Unit 2CAD and CAM.docx

Unit 2CAD and CAM.docx

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Unit2CADandCAM

Unit2CADandCAM

CAD/CAM

Computer-aideddesignandcomputer-aidedmanufacturingusedigitalcomputers,whichtheirhighspeedandaccuracy,astheintegratingforcethroughouttheentireprocessfromengineeringdesigntoproductmanufacture.CAD/CAMevolvesfromandbringstogethersuchtechnologiesasnumericalcontrolandcomputertechnology.Tiedtogetherbythecommonusageofdigitaldata,andfedbyacontinuousstreamofelectronicsdevelopments,numericalcontrolandthedigitalcomputerhavereachedthematuritytopermitalevelofintegrationthatbringsustothethresholdofthelong-envisionedautomaticfactory.

ThevariouscomputerizedfunctionsunderCAD/CAMfallintothreegeneralareas:

1.design/drafting,orcomputer-aidedengineering

2.Planning/scheduling,ormanagementinformationsystems

3.fabrication,ormanufacturingautomation

AconceptualizedoridealizedCAD/CAMsystemisshowninFig.6-1.Thisshowshowtheuserinteractswiththecomputerviaagraphicsterminal,designingandcontrollingthemanufacturingprocessfromstarttofinishwithinformationstoredinashareddatabase.Withtheadventofinteractivegraphics,theproblemoftheuserenteringdataandinstructionsintothecomputerwithstacksofcodedpunchedcardsorreelsoftapewentaway.Nolongerwastheuserrequiredtobeexperiencedincomputerprogrammingandoperationtousethemachine.Withinteractivegraphics,theusercommunicateswiththecomputerindisplay-screenpictures.Noknowledgeofcomputersisrequiredtooperatethesesystems,andthecommunicationisinrealtime,whichmeansthatthecomputer’sresponsetotheuser’sinstructionsisalmostinstantaneous.InCAD/CAM,therequirementisforthesolutionofthree-dimensionalmechanicaldesignandmanufacturingproblems.Interactingwiththecomputerviakeyboardandlightpenorotherpencil-likedevices,thedesignerspecifiespointsandlinesonthedisplayscreentoquicklyconstructadisplayscreendrawingormodel.Thisisinrealitytherepresentationofthediagram-storedinthecomputerdatabase.

Withastrokeofthepen,orbypushingabutton,thedesignercanmove,magnify,rotate,flip,copy,orotherwisemanipulatetheentiredesignoranypartofit.Forexample,bypushingabuttonthedesignermayissueaflipcommandtoproduceamirrorimageofadisplayformodelingsymmetricalparts,oratranslatecommandmaybeusedtocreatemodelsforlinearparts.Acrosssectionmaybedefinedandthentranslatedlinearlytocreateasurfacedmodel.Inasimilarmanner,circularpartscanbeeasilymodeledwitharotatecommandinwhichacrosssectionisrotatedaboutacentralaxistomodelparts.Theseoperationstakeadvantageofthecomputer’scapabilitytorepeatdetailedoperationsquicklyandflawlessly.

Inworkingwithacomplexmodel,thedesignercantemporarilyeraseportionsofitfromthescreentoseetheareaunderconstructionmoreclearly.Thedeletedareaisrecalledlatertocompletethemodel.Portionsofthemodelmaybeenlargedtoviewandaddminutedetailsaccurately.Themodelmaybemovedandrotatedonthescreenforviewingatanyangle.Mechanismssuchaslinkagesandgearsintheassemblymaybeanimatedonthescreentoensurepropercheckforinterferencesandeaseofoperation.Whenthedesigniscomplete,thesystemmaythenbeusedtoproducedetailedengineeringdrawings.

Afterpartgeometryisdefinedwiththecompletedmodel,thedesignercanhavethecomputercalculatephysicalpropertiessuchasweight,volume,surfacearea,momentofinertia,andcenterofgravity.Thefinite-elementmethodmaybeusedtodeterminethestress,deflections,andotherstructuralcharacteristics.Aftersuchananalysis,thedisplayscreenmayshowcolor-codedstressplots,thedeflectedshapeofapartsubjectedtoagivenload,orananimatedmodeshapethatshowshowthestructuremightvibrateanddeforminoperation.

TheresultisthatwithaCAD/CAMsystemengineersanddesignerscanviewcomplexformsfromvariousanglesatthepushofabuttoninsteadofhavingtoconstructcostly,time-consumingmodelsandmock-ups.Changescanbemadequicklyandinexpensivelyatthekeyboardwithoutrequiringdrawingchanges,ormodificationofaphysicalmodel.Inaddition,thecomputerdisplayscanproducerealisticsimulationsofproductoperationbeforeanyhardwareisactuallyproduced.

Afterdesigncompletion,theresultantgeometricdatastoredinthecomputermemorymaybeusedtoproducenumericalcontrolinstructionsformakingthepartsonautomatedmachinetools,ortogeneratetheartworkanddrilltapesforautomatedprintedcircuitcardfabrication.ManyCAD/CAMsystemcannowproduceNCinstructionsautomaticallyforarangeofdifferentparttypes.Toolpathsmaybesimulatedonthemonitortovalidate,verify,andmodifytheprogrammorerapidly.

