mg university S5 sem computer aided design and manufacturering (CAD/cam) note

1. COMPUTERAIDEDDESIGNAND
MANUFACTURING

2. 2
B.TECH.DEGREECOURSE
SCHEMEANDSYLLABUS
(2002-03ADMISSIONONWARDS)
MAHATMAGANDHIUNIVERSITY
KOTTAYAM,KERALA
COMPUTERAIDEDDESIGNANDMANUFACTURING
Module1
EvolutionofCAD/CAMandCIMsegmentsofgenericCIM,computers
andworkstation,elementsofinteractivegraphics,input/output
display,storagedevicesinCAD-anoverviewofCIMsoftware-2D
Graphics:linedrawingalgorithms,DDAlinealgorithm,bressnham`s
linedrawingalgorithm–2Dtranslation,rotation,scaling–clipping-.
Designprocess–CADprocess:wireframe,surface,solidmodeling;
Engineeringanalysis;designreview&evaluation,automateddrafting
–CADhardware,software,datapresentation,
Module2
Numericalcontrol:Need-advantages&disadvantages–
classifications–Pointtopoint,straightcut&contouringpositioning-
incremental&absolutesystems–openloop&closedloopsystems
Programmablelogiccontrollers(PLC):need–relays-logicladder
program–timers-Simpleexercisesonly.
Module3
NC partprogramming:partprogramming fundamentals-manual
programming–NCco-ordinatesystemsandaxes–tapeformat–
sequencenumber,preparatoryfunctions,dimensionwords,speed
word,feedworld,toolworld,miscellaneousfunctions–programming
exercises.
Computeraidedpartprogramming:concept&needofCAP–CNC
languages–APTlanguagestructure:geometrycommands,motion
commands, postprocessor commands, compilation control
commands–programmingexercises
Module4
Automatedprocessplanning:Processplanning,generalmethodology
ofgrouptechnology,codestructuresofvariant&generativeprocess
planningmethods,processplanningsoftware.
Module5
Robotics:Industrialrobotsandtheirapplicationsfortransformational
andhandlingactivities,configuration&motion,actuators,sensorsand
end effectors,featurelikeworkenvelop,precision ofmovement,
weightcarryingcapacity,robotprogramminglanguages.

3. 3
MODULE1
INTRODUCTION
CAD/CAM
CAD/CAM is a term which means computer-aided design and
computer-aidedmanufacturing.Itisthetechnologyconcernedwiththeuse
ofdigitalcomputerstoperform certainfunctionsindesignandproduction.
Thistechnologyismovinginthedirectionofgreaterintegrationofdesign
andmanufacturing,twoactivitieswhichhavetraditionallybeentreatedas
distinctandseparatefunctionsinaproductionfirm.Ultimately,CAD/CAM
willprovidethetechnologybaseforthecomputer-integratedfactoryofthe
future.
Computer-aideddesign(CAD)canbedefinedastheuseofcomputer
systemstoassistinthecreation,modification,analysis,oroptimizationofa
design.Thecomputersystemsconsistofthehardwareandsoftwareto
perform thespecializeddesignfunctionsrequiredbytheparticularuserfirm.
TheCADhardwaretypicallyincludesthecomputer,oneormoregraphics
displayterminals,keyboards,and otherperipheralequipment.TheCAD
software consists ofthe computerprograms to implementcomputer
graphics on the system plus application programs to facilitate the
engineeringfunctionsoftheusercompany.Examplesoftheseapplication
programsincludestress-strainanalysisofcomponents,dynamicresponseof
mechanisms, heat-transfer calculations, and numerical control part
programming.Thecollectionofapplicationprogramswillvaryfrom oneuser
firm tothenextbecausetheirproductlines,manufacturingprocesses,and
customermarketsaredifferent.ThesefactorsgiverisetodifferencesinCAD
systemrequirements.
Computer-aidedmanufacturing(CAM)canbedefinedastheuseof
computersystems to plan,manage,and controlthe operations ofa
manufacturingplantthrougheitherdirectorindirectcomputerinterfacewith
the plant's production resources.As indicated by the definition,the
applicationsofcomputer-aidedmanufacturingfallintotwobroadcategories:

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1. Computer monitoring and control. These are the direct
applicationsinwhichthecomputerisconnecteddirectlytothe
manufacturing process for the purpose of monitoring or
controllingtheprocess.
2.Manufacturing supportapplications.These are the indirect
applicationsinwhichthecomputerisusedinsupportofthe
productionoperationsintheplant,butthereisnodirectinterface
betweenthecomputerandthemanufacturingprocess.
Thedistinctionbetweenthetwocategoriesisfundamentaltoan
understandingofcomputer-aidedmanufacturing.Itseemsappropriateto
elaborateonourbriefdefinitionsofthetwotypes.
Computermonitoringandcontrolcanbeseparatedintomonitoring
applicationsandcontrolapplications.Computerprocessmonitoringinvolves
adirectcomputerinterfacewiththemanufacturingprocessforthepurpose
ofobservingtheprocessandassociatedequipmentandcollectingdatafrom
theprocess.Thecomputerisnotusedtocontroltheoperationdirectly.The
controloftheprocessremainsinthehandsofhumanoperators,whomaybe
guidedbytheinformationcompiledbythecomputer.
Computerprocesscontrolgoesonestepfurtherthanmonitoringby
notonly observing the process butalso controlling itbased on the
observations.Thedistinctionbetweenmonitoringandcontrolisdisplayedin
Figure.Withcomputermonitoringtheflowofdatabetweentheprocessand
thecomputerisinonedirectiononly,from theprocesstothecomputer.In
control,thecomputerinterfaceallowsforatwo-wayflowofdata.Signalsare
transmitted from theprocessto thecomputer,justasin thecaseof
computermonitoring.Inaddition,thecomputerissuescommandsignals
directlytothemanufacturingprocessbasedoncontrolalgorithmscontained
initssoftware.
Inadditiontotheapplicationsinvolvingadirectcomputer-processinterface
for the purpose of process monitoring and control,computer-aided
manufacturingalsoincludesindirectapplicationsinwhichthecomputer

5. 5
servesasupportroleinthemanufacturingoperationsoftheplant.Inthese
applications,thecomputerisnotlinkeddirectlytothemanufacturingprocess.
Computermonitoringversuscomputercontrol:(a)computermonitoring;
(b)computercontrol.
Instead,thecomputerisused"off-line"toprovideplans,schedules,
forecasts,instructions,and information bywhich the firm's production
resourcescanbemanagedmoreeffectively.Theform oftherelationship
betweenthecomputerandtheprocessisrepresentedsymbolicallyinFigure.
Dashedlinesareusedtoindicatethatthecommunicationandcontrollinkis
anoff-lineconnection,withhumanbeingsoftenrequiredtoconsumatethe
interface.Some examples ofCAM formanufacturing supportthatare
discussedinsubsequentchaptersofthisbookinclude:
Numerical control part programming by computers. Control
programsarepreparedforautomatedmachinetools.
Computer-automatedprocessplanning.Thecomputerpreparesa
listingoftheoperationsequencerequiredtoprocessaparticularproductor
component.
Computer-generateworkstandards.Thecomputerdeterminesthe
timestandardforaparticularproductionoperation.
Productionscheduling.Thecomputerdeterminesanappropriate
scheduleformeetingproductionrequirements.
Materialrequirementsplanning.Thecomputerisusedtodetermine
whentoorderraw materialsandpurchasedcomponentsandhow many

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shouldbeorderedtoachievetheproductionschedule.
Shopfloorcontrol.InthisCAM application,dataarecollectedfrom
thefactorytodetermineprogressofthevariousproductionshoporders.
Inalloftheseexamples,humanbeingsarepresentlyrequiredinthe
applicationeithertoprovideinputtothecomputerprogramsortointerpret
thecomputeroutputandimplementtherequiredaction.
CAMformanufacturingsupport.
THEPRODUCTCYCLEANDCAD/CAM
ForthereadertoappreciatethescopeofCAD/CAMintheoperations
ofamanufacturingfirm,itisappropriatetoexaminethevariousactivities
andfunctionsthatmustbeaccomplishedinthedesignandmanufactureofa
product.Wewillrefertotheseactivitiesandfunctionsastheproductcycle.
A diagram showing the various steps in the productcycle is
presentedinFigure.Thecycleisdrivenbycustomersandmarketswhich
demandtheproduct.Itisrealistictothinkoftheseasalargecollectionof
diverseindustrialandconsumermarketsratherthanonemonolithicmarket.
Dependingontheparticularcustomergroup,therewillbedifferencesinthe
waytheproductcycleisactivated.Insomecases,thedesignfunctionsare
performedbythecustomerandtheproductismanufacturedbyadifferent
firm.Inothercases,designandmanufacturingisaccomplishedbythesame
firm.Whateverthecase,theproductcyclebeginswithaconcept,anideafor
a product.This conceptis cultivated,refined,analyzed,improved,and
translatedintoaplanfortheproductthroughthedesignengineeringprocess.

