自动化课后答案ch07 nc3esi.docx
《自动化课后答案ch07 nc3esi.docx》由会员分享,可在线阅读,更多相关《自动化课后答案ch07 nc3esi.docx(50页珍藏版)》请在冰豆网上搜索。
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