数控专业外文翻译数控机床的组成部分精品.docx

数控专业外文翻译数控机床的组成部分精品.docx

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数控专业外文翻译数控机床的组成部分精品

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英文原文

N/CMachineToolElements

———From《EnglishforMachinery》

N/Cmachinetoolelementsconsistofdimensioningsystems,controlsystems,servomechanismsandopen-orclosed-loopsystems.Itisimportanttounderstandeachelementpriortoactualprogrammingofanumericallycontrolledport.

ThetermmeasuringsysteminN/Creferstothemethodamachinetoolusestomoveapartfromareferencepointtoatargetpoint.Atargetpointmaybeacertainlocatingfordrillingahole,millingaslot,orothermachineoperation.ThetwomeasuringsystemsusedonN/Cmachinesaretheabsoluteandincremental.Theabsolutemeasuringsystemusesafixedreferencepoint.Itisonthispointthatallpositionalinformationisbased.Inotherwords,allthelocationstowhichapartwillbemovedmustbegivendimensionsrelatingtothatoriginalfixedreferencepoint.FigureshowsanabsolutemeasuringsystemwithXandYdimensions,eachbasedontheorigin.Theincrementalmeasuringsystemhasafloatingcoordinatingsystem.Withtheincrementalsystem,thetimethepartismoved.Figure16.2showXandYvaluesusinganincrementalmeasuringsystem.Noticethatwiththissystem,eachnewlocationbasesitsvaluesinXandYfromtheprecedinglocation.Onedisadvantagetothissystemisthatanyerrorsmadewillberepeatedthroughouttheentireprogram,ifnotdetectedandcorrected.

TherearetwotypesofcontrolsystemscommonlyusedonN/Cequipment:

point-to-pointandcontinuouspath.Apoint-to-pointcontrolledN/Cmachinetool,sometimesreferredtoasapositioningcontroltype,hasthecapabilityofmovingonlyalongastraightline.However,whentwoaxesareprogrammedsimultaneouslywithequalvaluesa45anglewillbegenerated.Point-to-pointsystemsaregenerallyfoundondrillingandsimplemillingmachinewhereholelocationandstraightmillingjobsareperformed.Point-to-pointsystemscanbeutilizedtogenetatearcsandanglesbyprogrammingthemachinetomoveinaseriesofsmallsteps.Usingthistechnique,however,theactualpathmachinedisslightlydifferentfromthecuttingpathspecified.

Machinetoolsthathavethecapabilityofmovingsimultaneouslyintwoormoreaxesareclassifiedascontinuous-pathorcontouring.Thesemachinesareusedformachiningarcs,radii,circles,andanglesofanysizeintwoortheredimensions.Continuous-pathmachinesaremoreexpensivethanpoint-to-pointsystemsandgenerallyrequireacomputertoaidprogrammingwhenmachiningcomplexcontours.

N/Cservomechanismsaredevicesusedforproducingaccuratemovementofatableorslidalonganaxis.TwotypesofservosarecommonlyusedonN/Cequipment:

electricsteppingmotorsandhydraulicmotors.SteppingmotorservosarefrequentlyusedonlessexpensiveN/Cequipment.Thesemotorsaregenerallyhigh-torquepowerservosandmounteddirectlytoaleadscrewofatableortoolslide.Moststeppingmotorsareactuatedbymagneticpulsesfromthestatorandrotorassemblies.Thenetresultofthisactionisthatonerotationofthemotorshaftproduces200steps.Connectionthemotorshafttoa10-pitchleadscrewallows0.0005-in.movementstobemade.Hydraulicservosproduceafluidpressurethatflowsthroughgearsorpistonstoeffectshaftrotation.Mechanicalmotionofleadscrewsandslidesisaccomplishedthroughvariousvaluesandcontrolsfromthesehydraulicmotors.However,theyaremoreexpensiveandnoisy.MostlargerN/Cmachinesusehydraulicservos.

N/Cmachinesthatuseanopen-loopsystemcontainno-feedbacksignaltoensurethatamachineaxishastraveledtherequireddistance.Thatis,iftheinputreceivedwastomoveaparticulartableaxis1.000in,theservounitgenerallymovesthetable1.000in.Thereisnomeansforcomparingtheactualtablemovementwiththeinputsignal,however,Theonlyassurancethatthetablehasactuallymoved1.000in.isthereliabilityoftheservosystemused.Open-loopsystemsare,ofcourse,lessexpensivethanclosed-loopsystems.Aclosed-loopsystemcomparestheactualoutputwiththeinputsignalandcompensatesforanyerrors.Afeedbackunitactuallycomparestheamountthetablehasbeenmovedwiththeinputsignal.Somefeedbackunitsusedonclosed-loopsystemsaretransducers,electricalormagneticscales,andsynch.Closed-loopsystemsgreatlyincreasethereliabilityofN/Cmachines.

MachiningCenters

Manyoftoday’smoresophisticatedlathesarecalledmachiningcenterssincetheyarecapableofperforming,inadditiontothenormalturningoperations,certainmillinganddrillingoperations.Basically,amachiningcentercanbethoughtofasbeingacombinationturretlatheandmillingmachine.Additionalfeaturesaresometimesincludedbymanufacturerstoincreasetheversatilityoftheirmachines.