7. 7
TheplanisdocumentedbydraftingIisetofengineeringdrawingsshowing
howtheproductismadeandprovidingasetofspecificationsindicatinghow
theproductshouldperform.
Exceptforengineeringchangeswhichtypicallyfollow theproduct
throughoutitslifecycle,thiscompletesthedesignactivitiesinFigure.The
nextactivitiesinvolvethemanufactureoftheproduct.Aprocessplanis
formulatedwhichspecifiesthesequenceofproductionoperationsrequired
tomaketheproduct.Newequipmentandtoolsmustsometimesbeacquired
toproducethenew product.Schedulingprovidesaplanthatcommitsthe
companytothemanufactureofcertainquantitiesoftheproductbycertain
dates.Once allofthese plans are formulated,the productgoes into
production,followedbyqualitytesting,anddeliverytothecustomer.
Productcycle(designandmanufacturing).
TheimpactofCAD/CAMismanifestinallofthedifferentactivitiesin
the productcycle,as indicated in Figure.Computer-aided design and
automated drafting are utilized in the conceptualization,design,and
documentationoftheproduct.Computersareusedinprocessplanningand
schedulingtoperform thesefunctionsmoreefficiently.Computersareused
inproductiontomonitorandcontrolthemanufacturingoperations.Inquality

8. 8
control,computersareusedtoperform inspectionsandperformancetests
ontheproductanditscomponents.
AsillustratedinFigure,CAD/CAM isoverlaidonvirtuallyallofthe
activitiesandfunctionsoftheproductcycle.Inthedesignandproduction
operationsofamodem manufacturingfirm,thecomputerhasbecomea
pervasive,useful,andindispensabletool.Itisstrategicallyimportantand
competitivelyimperativethatmanufacturingfirmsandthepeoplewhoare
employedbythemunderstandCAD/CAM.
ProductcyclerevisedwithCAD/CAMoverlaid.
AUTOMATIONANDCAD/CAM
Automation is defined as the technology concerned with the
applicationofcomplexmechanical,electronic,andcomputer-basedsystems
intheoperationandcontrolofproduction.Itisthepurposeofthissectionto
establishtherelationshipbetweenCAD/CAMandautomation.
AsindicatedinpreviousSection,therearedifferencesinthewaythe

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productcycleisimplementedfordifferentfirmsinvolvedinproduction.
Productionactivitycanbedividedintofourmaincategories:
1.Continuous-flowprocesses
2.Massproductionofdiscreteproducts
3.Batchproduction
4.Jobshopproduction
ThedefinitionsofthefourtypesaregiveninTable.Therelationships
amongthefourtypesintermsofproductvarietyandproductionquantities
canbeconceptualizedasshowninFigure.Thereissomeoverlappingofthe
categoriesasthefigureindicates.Tableprovidesalistofsomeofthe
notable achievements in automation technology foreach ofthe four
productiontypes.
Onefactthatstandsoutfrom Tableistheimportanceofcomputer
technologyin automation.Mostofthe automated production systems
implementedtodaymakeuseofcomputers.Thisconnectionbetweenthe
digitalcomputerandmanufacturingautomationmayseem perfectlylogical
tothereader.However,thislogicalconnectionhasnotalwaysexisted.For
onething,automationtechnology
TABLE FourTypesofProduction
Category Description
1.Continuous-flowprocesses Continuous dedicated production of
large amounts of bulk product.
Examplesincludecontinuouschemical
plantsandoilrefineries
2.Mass production ofdiscrete
products
Dedicated production of large
quantitiesofoneproduct(withperhaps
limited modelvariations). Examples
include automobiles,appliances,and
engineblocks.
3.Batchproduction Productionofmedium lotsizesofthe
sameproductorcomponent.Thelots
may be produced once orrepeated

10. 10
periodically.Examples include books,
clothing, and certain industrial
machinery.
4.Jobshopproduction Productionoflowquantities,oftenone
ofakind,ofspecializedproducts.The
products are often customized and
technologically complex. Examples
include prototypes,aircraft,machine
tools,andotherequipment.
Fourproductiontypesrelatedtoquantityandproductvariation
TABLEAutomationAchievementsfortheFourTypesofProduction

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Category Automationachievements
1.Continuous-flow
processes
Flowprocessfrombeginningtoend
Sensortechnologyavailabletomeasureimportant
processvariables
Use of sophisticated controland optimization
strategies
Fullycomputer-automatedplants
2.Massproduction
of discrete
products
Automatedtransfermachines
Dialindexingmachines
Partiallyandfullyautomatedassemblylines
Industrialrobotsforspotwelding,partshandling,
machineloading,spraypainting,etc.
Automatedmaterialshandlingsystems
Computerproductionmonitoring
3.Batchproduction Numericalcontrol(NC),directnumericalcontrol
(DNC),computernumericalcontrol(CNC)
Adaptivecontrolmachining
Robotsforarcwelding,partshandling,etc.
Computer-integratedmanufacturingsystems
4. Job shop
production
Numericalcontrol,computernumericalcontrol
FUNDAMENTALSOFCAD
INTRODUCTION
Thecomputerhasgrowntobecomeessentialintheoperationsof
business,government,themilitary,engineering,andresearch.Ithasalso
demonstrateditself,especiallyinrecentyears,tobeaverypowerfultoolin
design and manufacturing.In thisand the following two chapters,we
considertheapplicationofcomputertechnologytothedesignofaproduct.
Thissectonprovidesanoverviewofcomputer-aideddesign.
TheCADsystemdefined
Asdefinedinprevioussection,computer-aideddesigninvolvesany

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typeofdesignactivitywhichmakesuseofthecomputertodevelop,analyze,
ormodifyanengineeringdesign.Modem CADsystems(alsooftencalled
CAD/CAM systems) are based on interactive computer graphics
(ICG).Interactivecomputergraphicsdenotesauser-orientedsystem inwhich
thecomputerisemployedtocreate,transform,anddisplaydataintheform
ofpicturesorsymbols.Theuserinthecomputergraphicsdesignsystem is
thedesigner,whocommunicatesdataandcommandstothecomputer
throughanyofseveralinputdevices.Thecomputercommunicateswiththe
userviaacathoderaytube(CRT).ThedesignercreatesanimageontheCRT
screenbyenteringcommandstocallthedesiredsoftwaresub-routines
storedinthecomputer.Inmostsystems,theimageisconstructedoutof
basicgeometricelements-points,lines,circles,andsoon.Itcanbemodified
accordingtothecommandsofthedesigner-enlarged,reducedinsize,
movedtoanotherlocationonthescreen,rotated,andothertransformations.
Throughthesevariousmanipulations,therequireddetailsoftheimageare
formulated.
ThetypicalICGsystem isacombinationofhardwareandsoftware.
Thehardwareincludesacentralprocessingunit,oneormoreworkstations
(includingthegraphicsdisplayterminals),andperipheraldevicessuchas
printers.Plotters,anddraftingequipment.Someofthishardwareisshownin
Figure.The software consists ofthe computerprograms needed to
implementgraphicsprocessingonthesystem.Thesoftwarewouldalso
typicallyincludeadditionalspecializedapplicationprogramstoaccomplish
theparticularengineeringfunctionsrequiredbytheusercompany.
ItisimportanttonotethefactthattheICGsystemisonecomponent
ofacomputer-aideddesignsystem.AsillustratedinFigure,theothermajor
componentisthehumandesigner.Interactivecomputergraphicsisatool
usedbythedesignertosolveadesignproblem.Ineffect,theICGsystem
magnifiesthepowersofthedesigner.Thisbasbeenreferredtoasthe
synergisticeffect.Thedesignerperformstheportionofthedesignprocess
thatis mostsuitable to human intellectualskills (conceptualization,
independentthinking);thecomputerperformsthetask:bestsuitedtoits

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capabilities(speedofcalcu1ations,visualdisplay,storageoflarge8IWWIts
ofdata),andtheresultingsystemexceedsthesumofitscomponents.
Thereareseveralfundamentalreasonsforimplementingacomputer
-aideddesignsystem.
1.Toincreasetheproductivityofthedesigner.Thisisaccomplished
byhelpingthedesignertotheproductanditscomponentsubassembliesand
parts;andbyreducingthetimerequiredinsynthesizing,analyzing,and
documentingthedesign.Thisproductivityimprovementtranslatesnotonly
intolowerdesigncostbutalsointoshorterprojectcompletiontimes.
2.Toimprovethequalityofdesign.ACADsystem permitsamore
thoroughengineeringanalysisandalargernumberofdesignalternativescan
beinvestigated.Designerrorsarealsoreducedthroughthegreateraccuracy
providedbythesystem.Thesefactorsleadtoabetterdesign.
3.Toimprovecommunications.UseofaCADsystemprovidesbetter
engineering drawings,more standardization in the drawings,better
documentationofthedesign,fewerdrawingerrorsandgreaterlegibility.
4.Tocreateadatabaseformanufacturing.Intheprocessofcreating
thedocumentationfortheproductdesign(geometriesanddimensionsofthe
productanditscomponents,materialspecificationsforcomponents,billof
materials,etc.),muchoftherequireddatabasetomanufacturetheproductis
alsocreated.
THEDESIGNPROCESS
Beforeexaminingtheseveralfacetsofcomputer-aideddesign,letus
firstconsiderthe generaldesign process.The process ofdesigning
something ischaracterized byShigleyasan iterativeprocedure,which
consistsofsixidentifiablestepsorphases:-
1.Recognitionofneed
2.Definitionofproblem
3.Synthesis

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4.Analysisandoptimization
5.Evaluation
6.Presentation
Recognitionofneedinvolvestherealizationbysomeonethata
problem existsforwhichsomecorrectiveactionshouldbetaken.Thismight
betheidentificationofsomedefectinacurrentmachinedesignbyan
engineerortheperceptionofanew productmarketingopportunitybya
salesperson.Definitionoftheproblem involvesathoroughspecificationof
theitem tobedesigned.Thisspecificationincludesphysicalandfunctional
characteristics,cost,quality,andoperatingperformance.
Synthesisandanalysisarecloselyrelatedandhighlyinteractivein
thedesignprocess.Acertaincomponentorsubsystem oftheoverallsystem
isconceptualizedbythedesigner,subjectedtoanalysis,improvedthrough
thisanalysisprocedure,andredesigned.Theprocessisrepeateduntilthe
designhasbeenoptimizedwithintheconstraintsimposedonthedesigner.
Thecomponentsandsubsystemsaresynthesizedinto thefinaloverall
systeminasimilarinteractivemanner.
Evaluation isconcerned with measuring the design againstthe
specificationsestablishedintheproblem definitionphase.Thisevaluation
oftenrequiresthefabricationandtestingofaprototypemodeltoassess
operatingperformance,quality,reliability,andothercriteria.Thefinalphase
in thedesign processisthepresentation ofthedesign.Thisincludes
documentationofthedesignbymeansofdrawings,materialspecifications,
assemblylists,andsoon.Essentially,thedocumentationrequiresthata
designdatabasebecreated.Figureillustratesthebasicstepsinthedesign
process,indicatingitsiterativenature.