Anengineeratacomputer-aideddesigngraphicsterminalcandesignapart,analyzestressesanddeflections,studypartdynamics,andthenproduceengineeringdrawingsautomatically.Fromthegeometricdescriptionprovidedbycomputer-aideddesign,manufacturingengineersmayproducenumericalcontrolinstructions,generateprocessplansandworkinstructions,programrobots,andprovideplantmanagementinformationwithacomputer-aidedmanufacturingsystem.WithanintegratedCAD/CAMsystem,adesigniscreatedandthemanufacturingprocessiscontrolledandexecutedwithasinglecomputersystem.Thesehighlysophisticatedsystemsarebecomingawayoflifeatincreasingnumbersofplantsthroughouttheindustrializedworld.AsCAD/CAMcapabilitieshaveincreased,costsforthesesystemshavedecreasedtothepointwhereacompanycaninstallasystemforafewhundredthousanddollars,whichputsthemwithinthebudgetofmanymanufacturers.

Insummary,CAD/CAMiscomprisedofdistinctfunctionalareas.TheexpertsgroupCADfunctionsintofourmajorcategories:

1.geometricmodeling

2.engineeringanalysis

3.kinematics

4.automateddrafting

CAMtechnologyalsocentersaroundfourmajorareasorcategories:

1.numericalcontrol

2.processplanning

3.robotics

4.factorymanagement

ForCAD/CAMtorealizeitsfullpotential,thetechnologyoftheCAMfunctionsmustbeadvancedtothelevelofCADsophisticationandCAD/CAMfunctionsmustbecombinedintoatrulyintegratedsystem.Effortstodothisareunderway,andthetechnologyischangingsorapidlythatitisdifficulttoremainabreastofthestateoftheartwithoutacontinuingandsustainedeffort.

numericalcontrol数字控制,数控

thresholdn.门槛,临界,界限

envisionvt.想象,预见,展望

Computer-aidedengineering计算机辅助工程

managementinformationsystem管理信息系统

conceptualizevt.构思,形成概念,是概念化

graphicsterminal图形终端

sharen.份,均分

database数据库

adventn.到来,出现,来临

interactivegraphics交互式图形学

stackn.堆,一堆,堆栈

reeln.卷盘,盘架,磁带盘

instantaneousadj.瞬间的,即刻的,即时的

flipvi.交换,翻转,翻动书页

flawlesslyadv.无瑕疵地,无缺点地,无裂缝地

animatevt.动画制作

momentofinertia转动惯量

finite-elementn.有限元

mock-upn.实体模型,实物的原尺寸模型

resultantadj.作为结果而发生的,合成的

artworkn.工艺图,布线图,照相原图图纸

roboticsn.机器人学

Computeranddesign

Thecomputerhashadamajorimpactonthewayeverydaytasksassociatedwithdesignareaccomplished.Itcanbeusedinmanywaystodomanythings.However,alldesigntasksaccomplishedusingacomputerfallintooneoffourbroadcategories:

1.designmodeling

2.designanalysis

3.designreview

4.designdocumentation

Designmodeling

InCAD/CAMdesignmodeling,ageometricmodelofaproductisdevelopedthatdescribesthepartmathematically.Thismathematicaldescriptionisconvertedtographicformanddisplayonacathoderaytube（CRT）.Thegeometricmodelalsoallowsthegraphicimagetobeeasilyeditedandmanipulatedoncedisplayed.

TheprocessbeginswhenadesignercreatesagraphicimageusingaspecialICGsoftwarepackage.Theimageiscreatedbyenteringcommandsthatcausethecomputertoconstructtheimageoutofpoints,lines,circles,andcurves.Tocreateagraphicimage,thecomputermusttranslatethegeometriccharactersintoacorrespondingmathematicalmodel.Whatappearstothedesignerasagraphicimageisstoredinthecomputerasaseriesofmathematicalcoordinates.

Asthedesignerissuescommandstoeditormanipulateagraphicimage,thecomputerrevisesthemathematicalmodel.Thecomputermustfirstchangethegeometricmodelbeforeitcanchangethegraphicimage,anditmustcalculatethemathematicalcoordinatesofageometriccharacterbeforeitcandisplaythatcharacter.

Graphicimagesaredisplayedinoneofthreeformats:

1.two-dimensionalor2-D

2.two-and-a-half-dimensionalor21/2-D

3.three-dimensionalor3-D

A2-Dgraphicimageshowsanorthographic（flat）representationandusuallyshowstwoormoreviews.A21/2-Dgraphicimageisanobliquerepresentation.A3-Dgraphicimagemaybeawireframemodeloratrue3-Dsolidmodel.

Communicationcanbeenhancedfurtherwhengraphicimagesaredisplayincolor.ManycapabilitiesofCAD/CAMincludingfinite-elementanalysis,wireframe3-D,andsolidsmodeling,aremoreeasilyunderstoodwhenthegraphicimagesthatresultaredisplayedincolor.Specificapplicationssuchaspipingdrawingsarealsoeasiertoreadwhendisplayedinanappropriatevarietyofcolors.

Designanalysis

Thecomputerhassimplifiedthedesignanalysisstageofthedesignprocesssignificantly.Onceaprocessdesignhasbeendeveloped,itisnecessarytoanalyzehowitwillstanduptotheconditionstowhichitwillbesubjected.Suchanalysismethodsasheattransferandstress-straincalculationsaretimeconsumingandcomplex.WithCAD/CAM,specialcomputerprogramswrittenspecificallyforanalysispurposesareavailable.

Onesuchprogramthathassimplifiedthea