机械毕业设计英文外文翻译446数控机床 2.docx

机械毕业设计英文外文翻译446数控机床 2.docx

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机械毕业设计英文外文翻译446数控机床2

附录

CNCmachine

WhilethespecificintentionandapplicationforCNCmachinesvaryfromonemachinetypetoanother,allformsofCNChavecommonbenefits.HerearebutafewofthemoreimportantbenefitsofferedbyCNCequipment.

ThefirstbenefitofferedbyallformsofCNCmachinetoolsisimprovedautomation.Theoperatorinterventionrelatedtoproducingworkpiecescanbereducedoreliminated.ManyCNCmachinescanrununattendedduringtheirentiremachiningcycle,freeingtheoperatortodoothertasks.ThisgivestheCNCuserseveralsidebenefitsincludingreducedoperatorfatigue,fewermistakescausedbyhumanerror,andconsistentandpredictablemachiningtimeforeachworkpiece.Sincethemachinewillberunningunderprogramcontrol,theskilllevelrequiredoftheCNCoperator（relatedtobasicmachiningpractice）isalsoreducedascomparedtoamachinistproducingworkpieceswithconventionalmachinetools.

ThesecondmajorbenefitofCNCtechnologyisconsistentandaccurateworkpieces.Today'sCNCmachinesboastalmostunbelievableaccuracyandrepeatabilityspecifications.Thismeansthatonceaprogramisverified,two,ten,oronethousandidenticalworkpiecescanbeeasilyproducedwithprecisionandconsistency.

AthirdbenefitofferedbymostformsofCNCmachinetoolsisflexibility.Sincethesemachinesarerunfromprograms,runningadifferentworkpieceisalmostaseasyasloadingadifferentprogram.Onceaprogramhasbeenverifiedandexecutedforoneproductionrun,itcanbeeasilyrecalledthenexttimetheworkpieceistoberun.Thisleadstoyetanotherbenefit,fastchangeover.Sincethesemachinesareveryeasytosetupandrun,andsinceprogramscanbeeasilyloaded,theyallowveryshortsetuptime.Thisisimperativewithtoday'sjust-in-time（JIT）productrequirements.

Motioncontrol-theheartofCNC

ThemostbasicfunctionofanyCNCmachineisautomatic,precise,andconsistentmotioncontrol.Ratherthanapplyingcompletelymechanicaldevicestocausemotionasisrequiredonmostconventionalmachinetools,CNCmachinesallowmotioncontrolinarevolutionarymanner2.AllformsofCNCequipmenthavetwoormoredirectionsofmotion,calledaxes.Theseaxescanbepreciselyandautomaticallypositionedalongtheirlengthsoftravel.Thetwomostcommonaxistypesarelinear（drivenalongastraightpath）androtary（drivenalongacircularpath）.

Insteadofcausingmotionbyturningcranksandhandwheelsasisrequiredonconventionalmachinetools,CNCmachinesallowmotionstobecommandedthroughprogrammedcommands.Generallyspeaking,themotiontype（rapid,linear,andcircular）,theaxestomove,theamountofmotionandthemotionrate（feedrate）areprogrammablewithalmostallCNCmachinetools.

ACNCcommandexecutedwithinthecontroltellsthedrivemotortorotateaprecisenumberoftimes.Therotationofthedrivemotorinturnrotatestheballscrew.Andtheballscrewdrivesthelinearaxis（slide）.Afeedbackdevice（linearscale）ontheslideallowsthecontroltoconfirmthatthecommandednumberofrotationshastakenplace3.Refertofig.1.

Fig.1

Thougharathercrudeanalogy,thesamebasiclinearmotioncanbefoundonacommontablevise.Asyourotatethevisecrank,yourotatealeadscrewthat,inturn,drivesthemovablejawonthevise.Bycomparison,alinearaxisonaCNCmachinetoolisextremelyprecise.Thenumberofrevolutionsoftheaxisdrivemotorpreciselycontrolstheamountoflinearmotionalongtheaxis.

Howaxismotioniscommanded-understandingcoordinatesystems

ItwouldbeinfeasiblefortheCNCusertocauseaxismotionbytryingtotelleachaxisdrivemotorhowmanytimestorotateinordertocommandagivenlinearmotionamount4.（Thiswouldbelikehavingtofigureouthowmanyturnsofthehandleonatablevisewillcausethemovablejawtomoveexactlyoneinch!

）Instead,allCNCcontrolsallowaxismotiontobecommandedinamuchsimplerandmorelogicalwaybyutilizingsomeformofcoordinatesystem.ThetwomostpopularcoordinatesystemsusedwithCNCmachinesaretherectangularcoordinatesystemandthepolarcoordinatesystem.Byfar,themorepopularofthesetwoistherectangularcoordinatesystem.

TheprogramzeropointestablishesthepointofreferenceformotioncommandsinaCNCprogram.Thisallowstheprogrammertospecifymovementsfromacommonlocation.Ifprogramzeroischosenwisely,usuallycoordinatesneededfortheprogramcanbetakendirectlyfromtheprint.

Withthistechnique,iftheprogrammerwishesthetooltobesenttoapositiononeinchtotherightoftheprogramzeropoint,X1.0iscommanded.Iftheprogrammerwishesthetooltomovetoapositiononeinchabovetheprogramzeropoint,Y1.0iscommanded.Thecontrolwillautomaticallydeterminehowmanytimestorotateeachaxisdrivemotorandballscrewtomaketheaxisreachthecommandeddestinationpoint.Thisletstheprogrammercommandaxismotioninaverylogicalmanner.Refertofig.2,3.