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ThegeneraldesignprocessasdefinedbyShigley.
Engineeringdesignhastraditionallybeenaccomplishedondrawingboards,
withthedesignbeingdocumentedintheform ofadetailedengineering
drawing.Mechanicaldesignincludesthedrawingofthecompleteproductas
wellasitscomponentsand subassemblies,and thetoolsand fixtures
requiredtomanufacturetheproduct.Electricaldesignisconcernedwiththe
preparationofcircuitdiagrams,specificationofelectroniccomponents,and
soon.Similarmanualdocumentationisrequiredinotherengineeringdesign
fields(structuraldesign,aircraftdesign,chemicalengineeringdesign,etc.).In
eachengineeringdiscipline,theapproachhastraditionallybeentosynthesize
apreliminarydesignmanuallyandthentosubjectthatdesigntosomeform

16. 16
ofanalysis.Theanalysismayinvolvesophisticatedengineeringcalculations
oritmay involve a very subjective judgmentofthe aesthete appeal
possessed by the design.The analysis procedure identifies certain
improvementsthatcanhemadeinthedesign.Asstatedpreviously,the
processisiterative.Eachiterationyieldsanimprovementinthedesign.The
trouble with this iterative process is thatitis time consuming.Many
engineeringlaborhoursarerequiredtocompletethedesignproject.
THEAPPLICATIONOFCOMPUTERSFORDESIGN
Thevariousdesign-relatedtaskswhichareperformedbyamodem
computer-aideddesign-systemcanbegroupedintofourfunctionalareas:
1.Geometricmodeling
2.Engineeringanalysis
3.Designreviewandevaluation
4.Automateddrafting
ThesefourareascorrespondtothefinalfourphasesinShigley's
general design process, illustrated in Figure. Geometric modeling
correspondstothesynthesisphaseinwhichthephysicaldesignproject
takesform ontheICGsystem.Engineeringanalysiscorrespondstophase4,
dealingwithanalysisandoptimization.Designreviewandevaluationisthe
fifthstepinthegeneraldesignprocedure.Automateddraftinginvolvesa
procedure forconverting the design image data residing in computer
memoryintoahard-copydocument.Itrepresentsanimportantmethodfor
presentation(phase6)ofthedesign.Thefollowingfoursectionsexplore
eachofthesefourCADfunctions.
Geometricmodeling
Incomputer-aideddesign,geometricmodelingisconcernedwiththe
computer-compatiblemathematicaldescriptionofthegeometryofanobject.
Themathematicaldescriptionallowstheimageoftheobjecttobedisplayed
andmanipulatedonagraphicsterminalthroughsignalsfrom theCPUofthe
CAD system.Thesoftwarethatprovidesgeometricmodelingcapabilities

17. 17
mustbedesignedforefficientusebothbythecomputerandthehuman
designer.
Tousegeometricmodeling,thedesignerconstructs,thegraphical
imageoftheobjectontheCRTscreenoftheICGsystem byinputtingthree
typesofcommandstothecomputer.Thefirsttypeofcommandgenerates
basicgeometricelementssuchaspoints,lines,andcircles.Thesecond
command type is used to accomplish scaling, rotating, or other
transformationsoftheseelements.Thethirdtypeofcommandcausesthe
variouselementstobejoinedintothedesiredshapeoftheobjectbeing
creaedontheICG system.Duringthegeometricmodelingprocess,the
computerconvertsthecommandsintoamathematicalmodel,storesitinthe

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computerdatafiles,anddisplaysitasanimageontheCRTscreen.The
modelcansubsequentlybecalledfrom thedatafilesforreview,analysis,or
alteration.
Thereareseveraldifferentmethodsofrepresentingtheobjectin
geometricmodeling.Thebasicform useswireframestorepresentthe
object.Inthisform,theobjectisdisplayedbyinterconnectinglinesasshown
inFigure.Wireframegeometricmodelingisclassifiedintothreetypes
dependingonthecapabilitiesoftheICGsystem.Thethreetypesare:
1.2D.Two-dimensionalrepresentationisusedforaflatobject.
2.2½D.Thisgoessomewhatbeyondthe2Dcapabilitybypermitting
athree-dimensionalobjecttoberepresentedaslongasithasnoside-wall
details.
3.3D.Thisallowsforfullthree-dimensionalmodelingofamore
complexgeometry.
Exampleofwire-framedrawingofapart.

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Even three-dimensionalwire-frame representations of an object are
sometimesinadequateforcomplicatedshapes.Wire-framemodelscanbe
enhanced byseveraldifferentmethods.Figureshowsthesameobject
showninthepreviousfigurebutwithtwopossibleimprovements.1befirst
usesdashedlinestoportraytherearedgesoftheobject,thosewhichwould
beinvisiblefrom thefront.1besecondenhancementremovesthehidden
linescompletely,thusprovidingalessclutteredpictureoftheobjectforthe
viewer.SomeCADsystemshaveanautomatic"hidden-lineremovalfeature,"
whileothersystemsrequiretheusertoidentifythelinesthataretobe
removedfrom view.Anotherenhancementofthewire-framemodelinvolves
providingasurfacerepresentationwhichmakestheobjectappearsolidto
theviewer.However,theobjectisstillstoredinthecomputerasawire-frame
model.
SameworkpartasshowninFigure4.4butwith(a)dashedlines10showrear
edgesofpart,and(b)hidden-lineremoval.(CourtesyofComputervision
Corp.)

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Solidmodelofyokepartasdisplayedonacomputergraphicssystem.
(CourtesyofComputervisionCorp.)
Themostadvancedmethodofgeometricmodelingissolidmodeling
inthreedimensions.Thismethod,illustratedinFigure,typicallyusessolid
geometryshapescalledprimitivestoconstructtheobject.
AnotherfeatureofsomeCADsystemsiscolorgraphicscapability.
Bymeansofcolour,itispossibletodisplaymoreinformationonthegraphics
screen.Colored imageshelp to clarifycomponentsinanassembly,or
highlightdimensions,orahostofotherpurposes.
Engineeringanalysis
Intheformulationofnearlyanyengineeringdesignproject,some
type of analysis is required.The analysis may involve stress-strain
calculations,heat-transfercomputations,ortheuseofdifferentialequations
to describe the dynamic behaviorofthe system being designed.The
computercanbeusedtoaidinthisanalysiswork.Itisoftennecessarythat
specificprogramsbedevelopedinternallybytheengineeringanalysisgroup
to solve a particulardesign problem.In othersituations,commercially
availablegeneral-purposeprogramscanbeusedtoperform theengineering
analysis.
TurnkeyCAD/CAM systemsoftenincludeorcanbeinterfacedto

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engineeringanalysissoftwarewhichcanbecalledtooperateonthecurrent
designmodel.
Wediscusstwoimportantexamplesofthistype:
Analysisofmassproperties
Finite-elementanalysis
TheanalysisofmasspropertiesistheanalysisfeatureofaCAD
system thathasprobablythewidestapplication.Itprovidespropertiesofa
solidobjectbeinganalyzed,suchasthesurfacearea,weight,volume,center
ofgravity,andmomentofinertia.Foraplanesurface(oracrosssectionofa
solidobject)thecorrespondingcomputationsincludetheperimeter,area,
andinertiaproperties.
ProbablythemostpowerfulanalysisfeatureofaCADsystem isthe
finite-elementmethod.Withthistechnique,theobjectisdividedintoalarge
numberoffiniteelements(usuallyrectangularortriangularshapes)which
form an interconnecting network ofconcentrated nodes.By using a
computerwithsignificantcomputationalcapabilities,theentireObjectcanbe
analyzed forstress-strain,heattransfer,and othercharacteristics by
calculating thebehaviorofeach node.Bydetermining theinterrelating
behaviorsofallthenodesinthesystem,thebehavioroftheentireobjectcan
beassessed.
SomeCADsystemshavethecapabilitytodefineautomaticallythe
nodesandthenetworkstructureforthegivenobject.1beusersimplydefines
certain parameters forthe finite-elementmodel,and the CAD system
proceedswiththecomputations.
Theoutputofthefinite-elementanalysisisoftenbestpresentedby
thesystem ingraphicalformatontheCRTscreenforeasyvisualizationby
theuser,Forexample,instress-strainanalysisofanobject,theoutputmay
be shown in the form ofa deflected shape superimposed overthe
unstressedobject.ThisisillustratedinFigure.Colorgraphicscanalsobe
usedtoaccentuatethecomparisonbeforeandafterdeflectionoftheobject.

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ThisisillustratedinFigureforthesameimageasthatshowninFigure.Ifthe
finite-elementanalysisindicatesbehaviorofthedesignwhichisundesirable,
thedesignercanmodifytheshapeandrecomputethefinite-elementanalysis
forthereviseddesign.
Finite-elementmodelingforstress-strainanalysis.Graphicsdisplayshows
strainedpartsuperimposedonunstrainedpartforcomparison.
Designreviewandevaluation
Checking the accuracy of the design can be accomplished
convenientlyonthegraphicsterminal.Semiautomaticdimensioningand
tolerancingroutineswhichassignsizespecificationstosurfacesindicated
bytheuserhelp to reducethepossibilityofdimensioning errors.The
designercanzoom inonpartdesigndetailsandmagnifytheimageonthe
graphicsscreenforclosescrutiny.
A procedurecalledlayeringisoftenhelpfulindesignreview.For
example,agoodapplicationoflayeringinvolvesoverlayingthegeometric
imageofthefinalshapeofthemachinedpartontopoftheimageofthe
roughcasting.Thisensuresthatsufficientmaterialisavailableonthe
castingtoacccomplishthefinalmachineddimensions.Thisprocedurecan
beperformedinstagestocheckeachsuccessivestepintheprocessingof
thepart.

