What is a CNC Machine? CNC : Computerised Numerical ...

1 WhatisaCNCM achine?CNC:ComputerisedNumericalControl( Computer+NumericalControl) Numericalcontrolisaprogrammableautomatio ninwhichprocessiscontrolledbyNumbers,Let ters,andsymbols. CNCM achiningisaprocessusedinthemanufacturing sectorthatinvolvestheuseofcomputerstocon trolmachinetoolslikelathes, Tomanufacturecomplexcurvedgeometriesin2 Dor3 Dwasextremelyexpensivebymechanicalmeans( whichusuallywouldrequirecomplexjigstocon trolthecuttermotions) MachiningcomponentswithhighRepeatability andPrecision Unmannedmachiningoperations Toimproveproductionplanningandtoincrease productivity However,sincethegrooveintheballscrewishe lical,itssteelballsrollalongthehelicalgr oove,and,then,theymaygooutoftheballnutun lesstheyarearrestedatacertainspot.

2 Itconsistsofascrewspindle,anut,ballsandi ntegratedballreturnmechanismashowninFigu re. , configuration3 Thus,itisnecessarytochangetheirpathafter theyhavereachedacertainspotbyguidingthem ,oneafteranother,backtotheir startingpoint (formationofarecirculationpath).Therecir culationpartsplaythatrole. Whenthescrewshaftisrotating,asshowninFig ure,asteelballatpoint(A)travels3turnsofs crewgroove,rollingalongthegroovesofthesc rewshaftandtheballnut,andeventuallyreach espoint(B). Then,theballisforcedtochangeitspathwayat thetipofthetube,passingbackthroughthetub e,untilitfinallyreturnstopoint(A). Wheneverthenutstrokesonthescrewshaft, Whendebrisorforeignmatterentertheinsideo fthenut,itcouldaffectsmoothnessinoperati onorcauseprematurewearing,eitherofwhichc ouldadverselyaffecttheballscrew'sfunctio ns.

3 Topreventsuchthingsfromoccurring, HighmechanicalefficiencyInballscrews,abo ut90% ,theamountofforceusedtorotatethescrewsha ftisaslowasonethirdofthatneededfortheacm ethreadleadscrew. LowinwearBecauseofrollingcontact, , forms (a) Gothic arc (b) Circular arc ThreadFormThethreadformusedinthesescrews caneitherbegothicarctype( )orcirculararctype( ). PreloadingInordertoobtainbidirectionalmo tionofthecarriagewithoutanypositionalerr or, (tensionorcompression) (spacer) (deflections) nut preloading Zerobacklash:Iteliminatesaxialplaybetwee nascrewshaftandaballnut. Itminimizeselasticdeformationcausedbyext ernalforce,thustherigidityenhances. Incasemountingerrors,misalignmentbetween thescrewshaftandthenutmayoccurthisfurthe rgeneratesdistortionforces.

4 Thiscouldleadtotheproblemssuchas,Shorten edservicelifeAdverseeffectonsmoothoperat ionReducedpositioningaccuracyGenerationo fnoiseorvibrationBreakageofscrewshaft9 Advantagesofballscrews Highlyefficientandreliable. Lessstartingtorque. Lowercoefficientoffrictioncomparedtoslid ingtypescrewsandrunatcoolertemperatures Powertransmissionefficiencyisveryhighand isoftheorderof95%. Couldbeeasilypreloadedtoeliminatebacklas h. Thefrictionforceisvirtuallyindependentof thetravelvelocityandthefrictionatrestisv erysmall;consequently,thestick-slippheno menonispracticallyabsent,ensuringuniform ityofmotion. Haslongerthreadlifehenceneedtobereplaced lessfrequently. Ballscrewsarewell-suitedtohighthroughout put,highspeedapplicationsorthosewithcont inuousorlongcycletimes.

5 Tendtovibrate. Requireperiodicoverhaulingtomaintainthei refficiency. Inclusionofdirtorforeignparticlesreduces thelifeofthescrews. Notasstiffasotherpowerscrews,thusdeflect ionandcriticalspeedcancausedifficulties. Theyarenotself-lockingscrewshencecannotb eusedinholdingdevicessuchasvices. Ballscrewsareemployedincuttingmachines,s uchasmachiningcenterandNClathewhereaccur atepositioningofthetableisdesired Usedintheequipment'ssuchaslithographiceq uipmentorinspectionapparatuswhereprecise positioningisvital Highprecisionballscrewsareusedinsteppers forsemiconductormanufacturingindustriesf orprecisionassemblyofmicroparts. Usedinroboticsapplicationwhereprecisionp ositioningisneeded.

