《机械专业英语》课件第21章

Unit21TEXT

LEARNTOSPEAKNEWWORDSANDEXPRESSIONSNOTESEXERCISES

READINGMATERIALPROFESSIONALWORDSANDPHRASES ComputerAidedDesign

ACADsystemisbasicallyadesigntoolinwhichthecomputerisusedtoanalyzevariousaspectsofadesignedproduct.TheCADsystemsupportsthedesignprocessatalllevels—conceptual,preliminary,andfinaldesign.Thedesignercanthentesttheproductinvariousenvironmentalconditions,suchastemperaturechanges,orunderdifferentmechanicalstresses.

AlthoughCADsystemsdonotnecessarilyinvolvecomputergraphics,thedisplayofthedesignedobjectonascreenisoneofthemostvaluablefeaturesofCADsystems.Thepictureoftheobjectisusuallydisplayedonthesurfaceofacathode-raytube(CRT).Computergraphicsenablesthedesignertostudytheobjectbyrotatingitonthecomputerscreen,separatingitintosegments,enlargingaspecificportionoftheobjectinordertoobserveitindetail,andstudyingthemotionofmechanismswiththeaidofkinematicprograms.

MostCADsystemsareusinginteractivegraphicssystems.Interactivegraphicsallowstheusertointeractdirectlywiththecomputerinordertogeneratemanipulate,andmodifygraphicdisplays.Interactivegraphicshasbecomeavaluabletool,ifnotanecessaryprerequisite,ofCADsystems.

TheendproductsofmanyCADsystemsaredrawingsgeneratedonaplotterinterfacedwiththecomputer.OneofthemostdifficultproblemsinCADdrawingsistheeliminationofhiddenlines.Thecomputerproducesthedrawingaswireframediagram.Sincethecomputerdefinestheobjectwithoutregardtoone’sperspective,itwilldisplayalltheobject’ssurfaces,regardlessofwhethertheyarelocatedonthesidefacingtheviewerorontheback,whichnormallytheeyecannotsee.

Variousmethodsareusedtogeneratethedrawingofthepartonthecomputerscreen.Onemethodistouseageometricmodelingapproach,inwhichfundamentalshapesandbasicelementsareusedtobuildthedrawing.Thelengthsandradiusoftheelementscanbemodified.Forexample,acylinderisabasicelement,thesubtractionofacylinderwithaspecificradiusandlengthwillcreateholeinthedisplayedpart.Eachvariation,however,maintainstheoverallgeometryofthepart.

OtherCADsystemsusegrouptechnology(GT)inthedesignofparts.Grouptechnologyisamethodofcodingandgroupingpartsonthebasisofsimilaritiesinfunctionorstructureorinthewaystheyareproduced.Applicationofgrouptechnologycanenableacompanytoreducethenumberofpartsinuseandtomaketheproductionofpartsandtheirmovementintheplantefficient.

RecentlyCADsystemsareusingthefinite-elementmethod(FEM)ofstressanalysis.Bythisapproachtheobjecttobeanalyzedisrepresentedbyamodelconsistingofsmallelements,eachofwhichhasstressanddeflectioncharacteristics.Theanalysisrequiresthesimultaneoussolutionofmanyequations;ataskwhichisperformedbythecomputer.Thedeflectionsoftheobjectcanbedisplayedonthecomputerscreenbygeneratinganimationofthemodel.

Withanyofthesemethods,orotherswhichareused,theCADsystemgeneratesatthedesignstageasinglegeometricdatabasewhichcanbeusedinallphasesofthedesignandlaterinthemanufacturing,assemble,andinspectionprocesses.Fig.21-1showsanexampleofvarioustypesofmodelingforCAD.Fig.21-1VarioustypesofmodelingforCAD LEARNTOSPEAK

Mr.Zhang: Hello,Mr.Cheng.

Mr.Cheng: Hi,Mr.Zhang.

Mr.Zhang: IwanttoknowsomeknowledgeoftheCAD.Couldyoutellmesomeabout

it?

Mr.Cheng: Noproblem.Whatdoyouwanttoknow?