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Anotherrelatedprocedurefordesignreviewisinterferencechecking.
Thisinvolvestheanalysisofanassembledstructureinwhichthereisarisk
thatthecomponentsoftheassemblymayoccupythesamespace.Thisrisk
occursinthedesignoflargechemicalplants,air-separationcoldboxes,and
othercomplicatedpipingstructures.
Oneofthemostinterestingevaluationfeaturesavailableonsome
computer-aided design systemsiskinematics.Theavailablekinematics
packagesprovidethecapabilitytoanimatethemotionofsimpledesigned
mechanismssuch ashinged componentsand linkages.Thiscapability
enhancesthedesigner’svisualizationoftheoperationofthemechanism and
helps to ensure againstinterference with othercomponents.Without
graphicalkinematicsonaCADsystem,designersmustoftenresorttothe
useofpin-and-cardboardmodelstorepresentthemechanism.commercial
softwarepackagesareavailabletoperformkinematicanalysis.Amongthese
areprogramssuchasADAMS(AutomaticDynamicAnalysisofMechanical
Systems),developedattheUniversityofMichigan.Thistypeofprogram can
beveryusefultothedesignerinconstructingtherequiredmechanism to
accomplishaspecifiedmotionand/orforce.
Automateddrafting
Automateddraftinginvolvesthecreationofhard-copyengineering
drawingsdirectlyfrom theCADdatabase.Insomeearlycomputer-aided
designdepartments,automationofthedraftingprocessrepresentedthe
principaljustificationforinvestingintheCADsystem.Indeed,CADsystems
canincreaseproductivityinthedraftingfunctionbyroughlyfivetimesover
manualdrafting.
Someofthegraphicsfeaturesofcomputer-aideddesignsystems
lendthem-selvesespeciallywelltothedraftingprocess.Thesefeatures
includeautomaticdimensioning,generationofcrosshatchedareas,scaling
ofthedrawing,andthecapabilitytodevelopsectionalviewsandenlarged
viewsofparticularpathdetails.Theabilitytorotatethepartortoperform
othertransformationsoftheimage(e.g.,oblique,isometric,orperspective

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views),asillustratedinFigure,canbeofsignificantassistanceindrafting.
MostCADsystemsarecapableofgeneratingasmanyassixviewsofthe
part.Engineeringdrawingscanbemadetoadheretocompanydrafting
standardsbyprogrammingthestandardsintotheCADsystem.Figureshows
an engineering drawing with fourviews displayed.This drawing was
producedautomaticallybyaCADsystem.Notehowmuchtheisometricview
promotesahigherlevelofunderstandingoftheobjectfortheuserthanthe
threeorthographicviews.
Partsclassificationandcoding
Inadditionto thefourCAD functionsdescribed above,another
featureoftheCAD databaseisthatitcanbeusedtodevelopaparts
classificationandcodingsystem.Partsclassificationandcodinginvolves
thegroupingofsimilarpartdesignsintoclasses,andrelatingthesimilarities
bymeanofacodingscheme.Designerscanusetheclassificationand
coding system to retrieve existing part designs rather than always
redesigningnewparts.
CREATINGTHEMANUFACTURINGDATABASE
AnotherimportantreasonforusingaCADsystemisthatitoffersthe
opportunitytodevelopthedatabaseneededtomanufacturetheproduct.In
theconventionalmanufacturingcyclepracticedforsomanyyearsinindustry,
engineeringdrawingswerepreparedbydesigndraftsmenandthenusedby
manufacturingengineerstodeveloptheprocessplan(i.e.,the"routesheets").
Theactivitiesinvolvedindesigningtheproductwereseparatedfrom the
activitiesassociatedwithprocessplanning.Essentially,atwo-stepprocedure
wasemployed.Thiswasbothtimeconsumingandinvolvedduplicationof
effortbydesignandmanufacturingpersonnel.InanintegratedCAD/CAM
system,a direct link is established between product design and
manufacturing:It"isthegoalofCAD/CAM notonlytoautomatecertain
phasesofdesignandcertainphasesofmanufacturing,butalsotoautomate
thetransitionfrom designtomanufacturing.Computer-basedsystemshave
beendevelopedwhichcreatemuchofthedataanddocumentationrequired

25. 25
toplanandmanagethemanufacturingoperationsfortheproduct.
ThemanufacturingdatabaseisanintegratedCAD/CAMdatabase.It
includesallthedataontheproductgeneratedduringdesign(geometrydata,
billofmaterialsandpartslists,materialspecifications,etc.)aswellas
additionaldatarequiredformanufacturingmuchofwhichisbased011the
productdesign.Figure4.10showshowtheCAD/CAMdatabaseisrelatedto
designandmanufacturinginatypicalproduction-orientedcompany.
FIGUREDesirablerelationshipofCAD/CAMdatabasetoCADandCAM.
BENERTSOFCOMPUTER-AIDEDDESIGN
Therearemanybenefitsofcomputer-aideddesign,onlysomeof
whichcanbeeasilymeasured.Someofthebenefitsareintangible,reflected
in improved work quality,more pertinentand usable information,and
improvedcontrol,allofwhicharedifficulttoquantify.Otherbenefitsare
tangible,butthe savings from them show up fardownstream in the
productionprocess,sothatitisdifficulttoassignadollarfiguretothem in
thedesignphase.Someofthebenefitsthatderivefrom implementing
CAD/CAM canbedirectlymeasured.Table providesachecklistofpotential
benefitsofanintegratedCAD/CAM system.Inthesubsectionsthatfollow,

26. 26
weelaborateonsomeoftheseadvantages.
Productivityimprovementindesign
Increasedproductivitytranslatesintoamorecompetitivepositionfor
thefirm becauseitwillreducestaffrequirementsonagivenproject.This
leadstolowercostsinadditiontoimprovingresponsetimeonprojectswith
tightschedules.
SurveyingsomeofthelargerCAD/CAM vendors,onefindsthatthe
Productivityimprovementratioforadesigner/draftsmanisusuallygivenasa
range,typicallyfrom alowendof3:1toahighendinexcessof10:1(often
farinexcessofthatfigure).Thereareindividualcasesinwhichproductivity
hasbeenincreasedbyafactorof100,butitwouldbeinaccuratetorepresent
thatfigureastypical.
TABLE PotentialBenefitsThatMayResultfrom implementingCAD
asPartofanIntegratedCAD/CAMSystem.
1.Improvedengineeringproductivity
2.Shorterleadtimes
3.Reducedengineeringpersonnelrequirements
4.Customermodificationsareeasiertomake
5.Fasterresponsetorequestsforquotations
6.Avoidanceofsubcontractingtomeetschedules
7.Minimizedtranscriptionerrors
8.Improvedaccuracyofdesign
9.Inanalysis,easierrecognitionofcomponentinteractions
10.Providesbetterfunctionalanalysistoreduceprototypetesting
11.Assistanceinpreparationofdocumentation
12.Designshavemorestandardization
13.Betterdesignsprovided
14.Improvedproductivityintooldesign
15.Betterknowledgeofcostsprovided
16.ReducedtrainingtimeforroutinedraftingtasksandNCpart
programming
17.FewererrorsinNCpartprogramming
18.Providesthepotentialforusing moreexisting partsand
tooling

27. 27
19. Helps ensure designs are appropriate to existing
manufacturingtechniques
20. Saves materials and machining time by optimization
algorithms
21.Providesoperationalresultsonthestatusofworkinprogress
22.Makesthemanagementofdesignpersonnelonprojectsmore
effective
23.Assistanceininspectionofcomplicatedparts
24.Bettercommunicationinterfacesandgreaterunderstanding
among engineers,designers,drafters,management,and
differentprojectgroups.
Productivityimprovementincomputer-aideddesignascomparedto
thetraditionaldesignprocessisdependentonsuchfactorsas:
Complexityoftheengineeringdrawing
Levelofdetailrequiredinthedrawing
Degreeofrepetitivenessinthedesignedparts
Degreeofsymmetryintheparts
Extensivenessoflibraryofcommonlyusedentities
Aseachofthesefactorsisincreased.theproductivityadvantageof
CADwilltendtoincrease
Shorterleadtimes
Interactive computer-aided design is inherently fasterthan the
traditionaldesign.Italsospeedsupthetaskofpreparingreportsandlists
(e.g.,the assembly lists)which are normally accomplished manually.
Accordingly,itispossiblewithaCADsystem toproduceafinishedsetof
componentdrawingsandtheassociatedreportsinarelativelyshorttime.
Shorterleadtimesindesigntranslateintoshorterelapsedtimebetween
receiptofacustomerorderanddeliveryofthefinalproduct.Theenhanced
productivityofdesignersworkingwithCADsystemswilltendtoreducethe
prominenceofdesign,engineeringanalysis,anddraftingascriticaltime
elementsintheoverallmanufacturingleadtime.

28. 28
Designanalysis
ThedesignanalysisroutinesavailableinaCAD system helpto
consolidatethedesignprocessintoamorelogicalworkpattern.Ratherthan
havingaback-and-forthexchangebetweendesignandanalysisgroups,the
samepersoncanperformtheanalysiswhileremainingataCADworkstation.
This helps to improve the concentration ofdesigners,since theyare
interactingwiththeirdesignsinareal-timesense.Becauseofthisanalysis
capability,designscanbecreatedwhichareclosertooptimum.Thereisa
timesavingtobederivedfrom thecomputerizedanalysisroutines,bothin
designertimeand in elapsed time.Thissaving resultsfrom therapid
responseofthedesignanalysisandfrom thetunenolongerlostwhilethe
design findsitswayfrom thedesigner'sdrawing board to thedesign
analyst'squeueandbackagain.
Fewerdesignerrors
InteractiveCADsystemsprovideanintrinsiccapabilityforavoiding
design,drafting,anddocumentationerrors.Dataentry,transposition,and
extensionerrorsthatoccurquitenaturallyduringmanualdatacompilation
forpreparationofabillofmaterialsarevirtuallyeliminated.Onekeyreason
forsuchaccuracyissimplythat
No manualhandling ofinformation is required once the initial
drawinghasbeendeveloped.Errorsarefurtheravoidedbecauseinteractive
CAD systemsperform time-consumingrepetitivedutiessuchasmultiple
symbolplacement,andsortsbyareaandbylikeitem,athighspeedswith
consistentandaccurateresults.Stillmoreerrorscanbeavoidedbecausea
CADsystem,withitsinteractivecapabilities,canbeprogrammedtoquestion
inputthatmaybeerroneous.Forexample,thesystem mightquestiona
toleranceof0.00002in.Itislikelythattheuserspecifiedtoomanyzeros.
ThesuccessofthischeckingwoulddependontheabilityoftheCADsystem
designerstodeterminewhatinputislikelytobeincorrectandhence,whatto
question.

