自动化课后答案ch07 nc3esi.docx

自动化课后答案ch07 nc3esi.docx

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自动化课后答案ch07nc3esi

Chapter7

NUMERICALCONTROL

REVIEWQUESTIONS

7.1Whatisnumericalcontrol?

Answer:

Asdefinedinthetext,numericalcontrol（NC）isaformofprogrammableautomationinwhichthemechanicalactionsofamachinetoolorotherequipmentarecontrolledbyaprogramcontainingcodedalphanumericdata.

7.2WhatarethethreebasiccomponentsofanNCsystem?

Answer:

Thethreecomponentsare

（1）thepartprogramofinstructions,

（2）themachinecontrolunit,and（3）theprocessingequipment（e.g.,machinetool）thataccomplishestheoperation.

7.3Whatistheright-handruleinNCandwhereisitused?

Answer:

Theright-handruleisusedtodistinguishpositiveandnegativedirectionsfortherotationalaxesinNC.Usingtherighthandwiththethumbpointinginthepositivelinearaxisdirection（x,y,orz）,thefingersofthehandarecurledinthepositiverotationaldirectionforthea,b,andcaxes.

7.4Whatisthedifferencebetweenpoint-to-pointandcontinuouspathcontrolinamotioncontrolsystem?

Answer:

Point-to-pointsystemsmovetheworktabletoaprogrammedlocationwithoutregardforthepathtakentogettothatlocation.Bycontrast,continuouspathsystemsarecapableofcontinuoussimultaneouscontroloftwoormoreaxes,thusprovidingcontrolofthetooltrajectoryrelativetotheworkpart.

7.5Whatislinearinterpolation,andwhyisitimportantinNC?

Answer:

Linearinterpolationisthecapabilitytomachinealongastraight-linetrajectorythatmaynotbeparalleltooneoftheworktableaxes.ItisimportantinNCbecausemanyworkpiecegeometriesrequirecutstobemadealongstraightlinestoformstraightedgesandflatsurfaces,andtheanglesofthelinesarenotbeparalleltooneoftheaxesinthecoordinatesystem.

7.6Whatisthedifferencebetweenabsolutepositioningandincrementalpositioning?

Answer:

Inabsolutepositioning,theworkheadlocationsarealwaysdefinedwithrespecttotheoriginoftheNCaxissystem.Inincrementalpositioning,thenextworkheadpositionisdefinedrelativetothepresentlocation.

7.7Howiscomputernumericalcontrol（CNC）distinguishedfromconventionalNC?

Answer:

CNCisanNCsystemwhosemachinecontrolunitisadedicatedmicrocomputerratherthanahard-wiredcontroller,asinconventionalNC.

7.8NamefiveofthetenfeaturesandcapabilitiesofamodernCNCmachinecontrolunitlistedinthetext.

Answer:

Thetenfeaturesandcapabilitiesidentifiedinthetextare

（1）storageofmorethanonepartprogram,

（2）variousformsofprograminput,suchaspunchedtape,magnetictape,floppydiskette,RS-232communicationswithexternalcomputers,andmanualdatainput,（3）programeditingatthemachinetool,（4）fixedcyclesandprogrammingsubroutines（macros）,（5）linearandcircularinterpolation,（6）workpiecepositioningfeaturesforsetup,（7）cutterlengthandsizecompensation,（8）accelerationanddecelerationcalculationswhenthecutterpathchangesabruptly,（9）communicationsinterface,and（10）diagnosticstodetectmalfunctionsanddiagnosesystembreakdowns.

7.9Whatisdistributednumericalcontrol（DNC）?

Answer:

DistributednumericalcontrolisadistributedcomputersysteminwhichacentralcomputercommunicateswithmultipleCNCmachinecontrolunits.Itevolvedfromdirectnumericalcontrolinwhichthecentralcomputerplayedtheroleofthetapereader,downloadingpartprogramsoneblockatatime.InamoderndistributedNCsystem,entirepartprogramsaredownloadedtotheMCUs.Alsoshopfloordataiscollectedbythecentralcomputertomeasureshopperformance.

7.10Whataresomeofthemachinetooltypestowhichnumericalcontrolhasbeenapplied?

Answer:

NChasbeenappliedtonearlyallmachinetoolstypes,includinglathes,boringmills,drillpresses,millingmachines,andcylindricalgrinders.

7.11Whatisamachiningcenter?

Answer:

Asdefinedinthetext,amachiningcenterisamachinetoolcapableofperformingmultiplemachiningoperationsonasingleworkpieceinonesetup.Theoperationsinvolverotatingcutters,suchasmillinganddrilling,andthefeaturethatenablesmorethanoneoperationtobeperformedinonesetupisautomatictool-changing.

7.12Namefourofthesixpartcharacteristicsthataremostsuitedtotheapplicationofnumericalcontrollistedinthetext.

Answer:

Thesixpartcharacteristicsidentifiedinthetextarethefollowing:

（1）batchproduction,

（2）repeatorders,（3）complexpartgeometry,（4）muchmetalneedstoberemoved,（5）manyseparatemachiningoperationsonthepart,and（6）thepartisexpensive.

7.13AlthoughNCtechnologyismostcloselyassociatedwithmachinetoolapplications,ithasbeenappliedtootherprocessesalso.Namethreeofthesixexampleslistedinthetext.

