电气工程专业英语实用教程 第9单元翻译完整.docx
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电气工程专业英语实用教程 第9单元翻译完整.docx
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电气工程专业英语实用教程第9单元翻译完整
本科毕业设计(论文)外文翻译
姓名:
王文超
学号:
200715010220
专业:
电气工程及其自动化
班级:
电气071502
指导教师:
智泽英
职称:
副教授
日期:
2011年6月12日
电子信息工程学院
ThebasicsofComputerNumericalControl
WhilethespecificintentionandapplicationforCNCmachinesvaryfrommachinetypeto
another,allformsofCNChavecommonbenefits.ThoughthethrustofthispresentationistoteachyouCNCusage,ithelpstounderstandwhythesesophisticatedmachineshavebecomesopopular.HerearebutafewofthemoreimportantbenefitsofferedbyCNCequipment.
ThefirstbenefitofferedbyallformsofCNCmachinetoolsisimprovedautomation.Theoperatorinterventionrelatedtoproducingworkpiecescanbereducedoreliminated.ManyCNCmachinescanrununattendedduringtheirentiremachiningcycle,freeingtheoperatortodoothertasks.ThisgivestheCNCuserseveralsidebenefitsincludingreducedoperatorfatigue,fewermistakescausedbyhumanerror,andconsistentandpredictablemachiningtimeforeachworkpiece.Sincethemachinewillberunningunderprogramcontrol,theskilllevelrequiredoftheaCNCoperator(relatedtobasicmachiningpractice)isalsoreducedascomparedtoamachinistproducingworkpieceswithconventionalmachinetools.
ThesecondmajorbenefitofCNCtechnologyisconsistentandaccurateworkpieces.Today’sCNCmachinesboastalmostunbelievableaccuracyandrepeatabilityspecifications.Thismeansthatonceaprogramisverified,two,ten,oronethousandidenticalworkpiecescanbeeasilyproducedwithprecisionandconsistency.
AthirdbenefitofferedbymostformsofCNCmachinetoolsisflexibility.Sincethesemachinesarerunfromprograms,runningadifferentworkpieceisalmostaseasyasloadingadifferentprogram.Onceaprogramhasbeenverifiedandexecutedforoneproductionrun,itcanbeeasilyrecalledthenexttimetheworkpieceistoberun.Thisleadstoyetanotherbenefit,fastchange-overs.Sincethesemachinesareveryeasytosetupandrun,andsinceprogramscanbeeasilyloaded,theyallowveryshortsetuptime.Thisisimperativewithtoday’sJust-In-Timeproductrequirements.
1.Motioncontrol-theheartofCNC
ThemostbasicfunctionofanyCNCmachineisautomatic,precise,andconsistentmotion.Ratherthanapplyingcompletelymechanicaldevicestocausemotionasisrequiredonmostconventionalmachinetools,CNCmachinesallowmotioncontrolinarevolutionarymanner.AllformsofCNCequipmenthavetwoormoredirectionsofmotion,calledaxes.Theseaxescanbepreciselyandautomaticallypositionedalongtheirlengthsoftravel.Thetwomostcommonaxistypesarelinear(drivenalongastraightpath)androtary(drivenalongacircularpath).
Insteadofcausingmotionbyturningcranksandhandwheelsasisrequiredonconventionalmachinetools,CNCmachinesallowmotionstobecommandedthroughprogrammedcommands.Generallyspeaking,themotiontype(rapid,linear,andcircular),theaxestomove,theamountofmotionandthemotionrate(feedrate)areprogrammablewithalmostallCNCmachinetools.
Accuratepositioningisaccomplishedbytheoperatorcountingthenumberofrevolutionsmadeonthehandwheelplusthegraduationsonthedial.Thedrivemotorisrotatedacorrespondingamount,whichinturndrivestheballscrew,causinglinearmotionoftheaxis.Afeedbackdeviceconfirmsthattheproperamountofballscrewrevolutionshasoccurred.
ACNCcommandexecutedwithinthecontrol(commonlythroughaprogram)tellsthedrivemotortorotateaprecisenumberoftimes.Therotationofthedrivemotorinturnrotatestheballscrew.Andtheballscrewcausesdrivesthelinearaxis.Afeedbackdeviceattheoppositeendoftheballscrewallowsthecontroltoconfirmthatthecommandednumberofrotationshastakenplace.
Thougharathercrudeanalogy,thesamebasiclinearmotioncanbefoundonacommontablevise.Asyourotatethevisecrank,yourotatealeadscrewthat,inturn,drivesthemovablejawonthevise.Bycomparison,alinearaxisonaCNCmachinetoolisextremelyprecise.Thenumberofrevolutionsoftheaxisdrivemotorpreciselycontrolstheamountoflinearmotionalongtheaxis.
Howaxismotioniscommanded-understandingcoordinatesystems.ItwouldbeinfeasiblefortheCNCusertocauseaxismotionbytryingtotelleachaxisdrivemotorhowmanytimestorotateinordertocommandagivenlinearmotionamount.(Thiswouldbelikehavingtofigureouthowmanyturnsofthehandleonatablevisewillcausethemovablejawtomoveexactlyoneinch!