29. 29
Greateraccuracyindesigncalculations
Thereisalsoahighlevelofdimensionalcontrol,farbeyondthe
levelsofaccuracyattainablemanually.Mathematicalaccuracyisoftento14
significantdecimalplaces.The accuracy delivered by interactive CAD
systemsinthree-dimensionalcurvedspacedesignsissofarbehindthat
providedbymanualcalculationmethodsthatthereisnorealcomparison.
Computer-basedaccuracypaysoffinmanyways.Partsarelabeled
bythesamerecognizablenomenclatureandnumberthroughoutalldrawings.
InsomeCAD systems,achangeenteredonasingleitem canappear
throughouttheentiredocumentationpackage,effectingthechangeonall
drawingswhichutilizethatpart.Theaccuracyalsoshowsupintheform of
more accurate materialand costestimates and tighterprocurement
scheduling.Theseitemsareespeciallyimportantinsuchcasesaslong-lead-
timematerialpurchases.
Standardizationofdesign,drafting,anddocumentationprocedures
The single data base and operating system is common to all
workstationsin theCAD system:Consequently,thesystem providesa
naturalstandardfordesign/draftingprocedure-Withinteractivecomputer-
aideddesign,drawingsare“standardized”astheyaredrawn;thereisno
confusionastoproperproceduresbecausetheentireformatis"builtinto"
thesystemprogram.
Drawingsaremoreunderstandable
Interactive CAD is equally adeptatcreating and maintaining
isometricsandobliquedrawingsaswellasthesimplerorthographies.All
drawingscanhegeneratedandupdatedwithequalease.Thusanup-to-date
versionofanydrawingtypecanalwayshemadeavailable.

30. 30
FIGUREImprovementinvisualizationofimagesforvariousdrawingtypes
andcomputergraphicsfeatures.
Ingeneral,easeofvisualizationofadrawingrelatesdirectlytothe
projectionused.Orthographicviewsarelesscomprehensiblethanisometrics.
Anisometricview isusuallylessunderstandablethanaperspectiveview.
Mostactualconstruction drawingsare"linedrawings."Theaddition of
shading increases comprehension. Different colors further enhance
understanding.Finally,animationoftheimagesontheCRTscreenallowsfor
evengreatervisualizationcapability.Thevariousrelationshipsareillustrated
inFigure..
Improvedproceduresforengineeringchanges
Controlandimplementationofengineeringchangesissignificantly
improvedwithcomputer-aideddesign.Originaldrawingsandreportsare
stored in the data base ofthe CAD system.This makes them more
accessiblethandocumentskeptinadrawingvault.Theycanbequickly

31. 31
checkedagainstnewinformation.Sincedatastorageisextremelycompact,
historicalinformationfrom previousdrawingscanbeeasilyretainedinthe
system'sdatabase,foreasycomparisonwithcurrentdesign/draftingneeds.
Benefitsinmanufacturing
Thebenefitsofcomputer-aideddesigncarryoverintomanufacturing.
As indicated previously,the same CAD/CAM data base is used for
manufacturing planning and control,as wellas for design.These
manufacturingbenefitsarefoundinthefollowingareas:
Toolandfixturedesignformanufacturing
Numericalcontrolpartprogramming
Computer-aidedprocessplanning
Assemblylists(generatedbyCAD)forproduction
Computer-aidedinspection
Roboticsplanning
Grouptechnology
Shortermanufacturingleadtimesthroughbetterscheduling
Thesebenefitsarederivedlargelyfrom theCAD/CAM database,
whoseinitialframeworkisestablishedduringcomputer-aideddesign.Wewill
discussthemanyfacetsofcomputer-aidedmanufacturinginlaterchapters.
Intheremainderofthischapter,letusexploreseveralapplicationsthat
utilize computer graphics technology to solve various problems in
engineeringandrelatedfields.
HARDWAREINCOMPUTER-AIDEDDESIGN
INTRODUCTION
Hardwarecomponentsforcomputer-aideddesignareavailableina
varietyofsizes,configurations,andcapabilities.Henceitispossibletoselect
a CAD system thatmeets the particularcomputationaland graphics
requirementsoftheuserfirm.Engineeringfirmsthatarenotinvolvedin

32. 32
productionwouldchooseasystem exclusivelyfordraftinganddesign-
relatedfunctions.Manufacturingfirmswouldchooseasystem tobepartof
acompany-wideCAD/CAM system.Ofcourse,theCADhardwareisoflittle
valuewithoutthesupportingsoftwareforthesystem,andweshalldiscuss
thesoftwareforcomputer-aideddesigninthefollowingchapter.
amodem computer-aideddesignsystem isbasedoninteractive
computergraphics(ICG).However,thescopeofcomputer-aideddesign
includesothercomputersystemsaswell.Forexample,computerizeddesign
hasalsobeenaccomplishedinabatchmode,ratherthaninteractively.Batch
designmeansthatdataaresuppliedtothesystem (adeckofcomputer
cardsistraditionallyusedforthispurpose)andthenthesystem proceedsto
developthedetailsofthedesign.Thedisadvantageofthebatchoperationis
thatthereisatimelagbetweenwhenthedataaresubmittedandwhenthe
answerisreceivedbackasoutput.Withinteractivegraphics,thesystem
providesanimmediateresponsetoinputsbytheuser.Theuserandthe
system areindirectcommunicationwitheachother,theuserentering
commandsandrespondingtoquestionsgeneratedbythesystem.
Computer-aideddesignalsoincludesnongraphicapplicationsofthe
computerindesignwork.Theseconsistofengineeringresultswhichare
bestdisplayedinotherthangraphicalform.Nongraphichardware(e.g.,line
printers)canbeemployedtocreateroughimagesonapieceofpaperby
appropriatecombinationsofcharactersandsymbols.However,theresulting
pictures,whiletheymaycreateinterestingwallposters,arenotsuitablefor
designpurposes.
The hardware we discuss in this chapteris restricted to CAD
systemsthatutilizeinteractivecomputergraphics.Typically,astand-alone
CADsystemwouldincludethefollowinghardwarecomponents:
Oneormoredesignworkstations.Thesewouldconsistof:
Agraphicsterminal
Operatorinputdevices

33. 33
Oneormoreplottersandotheroutputdevices
Centralprocessingunit(CPU)
Secondarystorage
Thesehardwarecomponentswouldbearrangedinaconfiguration
as illustrated in Figure.The following sections discuss these various
hardwarecomponentsandthealternativesandoptionsthatcanbeobtained
ineachcategory.
FIGURETypicalconfigurationofhardwarecomponentsinastand-aloneCAD
system.Therewouldlikelybemorethanonedesignworkstation.
THEDESIGNWORKSTATION
TheCADworkstationisthesystem interfacewiththeoutsideworld.
Itrepresentsasignificantfactorindetermininghowconvenientandefficient
itisforadesignertousetheCADsystem.Theworkstationmustaccomplish
fivefunctions:
1.Itmustinterfacewiththecentralprocessingunit.
2.Itmustgenerateasteadygraphicimagefortheuser.
3.Itmustprovidedigitaldescriptionsofthegraphicimage.
4.Itmusttranslatecomputercommandsintooperatingfunctions.
5.Itmustfacilitatecommunicationbetweentheuserandthesystem]

34. 34
Theuseofinteractivegraphicshasbeenfoundtobethebest
approach to accomplish these functions.A typicalinteractive graphics
workstationwouldconsistofthefollowinghardwareComponents:
Agraphicsterminal
Operatorinputdevices
A graphics design workstation showing these components is
illustratedinFigure.
FIGUREInteractivegraphicsdesignworkstationshowinggraphicsterminal
andtwoinputdevices:alphanumerickeyboardandelectronictabletandpen.
THEGRAPHICSTERMINAL
'Therearevarioustechnologicalapproacheswhichhavebeenapplied
tothedevelopmentofgraphicsterminals.Thetechnologycontinuesto
evolveasCADsystem manufacturesattempttoimprovetheirproductsand
reducetheircosts.Inthissectionwepresentadiscussionofthecurrent
technologyininteractivecomputergraphicsterminals.
Imagegenerationincomputergraphics
Nearlyallcomputergraphics terminals available todayuse the

35. 35
cathoderaytube(CRT)asthedisplaydevice.Televisionsetsuseaform of
thesamedeviceasthepicturetube.'TheoperationoftheCRTisillustrated
inFigure.A heated cathodeemitsahigh-speed electronbeam onto a
phosphor-coatedglassscreen.'Theelectronsenergizethephosphorcoating,
causingittoglowatthepointswherethebeam makescontact.Byfocusing
theelectronbeam,changingitsintensity,andcontrollingitspointofcontact
againstthephosphorcoatingthroughtheuseofadeflectorsystem,the
beamcanbemadetogenerateapictureontheCRTscreen.
Therearetwobasictechniquesusedincurrentcomputergraphics
terminalsforgeneratingtheimageontheCRTscreen.Theyare:
1.Strokewriting
2.Rasterscan
Othernamesforthestroke-writingtechniqueincludelinedrawing,
random position,vectorwriting,strokewriting,anddirectedbeam.Other
namesfortherasterscantechniqueincludedigitalTVandscangraphics.
FIGUREDiagramofcathoderaytube(CRT).