Answer:

Thesixnon-machinetoolapplicationslistedinthetextare

（1）electricalwire-wrapmachines,

（2）componentinsertionmachines,（3）draftingmachines（x-yplotters）,（4）coordinatemeasuringmachines,（5）tapelayingmachinesforpolymercomposites,and（6）filamentwindingmachinesforpolymercomposites.

7.14Whatarefouradvantagesofnumericalcontrolwhenproperlyappliedinmachinetooloperations?

Answer:

Thetextliststhefollowing11advantages:

（1）nonproductivetimeisreduced,

（2）greateraccuracyandrepeatability,（3）lowerscraprates,（4）inspectionrequirementsarereduced,（5）more-complexpartgeometriesarepossible,（6）engineeringchangescanbeaccommodatedmoregracefully,（7）simplerfixturesareneeded,（8）shortermanufacturingleadtimes,（9）reducedpartsinventory,（10）lessfloorspacerequiredduetofewermachines,and（11）operatorskillrequirementsarereduced.

7.15WhatarethreedisadvantagesofimplementingNCtechnology?

Answer:

Fourdisadvantagesareidentifiedinthetext:

（1）higherinvestmentcostbecauseNCmachinesaremoreexpensivethanconventionalmachinetools,

（2）highermaintenanceeffortduetogreatertechnologicalsophisticationofNC,（3）partprogrammingisrequired,and（4）equipmentutilizationmustbehightojustifythehigherinvestment,andthismightmeanadditionalworkshiftsarerequiredinthemachineshop.

7.16BrieflydescribethedifferencesbetweenthetwobasictypesofpositioningcontrolsystemsusedinNC?

Answer:

ThetwotypesofpositioningcontrolsystemsusedinNCsystemsareopenloopandclosedloop.Anopen-loopsystemoperateswithoutverifyingthattheactualpositionachievedinthemoveisthesameastheprogrammedposition.Aclosed-loopsystemusesfeedbackmeasurementstoconfirmthatthefinalpositionoftheworktableisthelocationspecifiedintheprogram.

7.17Whatisanopticalencoder,andhowdoesitwork?

Answer:

Anopticalencoderisadeviceformeasuringrotationalspeedthatconsistsofalightsourceandaphotodetectoroneithersideofadisk.Thediskcontainsslotsuniformlyspacedaroundtheoutsideofitsface.Theseslotsallowthelightsourcetoshinethroughandenergizethephotodetector.Thediskisconnectedtoarotatingshaftwhoseangularpositionandvelocityaretobemeasured.Astheshaftrotates,theslotscausethelightsourcetobeseenbythephotocellasaseriesofflashes.Theflashesareconvertedintoanequalnumberofelectricalpulses.

7.18WithreferencetoprecisioninanNCpositioningsystem,whatiscontrolresolution?

Answer:

Controlresolutionisdefinedasthedistanceseparatingtwoadjacentaddressablepointsintheaxismovement.Addressablepointsarelocationsalongtheaxistowhichtheworktablecanbespecificallydirectedtogo.Itisdesirableforcontrolresolutiontobeassmallaspossible.

7.19Whatisthedifferencebetweenmanualpartprogrammingandcomputer-assistedpartprogramming?

Answer:

Inmanualpartprogramming,theprogrammerpreparestheNCcodeusingalow-levelmachinelanguage.Incomputer-assistedpartprogramming,thepartprogramiswrittenusingEnglish-likestatementsthataresubsequentlyconvertedintothelow-levelmachinelanguage.

7.20Whatispostprocessingincomputer-assistedpartprogramming?

Answer:

PostprocessingconvertsthecutterlocationdataandmachiningcommandsintheCLDATAfileintolow-levelcodethatcanbeinterpretedbytheNCcontrollerforaspecificmachinetool.TheoutputofpostprocessingisapartprogramconsistingofG-codes,x-,y-,andz-coordinates,S,F,M,andotherfunctionsinwordaddressformat.Auniquepostprocessormustbewrittenforeachmachinetoolsystem.

7.21WhataresomeoftheadvantagesofCAD/CAM-basedNCpartprogrammingcomparedtocomputer-assistedpartprogramming?

Answer:

ThetextliststhefollowingadvantagesofCAD/CAMNCpartprogramming:

（1）thepartprogramcanbesimulatedoff-linetoverifyitsaccuracy;

（2）thetimeandcostofthemachiningoperationcanbedetermined;（3）themostappropriatetoolingcanbeautomaticallyselectedfortheoperation;（4）theCAD/CAMsystemcanautomaticallyinserttheoptimumvaluesforspeedsandfeeds;（5）inconstructingthegeometryorthetoolpath,theprogrammerreceivesimmediatevisualfeedbackontheCAD/CAMmonitor;（6）theCADdatabasecontainingthepartdesigncanbeusedtoconstructthetoolpathratherthanredefiningthepartgeometry;and（7）someofthestepsinthetoolpathconstructioncanbeautomated.

7.22WhatismanualdatainputoftheNCpartprogram?

Answer:

ManualdatainputiswhenthemachineoperatormanuallyentersthepartprogramdataandmotioncommandsdirectlyintotheMCUpriortorunningthejob.

PROBLEMS

NCApplications

7.1AmachinablegradeofaluminumistobemilledonanNCmachinewitha20mmdiameterfour-toothendmillingcutter.Cuttingspeed=120m/minandfeed=0.008mm/tooth.Convertthesevaluestorev/minandmm/rev,respectively.

Solution:

N=

=1909.9rev/min