NumericalControl

Oneofthemostfundamentalconceptsintheareaofadvancedmanufacturingtechnologiesisnumericalcontrol（NC）.PriortotheadventofNC,allmachinetoolsweremanuallyoperatedandcontrolled.Amongthemanylimitationsassociatedwithmanualcontrolmachinetools,perhapsnoneismoreprominentthanthelimitationofoperatorskills.Withmanualcontrol,thequalityoftheproductisdirectlyrelatedtoandlimitedtotheskillsoftheoperator.Numericalcontrolrepresentsthefirstmajorstepawayfromhumancontrolofmachinetools.

Numericalcontrolmeansthecontrolofmachinetoolsandothermanufacturingsystemsthroughtheuseofprerecorded,writtensymbolicinstructions.Ratherthanoperatingamachinetool,anNCtechnicianwritesaprogramthatissuesoperationalinstructionstothemachinetool.Foramachinetooltobenumericallycontrolled,itmustbeinterfacedwithadeviceforacceptinganddecodingtheprogrammedinstructions,knownasareader.

Numericalcontrolwasdevelopedtoovercomethelimitationofhumanoperators,andithasdoneso.Numericalcontrolmachinesaremoreaccuratethanmanuallyoperatedmachines,theycanproducepartsmoreuniformly,theyarefaster,andthelong-runtoolingcostsarelower.ThedevelopmentofNCledtothedevelopmentofseveralotherinnovationsinmanufacturingtechnology:

Electricaldischargemachining.

Lasercutting.

Electronbeamwelding.

Numericalcontrolhasalsomademachinetoolsmoreversatilethantheirmanuallyoperatedpredecessors.AnNCmachinetoolcanautomaticallyproduceawidevarietyofparts,eachinvolvinganassortmentofwidelyvariedandcomplexmachiningprocesses.Numericalcontrolhasallowedmanufacturerstoundertaketheproductionofproductsthatwouldnothavebeenfeasiblefromaneconomicperspectiveusingmanuallycontrolledmachinetoolsandprocesses.

Likesomanyadvancedtechnologies,NCwasborninthelaboratoriesoftheMassachusettsInstituteofTechnology.TheconceptofNCwasdevelopedintheearly1950swithfundingprovidedbytheU.S.Airforce.Initsearlieststages,NCmachineswereabletomakestraightcutsefficientlyandeffectively.

However,curvedpathswereaproblembecausethemachinetoolhadtobeprogrammedtoundertakeaseriesofhorizontalandverticalstepstoproduceacurve.Theshorteristhestraightlinesmakingupthesteps,thesmootheristhecurve.Eachlinesegmentinthestepshadtobecalculated.

Thisproblemledtothedevelopmentin1959oftheAutomaticallyProgrammedTools（APT）language.ThisisaspecialprogramminglanguageforNCthatusesstatementssimilartoEnglishlanguagetodefinethepartgeometry,describethecuttingtoolconfiguration,andspecifythenecessarymotions.ThedevelopmentoftheAPTlanguagewasamajorstepforwardinthefurtherdevelopmentofNCtechnology.TheoriginalNCsystemswerevastlydifferentfromthoseusedtoday.Themachineshadhardwiredlogiccircuits.Theinstructionalprogramswerewrittenonpunchedpaper,whichwaslatertobereplacedbymagneticplastictape.Atapereaderwasusedtointerprettheinstructionswrittenonthetapeforthemachine.Together,allofthisrepresentedagiantstepforwardinthecontrolofmachinetools.However,therewereanumberofproblemswithNCatthispointinitsdevelopment.

Amajorproblemwasthefragilityofthepunchedpapertapemedium.Itwascommonforthepapertapecontainingtheprogrammedinstructionstobreakortearduringamachiningprocess.Thisproblemwasexacerbatedbythefactthateachsuccessivetimeapartwasproducedonamachinetool,thepapertapecarryingtheprogrammedinstructionshadtobererunthroughthereader.Ifitwasnecessarytoproduce100copiesofagivenpart,itwasalsonecessarytorunthepapertapethroughthereader100separatetimes.Fragilepapertapessimplycouldnotwithstandtherigorsofashopfloorenvironmentandthiskindofrepeateduse.

Thisledtothedevelopmentofaspecialmagneticplastictape.Whereasthepapertapecarriedtheprogrammedinstructionsasaseriesofholespunchedinthetape,theplastictapecarriedtheinstructionsasaseriesofholespunchedinthetape,theplastictapecarriedtheinstructionsasaseriesofmagneticdots.Theplastictapewasmuchstrongerthanthepapertaps,whichsolvedtheproblemoffrequenttearingandbreakage.However,itstilllefttwootherproblems.

Themostimportantofthesewasthatitwasdifficultorimpossibletochangetheinstructionsenteredonthetape.Tomakeeventhemostminoradjustmentsinaprogramofinstructions,itwasnecessarytointerruptmachiningoperationsandmakeanewtape.Itwasalsostillnecessarytorunthetapethroughthereaderasmanytimesastherewerepartstobeproduced.Fortunately,computertechnologybecamearealityandsoonsolvedtheproblemsofNCassociatedwithpunchedpaperandplastictape.

Thedevelopmentofaconceptknownasdirectnumerical