,examplesofmachiningtwocopperandonealuminiumalloyaretakentoillustratethis.Figure3.8recordsthebehavioursofaCuNiandaCuZnalloy.TheCuNialloy,with80%Ni,mightbetterbeconsideredasaNialloy.However,itmachinesatahighershearplaneangleatagivencuttingspeedt
8、haneithercopperornickel,despiteitsstrain-hardeningcharacteristicbeingsimilartoormoreseverethanei