)Instead,allCNCcontrolsallowaxismotiontobecommandedinamuchsimplerandmorelogicalwaybyutilizingsomeformofcoordinatesystem.ThetwomostpopularcoordinatesystemsusedwithCNCmachinesaretherectangularcoordinatesystemandthepolarcoordinatesystem.Byfar,themovepopularofthesetwoistherectangularcoordinatesystem,andwe’lluseitforalldiscussionsmadeduringthispresentation.
Oneverycommonapplicationfortherectangularcoordinatesystemisgraphing.Almosteveryonehashadtomakeorinterpretagraph.Sincetheneedtoutilizegraphsissocommonplace,andsinceitcloselyresembleswhatisrequiredtocauseaxismotiononaCNCmachine,let’sreviewthebasicsofgraphing.
Aswithanytwodimensionalgraphs,thistypeofgraphhastwobaselines.Eachbaselineisusedtorepresentsomething.Whatthebaselinerepresentsisbrokenintoincrements.Also,eachbaselinehaslimits.Inourproductivityexample,thehorizontalbaselineisbeingusedtorepresenttime.Forthisbaseline,thetimeincrementisinmonths.Rememberthisbaselinehaslimits-itstartsatJanuaryandendwithDecember.Theverticalbaselineisrepresentingproductivity.Productivityisbrokenintotenpercentincrementsandstartsatzeropercentproductivityandendswithonehundredpercentproductivity.
Thepersonmakingthegraphwouldlookupthecompany’sproductivityforJanuaryoflastyearandattheproductivitypositiononthegraphforJanuary,apointisplotted.ThiswouldwhenberepeatedforFebruary,March,andeachmonthoftheyear.Onceallpointsareplotted,alineorcurvecanbedrawnthrougheachofthepointstomakeitmoreclearastohowthecompanydidlastyear.
Let’stakewhatwenowknowaboutgraphsandrelateittoCNCaxismotion.Insteadofplottingtheoreticalpointstorepresentconceptualideas,theCNCprogrammerisgoingtobeplottingphysicalendpointsforaxismotions.Eachlinearaxisofthemachinetoolcanbethoughtofaslikeabaselineofthegraph.Likegraphbaselines,axesarebrokenintoincrements.Butinsteadofbeingbrokenintoincrementsofconceptualideasliketimeandproductivity,eachlinearaxisofaCNCmachine’srectangularcoordinatesystemisbrokenintoincrementsofmeasurement.Intheinchmode,thesmallestincrementisusually0.0001inch(in).Inthemetricmode,thesmallestincrementis0.001millimeter(mm).(Bytheway,forrotaryaxestheincrementis0.001).
Justlikethegraph,eachaxiswithintheCNCmachine’scoordinatesystemmuststartsomewhere.Withthegraph,thehorizontalbaselinestartedatJanuaryandtheverticalbaselinestarteratzeropercentproductivity.Thisplacewheretheverticalandhorizontalbaselinescometogetheriscalledtheoriginpointofthegraph.ForCNCpurposes,thisoriginpointiscommonlycallertheprogramzeropoint(alsocalledworkzero,partzero,andprogramorigin).
Forthisexample,thetwoaxeswehappentobeshowingarelabelerasXandYbutkeepinminethatprogramzerocanbeappliedtoanyaxis.ThoughthenamesofeachaxiswillchangefromoneCNCmachinetypetoanother(othercommonnamesincludeZ,A,B,C,U,V,andW),thisexampleshouldworknicelytoshowyouhowaxismotioncanbecommanded.
TheprogramzeropointestablishesthepointofreferenceformotioncommandsinaCNCprogram.Thisallowstheprogrammertospecifymovementsfromacommonlocation.Ifprogramzeroischosenwisely,usuallycoordinatesneededfortheprogramcanbetakendirectlyfromtheprint.
Withthistechnique,istheprogrammerwishesthetooltobesenttoapositiononeinchtotherightoftheprogramzeropoint,X1.0iscommanded.Iftheprogrammerwishesthetooltomovetoapositiononeinchabovetheprogramzeropoint,Y1.0iscommanded.Thecontrolwillautomaticallydeterminehowmanytimestorotateeachaxisdrivemotorandballscrewtomaketheaxisreachthecommanderdestinationpoint.Thisletstheprogrammercommandaxismotioninaverylogicalmanner.
Withtheexamplesgivensofar,allpointshappenedtobeupandtotherightoftheprogramzeropoint.Thisareaupandtotherightoftheprogramzeropointiscalledaquadrant(inthiscase,quadrantnumberone).ItisnotuncommononCNCmachinesthatendpointsneededwithintheprogramfallinotherquadrants.Whenthishappens,atleastoneofthecoordinatesmustbespecifiedasminus.
2.Understandingabsoluteversusincrementalmotion
Alldiscussionstothispointassumethattheabsolutemodeofprogrammingisused.ThemostcommonCNCwordusedtodesignatetheabsolutemodeisG90.Intheabsolutemode,theendpointsforallmotionswillbespecifiedfromtheprogramzeropoint.Forbeginners,thisisusuallythebestandeasiestmethodofspecifyingendpointsformotioncommands.However,thereisanotherwayofspecifyingendpointsforaxismotion.
Intheincrementalmode(commonlyspecifiedbyG91),endpointsformotionsarespecifiedfromthetool’scurrentposition,notfromprogramzero.Withthismethodofcommandingmotion,theprogrammermustalwaysbeasking“HowfarshouldImovethetool?
”Whiletherearetimeswhentheincrementalmodecanbeveryhelpful,generallyspeaking
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