Unit_9_NC_Machines

Unit9NCMachines第9单元数控机床MachinemaybeclassifiedaccordingtothenumberofaxesofnumericallycontrolledmovementswithrespecttoCartesianX-Y-Zcoordinates.Theremaybeothermovementnotnumericallycontrolled.Atwo-axismachinewouldhavethetablemovedlengthwiseandcrosswiseinahorizontalplane;athree-axismachinewouldhaveanadditionalverticalmovementofthespindle,forexample,Four-,five-,orsix-axismachinesprovideadditionallinearorrotarymovements.机器根据多轴数控运动方面笛卡尔某某坐标可分为多种。有可能是其他运动的数字控制。一种双轴机将表的纵向和横向移动在水平面；三轴机将有额外的主轴垂直运动，例如，四-，五-，或提供额外的线性轴机或旋转运动。ThreeclassesofNCsystemsarecommonlyrecognized.Oneoftheseiscalledpoint-to-pointorpositioningNC,whereacuttingtoolandworkpiecearepositionedwithrespecttoeachotherbeforeacutistaken,asexemplifiedbyanNCdrillpress~.Thepathbetweenpointsisoflittleconcernandisnotparticularlycontrolled.Anotherclass,knownasastraight-cutNCsystem,involvesmovementsbetweenpointslikepoint-to-pointNCbutalongstraightorcurvedpathsdeterminedbythemachinewaysorslides.AnNCturretlathemayBeofthesecondclass.Athirdclassiscontinuous-pathNC,ormoreformallycontinuoustoolPathcontrol,whichdoescontouringorprofilingoflines,curves,orsurfaces,ofallshapes.OnewayofachievingcontinuouspathNCistomovetheworkpieceorcutterfrompointtopointalongastraightlineorcurve(aparabolicpathbetweenpointsisusedonsomemachines)withmanypointsspacedcloselyenoughtogethersothatthecompositionpathoftheCutapproximatesadesiredcurvewithinlimitsrequired~.Anotherwayistodrivemachinemembers,sayasaddleandthetableonit,alongcoordinateaxesatvaryingvelocitiescontrolledsothattheresultantmotionisalongaparticularlineorcurve.在三类数控系统中。其中一类被称为逐点或位置调节数控，在这类数控系统中，进行切削之前，切削刀具和工件彼此之间的位置就被确定了，数控钻床就是一个例子。点之间的路径不太重要，不是特别控制。另一类，称为数控纵切系统，涉及运动点像点位数控但沿直线或曲线路径决定的方式或过程。数控转塔车床可能是二类。三分之一类是连续轨迹数控，或更正式的连续轨迹控制，其轮廓或轮廓线，曲线或表面，各种形状。得到数控连续路径的一种方式是使工件或刀具沿一直线或曲线做点到点的运动（在某些机床上，点之间采用抛物线路径）许多点的间隔足够小，一边在要求的限度内使切削组合的路径接近一预想的曲线。另一种方法是驱动机器的部件，像一个鞍状的结构，沿坐标轴受控于不同的速度，由此产生的运动是一个特定的直线或曲线。AnNCmachinemaybecontrolledthroughanopen-looporclosed-loopcircuitasdepictedinFig.9.1.Theopen-loopsystemisthesimplestandcheapestbutdoesnotassureaccuracy.Asignalthatisanorderforacertainaction(suchastoturnonthecoolant,startthespindle,ormovethetabletoacertainposition)isissuedbythecontrolunit.Thistravelsthroughthedrivemechanismwhichimposestheactionuponthecontrolledmemberofthemachine.Thebasicdrivemechanismforatableorothermachinememberinanopen-loopsystemisasteppingmotor,alsocalledadigitalorpulsemotor.Ithas,forexample,49polesaroundthearmatureinsideof50polesaroundthestator.Twopolesarealwaysaligned.Polesareenergizedtoalignthenexttwopolesandturnthearmaturethroughadefinitesmallangleeachtimeapulseofelectricityisreceived.Themotorturnstheleadscrew,whichmovesthetableacorrespondingdistance.If,forexample,apulsestandsfora0.001mmmovement,andamovementof0.05mmisdesired,thecontrolunitissues50pulses;themotorturnsthrough50stepsandturnstheleadscrew,andthetable(orothermember)isdriventhe0.05mmrequired.Therateatwhichthepulsesareissueddeterminestherateoffeed.Thepowerofasteppingmotorislimited,andiftheresistancetomovementislarge,themotormayjuststallandmisssteps.Thereisnofeedbacktoreporttheomission,andaccuracyislost.Directdrivewithasteppingmotorisconfinedtolightserviceatfractionalhorsepower.Formorepowerandheavyservice,asteppingmotorismatchedwithahydraulicmotorthroughaservovalvefortabledrivesupto7.5kW(10hp)andmore.数控机床可控制通过一个开环或闭环电路如图9.1所。开环系统是最简单和便宜但不保证准确性。一个信号是一个特定操作(例如打开冷却剂,启动主轴,或的表移动到某一个位置)的控制单元。这个过程,通过驱动机制规定的行为来控制机器的部件。在开口回路系统中，一工作台或其他机器构件的基本的驱动机制是步进电动机驱动,也称为数字或脉冲发动机。比如,它在电枢内部49极点在定子内部有50的极点。两个极点总是一致的。驱动有足够的力量来调整下两个极点,将通过电枢每次收到的脉冲电产生的一个明确的小角度来进行控制。马达转动的距离是对应于移动表的距离。比如说,控制单元都为50脉冲时，一个脉冲代表0.001毫米的运动和一个脉冲代表0.05毫米的运动效果是很明显的，马达转动到50步产生的0.05毫米的距离是自身驱动所需要的。速度进给是由脉冲的速率决定的。步进电机的力量是有限的,如果要带动大型汽车运动是不可能的。这种方式与步进电机直接驱动相比仅限于光服务在分马力。如果需要更多的力和完成更重的工作，,一个步进电机配个通过伺服阀液压马达就可以驱动7.5千瓦(10点)的力，甚者更多的力。Itiscommoninclosed-loopsystemsforthepulsedcommandsignalforthemovementofthetable(orothermember)ofthemachinetobeconvertedtoasteadyanalogsignal.Thatsignalturnsonthepowertothedrivemechanism.HydraulicandacbutmostlydcelectricmotorsareallusedforNCmachinedrives.Theintensityofthesignalisdeterminedbythefeedvelocityordered.Forahigherfeed,pulsesareissuedatafasterrateand/ortheanalogsignalislarger,whichturnsonmorepowertothemotortodrivethetablefaster.Atachometerinthemotordrivesendsbackasignalthatiscomparedtothefeedrateordertoassurethattherequiredfeedrateisbeingdeliveredandtomakecorrectionsifitisnot.Commonly,aresolverisdrivenbytheleadscrewandreturnsasignalshowinghowfarthetablemoves.Moreaccuracyforalongerperiodoftimeisobtainedwiththefeedbackdeviceattacheddirectlytothetable,butsuchdevicesaremorecostly.Thefeedbacksignaliscomparedtotheorder,andwherethefeedbackequalstheorderandsignifiestherequiredmovementhasbeencompleted,thepowerisshutoff,andthemovementstops.Sincetheyarecontrolledthroughseparatechannels,allmovementsmayactatthesametime.Indeed,itisnecessarythattheyactsimultaneouslyandbesynchronizedaccuratelyforcontouring.这是脉冲信号指挥的常见的闭环系统的运动，其他的机器需要转换到一个稳定的模拟信号来进行运动。信号是打开电源的驱动机制。大多数的数控机床都是采用直流电机驱动的。发出脉冲的频率能确定进给的频率。要实现一个较高的进给需要脉冲发出的速度更快