Mr.Zhang: WhatdoestheCADmean?

Mr.Cheng: Computeraideddesign(CAD)meanstheuseofacomputertoassistinthedesignofanindividualpartorasystem.Thedesignprocessusuallyinvolvescomputergraphics.

Mr.Zhang: WhatistheCADsystem?

Mr.Cheng: Itisbasicallyadesigntoolinwhichthecomputerisusedtoanalyzeaspectsofadesignedproduct.

Mr.Zhang: WhatcantheCADsystemsupport?

Mr.Cheng: Itcansupportthedesignprocessatalllevels:conceptual,preliminary,andfinaldesign.

Mr.Zhang: CanthedesignertesttheproductbytheCADsystem?

Mr.Cheng: Yes.Itcando.

Mr.Zhang: WhatisoneofthemostvaluablefeaturesofCADsystems?

Mr.Cheng: Thedisplayofthedesignedobjectonascreen.

Mr.Zhang: WhatsystemsarethemostCADsystemsusing?

Mr.Cheng: Interactivegraphicssystems.

Mr.Zhang: DoyouthinktheeliminationofhiddenlinesisoneofthemostdifficultproblemsinCADdrawings?

Mr.Cheng: Yes,Ithinkso.

Mr.Zhang: Oh,Ihavelearnedsomuchfromyou.Thanksamillion.

Mr.Cheng: Atyourservice.NEWWORDSANDEXPRESSIONS

conceptual [kEn5septjuEl]adj.概念的，构想的

preliminary [prI5lImInErI]adj.预备的，初步的

graphics [5^rAfIks]n.制图学，图形

segment [5se^mEnt]n.段，节

environmental [In

vaIErEn

ment(E)l] adj.周围的，环境的

subtraction [sEb5trAkF(E)n] n.减少

perspective [pE5spektIv] n.透视，透视法，透视画

geometric [dVIE5metrIk] adj.几何的，几何学的

cylinder [5sIlIndE(r)] n.圆筒，圆柱体

animation [AnI5meIF(E)n] n.动画片，动画

computeraideddesign(CAD) 计算机辅助设计

cathode-raytube(CRT) 阴极射线管

regardlessof 不管，不顾

grouptechnology(GT) 成组技术

finite-elementmethod(FEM) 有限元方法 NOTES

1．Sincethecomputerdefinestheobjectwithoutregardtoone’sperspective,itwilldisplayalltheobject’ssurfaces,regardlessofwhethertheyarelocatedonthesidefacingtheviewerorontheback,whichnormallytheeyecannotsee.由于计算机定义物体时没有考虑其透视效果，因此它会显示出物体的所有面，而不管这些面是位于面向观察者的一面，还是位于人眼通常无法看到的背面。

此句中，since引导原因状语从句。since还可引导时间状语从句，要注意区分。

2．Grouptechnologyisamethodofcodingandgroupingpartsonthebasisofsimilaritiesinfunctionorstructureorinthewaystheyareproduced.成组技术是在零件具有相同功能、结构或加工方法的基础上对零件分组编码的一种方法。

此句中，“成组技术是一种方法”是句子的主干，ofcodingandgroupingpartsonthebasisofsimilaritiesinfunctionorstructureorinthewaystheyareproduced是介词短语做定语，修饰method。 EXERCISES

1．DiscussthefollowingquestionsinEnglish:

(1) WhatdoesCADmean?Whichworkcanitdo?

(2) WhatdoesCAMmean?Whichworkcanitdo?

(3) Ifwewanttousecomputertodesignaproduct,whichworkdowehavetodo?

2．UseEnglishforreplacingChineseoruseChineseforreplacingEnglish.

(1)计算机辅助设计技术是一种先进的设计技术。

(2)计算机辅助制造可以大大提高产品质量。

(3)成组技术是一种有效的机械加工技术。

(4) Computeraidedprocessplanning(CAPP)isanefficientproductmachiningmethod.

(5) Computeraidedtesting(CAT)isahighefficientlyproductmeasuringmethod.