36. 36
FIGUREStrokewritingforgeneratingimagesincomputergraphics.
Thestroke-writingsystemusesanelectronbeamwhichoperateslike
apenciltocreatealineimageontheCRTscreen.Theimageisconstructed
outofasequenceofstraight-linesegments.Eachlinesegmentisdrawnon
thescreenbydirectingthebeamtomovefromonepointonthescreentothe
next,whereeachpointisdefinedbyitsxandycoordinates.Theprocessis
portrayedinFigure.Althoughtheprocedureresultsinimagescomposedof
onlystraightlines,smoothcurvescanbeapproximatedbymakingthe
connectinglinesegmentsshortenough.
Intherasterscanapproach,theviewingscreenisdividedintoalarge
numberofdiscretephosphorpictureelements,calledpixels.Thematrixof
pixelsconstitutestheraster.Thenumberofseparatepixelsintheraster
displaymighttypicallyrangefrom256×256(atotalofover65,(00)to1024×
1024(atotalofover1,000,000points).Eachpixelonthescreencanbemade
toglowwithadifferentbrightness.Colorscreensprovideforthepixelsto
havedifferentcolorsaswellasbrightness.Duringoperation,anelectron
beam createstheimagebysweepingalongahorizontallineonthescreen
fromlefttorightandenergizingthepixelsinthatlineduringthesweep.When
thesweepofonelineiscompleted,theelectronbeam movestothenextline
belowandproceedsinafixedpatternasindicatedinFigure.Aftersweeping

37. 37
theentirescreentheprocessisrepeatedatarateof30to60entirescansof
thescreenpersecond:)
FIGURERasterscanapproachforgeneratingimagesincomputergraphics.
Graphicsterminalsforcomputer-aideddesign
Thetwoapproachesdescribedaboveareusedintheoverwhelming
majorityofcurrent-dayCADgraphicsterminals.Therearealsoavarietyof
othertechnicalfactorswhichresultindifferenttypesofgraphicsterminals.
Thesefactorsincludethetypeofphosphorcoatingonthescreen,whether
colorisrequired,thepixeldensity,andtheamountofcomputermemory
availabletogeneratethepicture.Wewilldiscussthreetypesofgraphics
terminals,whichseem tobethemostimportanttodayincommercially
availableCADsystems.Thethreetypesare:
1.Directed-beamrefresh
2.Direct-viewstoragetube(DVST)
3.Rasterscan(digitalTV)
Thefollowingparagraphsdescribethethreebasictypes.Wethen
discusssomeofthepossibleenhancements,suchascolorandanimation.
DIRECTED-BEAM REFRESH.The directed-beam refresh terminal

38. 38
utilizesthestroke-writingapproachtogeneratetheimageontheCRTscreen.
Theterm “refresh”inthenamereferstothefactthattheimagemustbe
regeneratedmanytimespersecondinordertoavoidnoticeableflickerofthe
image.The phosphorelements on the screen surface are capable of
maintainingtheirbrightnessforonlyashorttime(sometimesmeasuredin
microseconds).Inorderfortheimagetobecontinued,thesepicturetubes
mustberefreshedbycausingthedirectedbeam to retracetheimage
repeatedly.Ondenselyfilledscreens(verydetailedlineimagesormany
charactersoftext),itisdifficulttoavoidflickeringoftheimagewiththis
process.Ontheotherhand,thereareseveraladvantagesassociatedwiththe
directed-beam refreshsystems.Becausetheimageisbeingcontinually
refreshed,selective erasure and alteration of the image is readily
accomplished.Itisalsopossibletoprovideanimationoftheimagewitha
refreshtube.
Thedirected-beam refresh system istheoldestofthemodem
graphicsdisplaytechnologies.Othernamessometimesusedtoidentifythis
system includevectorrefreshandstroke-writingrefresh.Earlyrefreshtubes
wereveryexpensive.butthesteadilydecreasingcostofsolid-statecircuitry
hasbroughtthepriceofthesegraphicssystemsdowntoalevelwhichis
competitivewithothertypes.
DIRECT-VIEW STORAGETUBE(DVST).DVSTterminalsalsousethe
stroke-writingapproachtogeneratetheimageontheCRTscreen.Theterm
storagetubereferstotheabilityofthescreentoretaintheimagewhichhas
beenprojectedagainstit,thusavoidingtheneedtorewritetheimagewhich
hasbeenprojectedagainstit,thusavoidingtheneedtorewritetheimage
constantly.Whatmakesthispossibleistheuseofanelectronfloodgun
directedatthephosphorcoatedscreenwhichkeepsthephosphorelements
illuminatedoncetheyhavebeenenergizedbythestroke-writingelectron
beam.TheresultingimageontheCRTscreenisflicker-free.Linesmaybe
readilyaddedtotheimagewithoutconcernovertheireffectonimagedensity
orrefreshrates.However,thepenaltyassociatedwiththestoragetubeis
thatindividuallinescannotbeselectivelyremovedfromtheimage.

39. 39
Storagetubeshavehistoricallybeenthelowest-costterminalsand
arecapableofdisplayinglargeamountsofdata,eithergraphicalortextual.
Becauseofthesefeatures,thereareprobablymorestoragetubeterminalsin
serviceinindustryatthetimeofthiswritingthananyothergraphicsdisplay
terminal.TheprincipaldisadvantageofastorageCRT isthatselective
erasureisnotpossible.Instead,iftheuserwantstochangethepicture,the
changewillnotbemanifestedonthescreenuntiltheentirepictureis
regenerated.Otherdisadvantagesincludeitslackofcolorcapability,the
inabilitytousealightpenasadataentry,anditslackofanimationcapability.
RASTER SCAN TERMINALS.Rasterscan terminals operate by
causinganelectronbeamtotraceazigzagpatternacrosstheviewingscreen,
asdescribed earlier.Theoperation issimilarto thatofa commercial
televisionset.ThedifferenceisthataTVsetusesanalogsignalsoriginally
generatedbyavideocameratoconstructtheimageontheCRTscreen,while
therasterscanICGterminalusesdigitalsignalsgeneratedbyacomputer.
Forthisreason,therasterscanterminalsusedincomputergraphicsare
sometimescalleddigitalTVs.
Theintroductionoftherasterscangraphicsterminalusingarefresh
tubehadbeenlimitedbythecostofcomputermemory.Forexample,the
simplestandlowest-costterminalinthiscategoryusesonlytwobeam
intensitylevels,onoroff.Thismeansthateachpixelintheviewingscreenis
eitherilluminatedordark.Apicturetubewith256linesofresolutionand256
addressablepointsperlinetoform theimagewouldrequire256×256or
over65,000bitsofstorage.Eachbitofmemorycontainstheon/offstatusof
thecorrespondingpixelontheCRTscreen.Thismemoryiscalledtheframe
bufferorrefreshbuffer.Thepicturequalitycanbeimprovedintwoways:by
increasingthepixeldensityoraddingagrayscale(orcolor).Increasingpixel
densityforthesamesizescreenmeansaddingmorelinesofresolutionand
moreaddressablepointsperline.A1024×1024rasterscreenwouldrequire
morethan1millionbitsofstorageintheframebuffer.A grayscaleis
accomplishedbyexpandingthenumberofintensitylevelswhichcanbe
displayedoneachpixel.Thisrequiresadditionalbitsforeachpixeltostore

40. 40
theintensitylevel.Twobitsarerequiredforfourlevels,threebitsforeight
levels,and so forth.Five orsixbits would be needed to achieve an
approximationofacontinuousgrayscale.Foracolordisplay,threetimesas
manybitsarerequiredtogetvariousintensitylevelsforeachofthethree
primarycolors:red,blue,andgreen.(Wediscusscolorinthefollowing
section.)Arasterscangraphicsterminalwithhighresolutionandgrayscale
canrequireaverylargecapacityrefreshbuffer.Untilrecentdevelopmentsin
memorytechnology,thecostofthisstoragecapacitywasprohibitivefora
terminalwith good picturequality.Thecapabilityto achievecolorand
animationwasnotpossibleexceptforverylowresolutionlevels.
TABLEComparisonofGraphicsTerminalFeatures
Directed-beam
refresh
DVST Rasterscan
Imagegeneration Strokewriting Strokewriting Rasterscan
Picturequality Excellent Excellent Moderatetogood
Datacontent Limited High High
Selectiveerase Yes No Yes
Grayscale Yes No Yes
Colorcapability Moderate No Yes
Animation
capability
Yes No Moderate
ItisnowpossibletomanufacturedigitalTVsystemsforinteractive
computergraphicsatpriceswhicharecompetitivewiththeothertwotypes.
Theadvantagesofthepresentrasterscanterminalsincludethefeasibilityto
uselow-costTVmonitors,colorcapability,andthecapabilityforanimationof
theimage.Thesefeatures,plusthecontinuingimprovementsbeingmadein
rasterscantechnology,makeitthefastest-growingsegmentofthegraphics

41. 41
displaymarket.
ThetypicalcolorCRTusesthreeelectronbeamsandatriadofcolor
dotsanthephosphorscreentoprovideeachofthethreecolors,red,green,
andblue.Bycombiningthethreecolorsatdifferentintensitylevels,avariety
ofcolorscanbecreatedonthescreen.Itismaredifficulttofabricatea
stroke-writing tube which is precise enough farcolorbecause ofthe
technicalproblem ofgettingthethreebeamsto.convergeproperlyagainst
thescreen.
Therasterscanapproachhassuperiorcolorgraphicscapabilities
becauseofthedevelopmentswhichhavebeenmadeovertheyearsinthe
colortelevisionindustry.Colorrasterscanterminalswith1024×1024
resolutionarecommerciallyavailableforcomputergraphics.Theproblem in
therasterterminalsisthememoryrequirementsoftherefreshbuffer.Each
pixelontheviewingscreen'mayrequireupto24bitsofmemoryinthe
refresh bufferin orderto displaythe fullrange ofcolortones.When
multipliedbythenumberofpixelsinthedisplayscreen,thistranslatesintoa
verylargestoragebuffer.
Thecapabilityforanimation in computergraphicsislimited to
displaymethodsinwhichtheimagecanbequicklyredrawn.Thislimitation
excludesthestoragetubeterminalsBoththedirected-beam refreshandthe
rasterscansystemsarecapableofanimation.However,thiscapabilityisnot
automaticallyacquired.withthesesystems.Itmustbeaccomplishedby
meansofapowerfulandfastCPUinterfacedtothegraphicsterminalto
processthelargevolumesofdatarequiredforanimatedimagesIncomputer
-aideddesign,animationwouldbeapowerfulfeatureinapplicationswhere
kinematicsimulationisrequired.Theanalysisoflinkagemechanismsand
other mechanical behavior would be examples. In computer-aided
manufacturing,theplanningofaroboticworkcyclewouldbeimproved
throughtheuseofananimatedimageoftherobotsimulatingthemotionof
thearm duringthecycle.ThepopularvideogamesmarketedbyAtariand
othermanufacturersforusewithhomeTVsetsareprimitiveexamplesof