Fig.2

Fig.3

Alldiscussionstothispointassumethattheabsolutemodeofprogrammingisused6.ThemostcommonCNCwordusedtodesignatetheabsolutemodeisG90.Intheabsolutemode,theendpointsforallmotionswillbespecifiedfromtheprogramzeropoint.Forbeginners,thisisusuallythebestandeasiestmethodofspecifyingendpointsformotioncommands.However,thereisanotherwayofspecifyingendpointsforaxismotion.

Intheincrementalmode（commonlyspecifiedbyG91）,endpointsformotionsarespecifiedfromthetool'scurrentposition,notfromprogramzero.Withthismethodofcommandingmotion,theprogrammermustalwaysbeasking"HowfarshouldImovethetool?

"Whiletherearetimeswhentheincrementalmodecanbeveryhelpful,generallyspeaking,thisisthemorecumbersomeanddifficultmethodofspecifyingmotionandbeginnersshouldconcentrateonusingtheabsolutemode.

Becarefulwhenmakingmotioncommands.Beginnershavethetendencytothinkincrementally.Ifworkingintheabsolutemode（asbeginnersshould）,theprogrammershouldalwaysbeasking"Towhatpositionshouldthetoolbemoved?

"Thispositionisrelativetoprogramzero,NOTfromthetoolscurrentposition.

Asidefrommakingitveryeasytodeterminethecurrentpositionforanycommand,anotherbenefitofworkingintheabsolutemodehastodowithmistakesmadeduringmotioncommands.Intheabsolutemode,ifamotionmistakeismadeinonecommandoftheprogram,onlyonemovementwillbeincorrect.Ontheotherhand,ifamistakeismadeduringincrementalmovements,allmotionsfromthepointofthemistakewillalsobeincorrect.

Assigningprogramzero

KeepinmindthattheCNCcontrolmustbetoldthelocationoftheprogramzeropointbyonemeansoranother.HowthisisdonevariesdramaticallyfromoneCNCmachineandcontroltoanother8.One（older）methodistoassignprogramzerointheprogram.Withthismethod,theprogrammertellsthecontrolhowfaritisfromtheprogramzeropointtothestartingpositionofthemachine.ThisiscommonlydonewithaG92（orG50）commandatleastatthebeginningoftheprogramandpossiblyatthebeginningofeachtool.

Another,newerandbetterwaytoassignprogramzeroisthroughsomeformofoffset.Refertofig.4.Commonlymachiningcentercontrolmanufacturerscalloffsetsusedtoassignprogramzerofixtureoffsets.Turningcentermanufacturerscommonlycalloffsetsusedtoassignprogramzeroforeachtoolgeometryoffsets.

Fig.4

Flexiblemanufacturingcells

Aflexiblemanufacturingcell（FMC）canbeconsideredasaflexiblemanufacturingsubsystem.ThefollowingdifferencesexistbetweentheFMCandtheFMS:

1.AnFMCisnotunderthedirectcontrolofthe

centralcomputer.Instead,instructionsfromthecentral

computerarepassedtothecellcontroller.

2.Thecellislimitedinthenumberofpartfamiliesit

canmanufacture.

ThefollowingelementsarenormallyfoundinanFMC:

•Cellcontroller

•Programmablelogiccontroller（PLC）

•Morethanonemachinetool

•Amaterialshandlingdevice（robotorpallet）

TheFMCexecutesfixedmachiningoperationswithpartsflowingsequentiallybetweenoperations.

Highspeedmachining

ThetermHighSpeedMachining（HSM）commonlyreferstoendmillingathighrotationalspeedsandhighsurfacefeeds.Forinstance,theroutingofpocketsinaluminumairframesectionswithaveryhighmaterialremovalrate1.Overthepast60years,HSMhasbeenappliedtoawiderangeofmetallicandnon-metallicworkpiecematerials,includingtheproductionofcomponentswithspecificsurfacetopographyrequirementsandmachiningofmaterialswithhardnessof50HRCandabove.Withmoststeelcomponentshardenedtoapproximately32-42HRC,machiningoptionscurrentlyinclude:

Roughmachiningandsemi-finishingofthematerialinitssoft（annealed）conditionheattreatmenttoachievethefinalrequiredhardness=63HRCmachiningofelectrodesandElectricalDischargeMachining（EDM）ofspecificpartsofdiesandmoulds（specificallysmallradiianddeepcavitieswithlimitedaccessibilityformetalcuttingtools）finishingandsuper-finishingofcylindrical/flat/cavitysurfaceswithappropriatecementedcarbide,cermet,solidcarbide,mixedceramicorpolycrystallinecubicboronnitride（PCBN）

Formanycomponents,theproductionprocessinvolvesacombinationoftheseoptionsandinthecaseofdiesandmouldsitalsoincludestimeconsuminghandfinishing.Consequently,productioncostscanbehighandleadtimesexcessive.

Itistypicalinthedieandmouldindustrytoproduceoneorjustafewtoolsofthesamedesign.Theprocessinvolvesconstantchangestothedesign,andbecauseofthesechangesthereisalsoacorrespondingneedformeasurin