(6) WecanrotatetheobjectonthecomputerscreenwhenweuseCADtodesignaproduct.

3．ChoosethetermwhichbestmatchesthedefinitiongiveninColumn2fromColumn1.

Column1

(1) computergraphics

(2) CRT

(3) Interactivegraphic

(4) Thecomputerdefinestheobjectwithoutregardtoone’sperspective

(5) Usingthefinite-elementmethod

(6) Computeraidedmanufacturing

Column2

(a) itwilldisplayalltheobjectsurfaces.

(b) theobjecttobeanalyzedisrepresentedbyamodelconsistingofsmallelements.

(c) thedisplayofthedesignedobjectonascreen.

(d) allowtheusertointeractdirectlywiththecomputer.

(e) meanstheuseofacomputertoassistinthemanufactureofapart.

(f) cathode-raytube.

4．TranslatethefollowingpassageintoChinese:

CADsystemcancreatedanddisplayedgraphicinformationonacathode-raytube(CRT),anditallowsthedesignertoproducefinishedengineeringdrawingsfromsinglepensilesketchesorfrommodelsandmodifythesedrawingsonthescreeniftheydonotseemfunctional.Fromthree-drawing,orthographicviews,thedesignercantransformdrawingsintoathree-dimensionalviews.Thisenablesadesignertoredesignapartonthescreen,projecthowthepartwilloperateinuse,andmakesuccessivedesignchangesinamatterofminutes.READINGMATERIAL

CAD/CAMSystems

Computeraideddesign(CAD)meanstheuseofacomputertoassistinthedesignofanindividualpartorasystem,suchasanaircraft.Thedesignprocessusuallyinvolvescomputergraphics.

Computeraidedmanufacturing(CAM)meanstheuseofacomputertoassistinthemanufactureofapart.CAMcanbedividedintotwomainclasses:(1)On-lineapplications,namely,theuseofthecomputertocontrolmanufacturingsystemsinrealtime,suchasCNCandACsystemsofmachinetools.(2)Off-lineapplications,namely,theuseofthecomputerinproductionplanningandnon-real-timeassistanceinthemanufacturingofparts.

CAD/CAMisaunifiedsoftwaresystem,inwhichtheCADportionisinterfacedinsidethecomputerwiththeCAMsystem,shownasFig.21-2andFig.21-3.TheendresultofcurrentCAD/CAMsystemsisusuallyapartprogramintheformofalistorapunchedtape.InadvancedCAD/CAMsystemspartprogramscanbedirectlyfedintothecontrolcomputersofCNCmachinesandinspectionstations.Fig.21-2InformationflowchartinCAD/CAMapplicationFig.21-3TheblockdiagramofaCAD/CAMsystem

ThemainconceptofCAD/CAMsystemsisthegeneratingofacommondatabasewhichisusedforallthedesignandmanufacturingactivities.Theseincludespecificationsoftheproduct,conceptualdesign,finaldesign,drafting,manufacturing,andinspection.Ateachstageofthisprocess,datacanbeadded,modified,used,anddistributedovernetworksofterminalsandcomputers.Thesingledatabaseprovidesasubstantialreductioninhumanerrorsandasignificantshorteningofthetimerequiredfromtheintroductionofaconceptofaproducttothemanufacturingofthefinalphysicalproduct.

Thesizeandcapabilityoftherequiredcomputersystemdependonthecomplexityoftheproduct.Intheaerospaceandaeronauticsindustry,whereacompleteaircraftcanbedesignedwithaCAD/CAMprocessor,thesystemmustaccommodatenewdataandchangesindataarrivingfromavarietyofusers.Thereforethesesystemsmusthaveastrongdatamanagementcapability.Bycontrast,ifacompanydesignedsimpleproducts,therequiredCAD/CAMsystemwouldneedonlyonecomputerterminal.TodaythemajorusersofCAD/CAMsystemsaretheaerospaceandautomotiveindustries,butthedecliningpriceofthesesystemsenlargethenumberofotherusers.

AdvancedCAD/CAMsyst