42. 42
animationincomputergraphics.AnimationintheseTV gamesismade
possiblebysacrificingthequalityofthepicture.Thiskeepsthepriceofthese
gameswithinanaffordablerange.
OPERATORINPUTDEVICES
Operatorinputdevicesareprovidedatthegraphicsworkstationto
facilitateconvenientcommunication between theuserand thesystem.
Workstationsgenerallyhaveseveraltypesofinputdevicestoallow the
operatorto selectthe various preprogrammed inputfunctions.These
functionspermittheoperatortocreateormodifyanimageontheCRT
screenortoenteralphanumericdataintothesystem.Thisresultsina
completepartontheCRTscreenaswellascompletegeometricdescription
ofthepartmtheCADdatabase.
DifferentCAGsystem vendorsofferdifferenttypesofoperatorinput
devices.Thesedevicescanbedividedintothreegeneralcategories:
1.Cursorcontroldevices
2.Digitizers
3.Alphanumericandotherkeyboardterminals
Ofthethree,cursorcontroldevicesanddigitizersarebothusedfor
graphicalinteractionwiththesystem.Keyboardterminalsareusedasinput
devicesforcommandsandnumericaldata.
Therearetwobasictypesofgraphicalinteractionaccomplishedby
meansofcursorcontrolanddigitizing:
CreatingandpositioningnewitemsontheCRTscreen
Pointingatorotherwiseidentifyinglocationsonthescreen,usually
associatedwithexistingimages
Ideally,agraphicalinputdeviceshouldlenditselftobothofthese
functions.However,thisisdifficulttoaccomplishwithasingleunitandthat
iswhymostworkstationshaveseveraldifferentinputdevices.

43. 43
Cursorcontrol
Thecursornormallytakestheform ofabrightspotontheCRT
screenthat,indicateswhereletteringordrawingwilloccur.Thecomputeris
capableofreadingthecurrentpositionofthecursor.Hencetheuser's
capabilitytocontrolthecursorpositionallowslocationaldatatobeentered
intotheCADsystem database.Atypicalexamplewouldbefortheuserto
locatethecursortoidentifythestartingpointofaline.Another,more
sophisticatedcase,wouldbefortheusertopositionthecursortoselectan
item from amenuoffunctionsdisplayedonthescreen.Forinstance,the
screenmightbedividedintotwosections,oneofwhichisanarrayofblocks
whichcorrespondtooperatorinputfunctions.Theusersimplymovesthe
cursortothedesiredblocktoexecutetheparticularfunction.
Therearea varietyofcursorcontroldeviceswhich havebeen
employedinCADsystems.Theseinclude:
Thumbwheels
Directionkeysonakeyboardterminal
Joysticks
Trackerball
Lightpen
Electronictablet/pen
Thefirstfouritemsinthelistprovidecontroloverthecursorwithout
anydirectphysicalcontactofthescreenbytheuser.Thelasttwodevicesin
thelistrequiretheusertocontrolthecursorbytouchingthescreen(orsome
otherflatsurfacewhichisrelatedtothescreen)withapen-typedevice.
Thethumbwheeldeviceusestwothumbwheels,onetocontrolthe
horizontalpositionofthecursor,theothertocontroltheverticalposition.
ThistypeofdeviceisoftenmountedasanintegralpartoftheCRTterminal.
Thecursorinthisarrangementisoftenrepresentedbytheintersectionofa
verticallineandahorizontallinedisplayedontheCRTscreen.Thetwolines
arelikecrosshairsinagunsightwhichspantheheightandwidthofthe

44. 44
screen.
Directionkeysonthekeyboardareanotherbasicform ofcursor
controlusednotonlyforgraphicsterminalsbutalsoforCRTterminals
withoutgraphicscapabilities.Fourkeysareused foreach ofthefour
directionsinwhichthecursorcanbemoved(rightorleft,andupordown).
ThejoystickapparatusispicturedinFigure.Itconsistsofaboxwith
averticaltogglestickthatcanbepushedinanydirectiontocausethecursor
tobemovedinthatdirection.Thejoystickgetsitsnamefrom thecontrol
stickthatwasused10oldairplanes.
ThetrackerballispicturedinFigure.Itsoperationissimilartothatof
thejoystickexceptthatanoperator-controlledballisrotatedtomovethe
cursorinthedesireddirectiononthescreen.
Thelightpenisapointingdeviceinwhichthecomputerseeksto
identifythe
FIGUREJoystickinputdeviceforinteractivecomputergraphics

45. 45
FIGURETrackerballinputdeviceforinteractivecomputergraphics.
positionwherethelightpenisincontactwiththescreen.Contraryto
whatitsnamesuggests,thelightpendoesnotprojectlight.Instead,itisa
detectoroflightontheCRTscreenandusesaphotodiode,phototransistor,
orsomeotherform oflightsensor.Thelightpencanbeutilizedwitha
refresh-typeCRTbutnotwithastoragetube.Thisisbecausetheimageon
therefreshtubeisbeinggeneratedintimesequence.Thetimesequenceis
soshortthattheimageappearscontinuoustothehumaneye.However,the
computeriscapableofdiscerningthetimesequenceanditcoordinatesthis
timingwiththepositionofthepenagainstthescreen.Inessence,thesystem
isperformingasanopticaltrackinglooptolocatethecursorortoexecute
some otherinputfunction.The tabletand pen in computergraphics
describesan electronicallysensitivetabletused in conjunction with an
electronicstylus.Thetabletisaflatsurface,separatefrom theCRTscreen,
onwhichtheuserdrawswiththepenlikestylustoinputinstructionsorto
controlthecursor
Itshouldbenotedthatthumbwheels,directionkeys,joysticks,and
trackerballsaregenerallylimitedintheirfunctionstocursorcontrol.The
lightpenandtablet/penaretypicallyusedforotherinputfunctionsaswellas

46. 46
cursorcontrol.Someofthesefunctionsare:
Selectingfromafunctionmenu
Drawingonthescreenormakingstrokesonthescreenortablet
whichindicatewhatimageistobedrawn
Selectingaportionofthescreenforenlargementofanexisting
image
Digitizers
Thedigitizerisanoperatorinputdevicewhichconsistsofalarge,
smoothboard(theappearanceissimilartoamechanicaldrawingboard)and
anelectronictrackingdevicewhichcanbemovedoverthesurfacetofollow
existinglines.ItisacommontechniqueinCAD systemsfortakingx,y
coordinatesfrom apaperdrawing.Theelectronictrackingdevicecontainsa
switchfortheusertorecordthedesiredxandycoordinatepositions.The
coordinatescanbeenteredintothecomputermemoryorstoredonanoff-
line storage medium such as magnetic tape.High-resolution digitizers,
typicallywithalargeboard(e.g.,42inby60in.)canprovideresolutionand
accuracyontheorderof0.001in.Itshouldbementionedthattheelectronic
tabletandpen,previouslydiscussedasacursorcontroldevice,canbe
consideredtobeasmall,low-resolutiondigitizer.
NotallCADsystemswouldincludeadigitizeraspartofitscoreof
operatorinputdevices.Itwould be inadequate,forexample,in three-
dimensionalmechanicaldesignworksincethedigitizerislimitedtotwo
dimensions.Fortwo-dimensionaldrawings,drafterscanreadilyadapttothe
digitizerbecauseitissimilartotheirdraftingboards.Itcanbetilted,raised,
orloweredtoassumeacomfortablepositionforthedrafter.
Thedigitizercanbeusedtodigitizelinedrawings.Theusercaninput
datafromaroughschematicorlargelayoutdrawingandeditthedrawingsto

47. 47
thedesiredlevelofaccuracyanddetail.Thedigitizercanalsobeusedto
freehandanewdesignwithsubsequenteditingtofinalizethedrawing.
Keyboardterminals
SeveralformsofkeyboardterminalsareavailableasCAD input
devices.Themostfamiliartypeisthealphanumericterminalwhich is
availablewithnearlyallinteractivegraphicssystems.Thealphanumeric
terminalcanbeeitheraCRTorahardcopyterminal,whichprintsonpaper.
Forgraphics,theCRThastheadvantagebecauseofitsfasterspeed,the
abilitytoeasilyedit,andtheavoidanceoflargevolumesofpaper.Onthe
otherhand,apermanentrecordissometimesdesirableandthisismost
easilycreated with a hard-copyterminal.ManyCAD systems use the
graphicsscreentodisplaythealphanumericdata,butthereisanadvantage
inhavingaseparateCRTterminalsothatthealphanumericmessagescanbe
createdwithoutdisturbingoroverwritingtheimageonthegraphicsscreen.
Thealphanumericterminalisusedtoentercommands,functions,
andsupplementaldatatotheCADsystem.Thisinformationisdisplayedfor
verificationontheCRTortypedonpaper.Thesystem alsocommunicates
backtotheuserinasimilarmanner.Menulistings,program listings,error
messages,andsoforth,canbedisplayedbythecomputeraspartofthe
interactiveprocedure.
Thesefunctionkeyboardsareprovidedtoeliminateextensivetyping
ofcommands,orcalculatecoordinatepositions,andotherfunctions.The
numberoffunctionkeysvariesfrom about8to80.Theparticularfunction
correspondingwitheachbuttonisgenerallyundercomputercontrolsothat
thebuttonfunctioncanbechangedastheuserproceedsfrom onephaseof
thedesigntothenext.Inthiswaythenumberofalternativefunctionscan
easilyexceedthenumberofbuttonsonthekeyboard.
Also,lightedbuttonsareusedonthekeyboardstoindicatewhich
functionsarepossibleinthecurrentphaseofdesignactivity.Amenuofthe