6 Usedinmedicalexaminationequipment' Constructionaldetails: Basicallyconventionalmachinehave2axes,kn ownasX&Yaxis. ThereisalsoaZaxislongwhichonlythebedmove svertically. , CNCmachineshavemorerigidconstructionwhen comparedtotheconventionalmachine. Theslideways, Recirculatingballleadscrewsandantifricti onslideways Theslidewaysonaconventionalmachineoperat eundertheconditionsofslidingfriction. TheleadscrewsareusuallyoftheAcmethreadfo rm, , Rollingfrictioncanbeusedinsteadofsliding friction,wherere-circulatingrollerbearin gsarepositionedundertheslideways. Arecirculatingballleadscrew, ,lessfrictionalresistance,lowertorquereq uired,moreprecisepositioningofslides, Use of Stepping Motors in Slide Movement Adigitalsignalissentfromthecontrollertot hemotorintheformofpulses,whichwillcauset hemotortorotatethroughaspecifiedangle, :Iffivedigitalpulsesaresenttothestepperm otorthenitwillrotatebyfivesteps, ,thereisnoneedforpositionalorvelocityfee dback18 MAJORCOMPONENTSRELATEDTOCNCMACHINETOOLSA nyCNCmachinetoolessentiallyconsistsofthe followingparts: Partprogram: Aseriesofcodedinstructionsrequiredtoprod uceapart.

7 Controlsthemovementofthemachinetoolandon /offcontrolofauxiliaryfunctionssuchasspi ndlerotationandcoolant. Thecodedinstructionsarecomposedofletters , Programinputdevice Theprograminputdeviceisthemeansforpartpr ogramtobeenteredintotheCNCcontrol. Threecommonlyusedprograminputdevicesarep unchtapereader,magnetictapereader, Machine Control UnitThe machine control unit (MCU) is the heart of a CNC system. It is used to perform the following functions: To read the coded instructions. To decode the coded instructions. To implement interpolations ( linear , circular, and helical) to generate axis motion commands. To feed the axis motion commands to the amplifier circuits for driving the axis mechanisms.

8 To receive the feedback signals of position and speed for each drive axis. To implement auxiliary control functions such as coolant or spindle on/off and tool MachineTool CNCcontrolsareusedtocontrolvarioustypeso fmachinetools. Regardlessofwhichtypeofmachinetooliscont rolled,italwayshasaslidetableandaspindle tocontrolpositionandspeed. ThemachinetableiscontrolledintheXandYaxe s, FeedBackSystem Thefeedbacksystemisalsoreferredtoastheme asuringsystem. Itusespositionandspeedtransducerstoconti nuouslymonitorthepositionatwhichthecutti ngtoolislocatedatanyparticularinstant. DriveSystem Drivesareusedtoprovidecontrolledmotionto CNCelements Adrivesystemconsistsofamplifiercircuits, drivemotors,andballlead-screws.

9 TheMCUfeedsthecontrolsignals(positionand speed)ofeachaxistotheamplifiercircuits. Inmachinetools,powerisgenerallyrequiredf ordrivingthemainspindle,saddlesandcarria gesandtosomeauxiliaryunits. ThemotorsusedforCNCsystemareoftwokinds Electrical-AC,DCorSteppermotors Fluid-HydraulicorPneumatic InCNC,usuallystepperandservoelectricaldr ivesareused Theyexhibitfavourabletorque-speedcharact eristicsandarerelativelyinexpensive. POWER DRIVES25 ,itiswidelyusedinlowcost, :oPermanentMagnetEmploypermanentmagnetLo wspeed,relativelyhightorqueoVariableRelu ctanceDoesnothavepermanentmagnetLowtorqu e26 Rotorisapermanentmagnet. (PM)steppermotor to15 Figureshowsasimple,90 PMmotorwithfourphases(A-D).

10 Applyingcurrenttoeachphaseinsequencewill causetherotortorotatebyadjustingtothecha ngingmagneticfields. Althoughitoperatesatfairlylowspeed, ,3,1,andsoon,themotorwillrotateina30 stepangle. Inthenon-energizedcondition,thereisnomag neticfluxintheairgap,asthestatorisanelec tromagnetandtherotorisapieceofsoftiron;h ence, Variable reluctance stepper motorVariableReluctanceMotor Thecylindricalrotorismadeofsoftsteelandh asfourpoles Ithasfourrotorteeth,90 apartandsixstatorpoles,60 apart. Hybridsteppingmotorscombineapermanentmag netandarotorwithmetalteethtoprovidefeatu resofthevariablereluctanceandpermanentma gnetmotorstogether. Thenumberofrotorpolepairsisequaltothenum berofteethononeoftherotor' stepper29 Rotationofahybridsteppingmotorisproduced inthesimilarfashionasapermanentmagnetste ppingmotor,byenergizingindividualwinding sinapositiveornegativedirection.