48. 48
variousfunctionalternativesistypicallydisplayedontheCRTscreenforthe
usertoselectthedesiredfunction.
PLOTTERSANDOTHEROUTPUTDEVCES
Therearevarioustypesofoutputdevicesusedinconjunctionwitha
computer-aideddesignsystem.Theseoutputdevicesinclude:
Penplotters
Hard-copyunits
ElectrostaticplottersComputer-output-to-microfilm(COM)units
Wediscussthesedevicesinthefollowingsections.
Penplotters
Theaccuracyandqualityofthehard-copyplotproducedbyapen
plotterisconsiderablygreaterthantheapparentaccuracyandqualityofthe
correspondingimageontheCRTscreen.InthecaseoftheCRTimage,the
qualityofthepictureisdegradedbecauseoflackofresolutionandbecause
oflossesinthedigital-to-analogconversionthrough:thedisplaygenerators.
Ontheotherhand,ahigh-precisionpenplotteriscapableofachievingahard-
copydrawingwhoseaccuracyisnearlyconsistentwiththedigitaldefinitions
intheCADdatabase.
Thepenplotterusesamechanicalinkpen(eitherwetinkorballpoint)
towriteonpaperthroughrelativemovementofthepenandpaper.Thereare
twobasictypesofpenplotterscurrentlyinuse:
Drumplotters
Fiat-bedplotters
Hard-copyunit
Ahard-copyunitisamachinethatcanmakecopiesfrom thesame
imagedatalayedontheCRTscreen.Theimageonthescreencanbe
duplicatedinamatterofseconds.Thecopiescanbeusedasrecordsof
intermediatestepsinthedesignprocessorwhenroughhardcopiesofthe
screenareneededquickly.Thehardcopiesproducedfrom theseunitsare

49. 49
notsuitableasfinaldrawingsbecausetheaccuracyandqualityofthe
reproductionisnotnearlyasgoodastheoutputofapenplotter.
Mosthard-copyunitsaredrysilvercopiersthatuselight-sensitive
paperexposedthroughanarrowCRTwindowinsidethecopier.Thewindow
istypically8½ in.(216mm),correspondingtothewidthofthepaper,by
about½ in.(12mm)wide.Thepaperisexposedbymovingitpastthe
windowandcoordinatingtheCRTbeam tograduallytransfertheimage.A
heatedrollerinsidethecopierisusedtodeveloptheexposedpaper.Thesize
ofthepaperisusuallylimitedonthesehard-copyunitsto8½byIIin.Another
drawbackisthatthedrysilvercopieswilldarkenwithtimewhentheyareleft
exposedtonormallight.
Electrostaticplotters
Hard-copyunitsarerelativelyfastbuttheiraccuracyandresolution
arepoor.Penplottersarehighlyaccuratebutplottingtimecantakemany
minutes (up to a half-hourorlongerforcomplicated drawings).The
electrostaticplotteroffersacompromisebetweenthesetwotypesinterms
ofspeedandaccuracy.Itisalmostasfastasthehard-copyunitandalmost
asaccurateasthepenplotter.
Theelectrostaticcopierconsistsofaseriesofwirestylimountedon
abarwhichspansthewidthofthecharge-sensitivepaper.Thestylihavea
densityofupto200perlinearinch.Thepaperisgraduallymovedpastthe
barandcertainstyliareactivatedtoplacedotsonthepaper.Bycoordinating
thegenerationofthedotswiththepapertravel,theimageisprogressively
transferredfrom thedatabaseintohard-copyform.Thedotsoverlapeach
otherslightlytoachievecontinuity.Forexample,aseriesofadjacentdots
givestheappearanceofacontinuousline.
Alimitationoftheelectrostaticplotteristhatthedatamustbeinthe
rasterformat(i.e.,inthesameformatusedtodrivetheraster-typeCRT)in
ordertobereadilyconvertedintohardcopyusingtheelectrostaticmethod.If
thedataarenotinrasterformat,sometypeofconversionisrequiredto
changethem intotherequiredformat.Theconversionmechanism isusually

50. 50
basedonacombinationofsoftwareandhardware.
Anadvantageoftheelectrostaticplotterwhichissharedwiththe
drum-typepenplotteristhatthelengthofthepaperisvirtuallyunlimited.
Typicalplottingwidthsmightbeupto6ft(1.83m).Anotheradvantageis
thattheelectrostaticplottercanbeutilizedasahigh-speedlineprinter,
capableofupto1200linesoftextperminute.
THECENTRALPROCESSINGUNIT
TheCPUoperatesasthecentral"brain"ofthecomputer-aideddesign
system.Itistypicallyaminicomputer.Itexecutesallthemathematical
computationsneededtoaccomplishgraphicsandotherfunctions,andit
directsthevariousactivitieswithinthesystem.
COMPUTERGRAPHICSSOFTWAREANDDATABASE
INTRODUCTION
Thegraphicssoftwareisthecollectionofprogramswrittentomake
itconvenientforausertooperatethecomputergraphicssystem.Itincludes
ProgrammestogenerateimagesontheCRT screen,tomanipulatethe
images,andtoaccomplishvarioustypesofinteractionbetweentheuserand
thesystem.Inadditiontothegraphicssoftware,theremaybeadditional
programs for implementing certain specialized functions related to
CAD/CAM.These include design analysis programs(e.g.,finite-element
analysisandkinematicsimulation)andManufacturingplanningprograms
(e.g.,automatedprocessplanningandnumericalcontrolpartprogramming).
Thischapterdealsmainlywiththegraphicssoftware.
Thegraphicssoftwareforaparticularcomputergraphicssystem is
verymuchafunctionofthetypeofhardwareusedinthesystem.The
softwaremustbewrittenspecificallyforthetypeofCRTandthetypesof
inputdevicesusedinthesystem.Thedetailsofthesoftwareforastroke-
writingCRTwouldbedifferentthanforarasterscanCRT.Thedifferences
betweenastoragetubeandarefreshtubewouldalsoinfluencethegraphics
software.Althoughthesedifferencesinsoftwaremaybeinvisibletotheuser

51. 51
to someextent,theyareimportantconsiderationsinthedesignofan
interactivecomputergraphicssystem.
NewmanandSpoulllistsix“groundrules”thatshouldbeconsidered
indesigninggraphicssoftware:
1.Simplicity.Thegraphicssoftwareshouldbeeasytouse.
2.Consistency.Thepackageshouldoperateinaconsistentand
predict-
ablewaytotheuser.
3.Completeness.Thereshouldbenoinconvenientomissionsinthe
setof
graphicsfunctions.
4.Robustness.Thegraphicssystem shouldbetolerantofminor
instances
ofmisusebytheoperator.
5.Performance.Withinlimitationsimposedbythesystem hardware,
the
performanceshouldbeexploitedasmuchaspossiblebysoftware.
Graphicsprogramsshouldbeefficientandspeedofresponse
shouldbefastandconsistent.
6.Economy.Graphicsprogramsshouldnotbesolargeorexpensive
astomaketheiruseprohibitive.
THESOFTWARECONFIGURATIONOFAGRAPHICSSYSTEM
Intheoperationofthegraphicssystem bytheuser,avarietyof
activitiestakeplace,whichcanbedividedintothreecategories:
1.Interactwiththegraphicsterminaltocreateandalterimageson
thescreen
2.Constructamodelofsomethingphysicaloutoftheimagesonthe
screen.themodelsaresometimescalledapplicationmodels.

52. 52
3.Enterthemodelintocomputermemoryand/orsecondarystorage.
Inworkingwiththegraphicssystem theuserperformsthesevarious
activitiesincombinationratherthansequentially.Theuserconstructsa
physicalmodelandinputsittomemorybyinteractivelydescribingimagesto
thesystem.Thisisdonewithoutanythoughtaboutwhethertheactivityfalls
intocategory1,2,or3.
Thereasonforseparatingtheseactivitiesinthisfashionisthatthey
correspondtothegeneralconfigurationofthesoftwarepackageusedwith
theinteractivecomputergraphics(ICG)system.Thegraphicssoftwarecan
bedividedintothreemodulesaccordingtoaconceptualmodelsuggestedby
FoleyandVanDam:
1.Thegraphicspackage(FoleyandVanDamcalledthisthegraphics
system)
2.Theapplicationprogram
3.Theapplicationdatabase
This software configuration is illustrated in Figure.The central
moduleistheapplicationprogram.Itcontrolsthestorageofdataintoand
retrievesdataoutoftheapplicationdatabase.Theapplicationprogram is
drivenbytheuserthroughthegraphicspackage.
Theapplicationprogram isimplementedbytheusertoconstructthe
modelofaphysicalentitywhoseimage'istobeviewedonthegraphics-
screen.Application programs are written forparticularproblem areas.
Problemareasinengineeringdesignwouldincludearchitecture,construction,
mechanicalcomponents,electronics,chemicalengineering,andaerospace
engineering.Problemareasotherthandesignwouldincludeflightsimulators,
graphicaldisplayofdata,mathematicalanalysis,andevenartwork.Ineach
case,the application software is developed to dealwith images and
conventionswhichareappropriateforthatfield.

53. 53
Thegraphicspackageisthesoftwaresupportbetweentheuserand
thegraphicsterminal.Itmanagesthegraphicalinteractionbetweentheuser
andthesystem.Italsoservesastheinterfacebetweentheuserandthe
applicationsoftware.Thegraphicspackageconsistsofinputsubroutines
andoutputsubroutines.Theinputroutinesacceptinputcommandsanddata
from theuserandforwardthem totheapplicationprogram.Theoutput
subroutinescontrolthedisplayterminal(orotheroutputdevice)andconverts
theapplicationmodelsintotwo-dimensionalorthree-dimensionalgraphical
pictures.
ThethirdmoduleintheICGsoftwareisthedatabase.Thedatabase
containsmathematical,numerical,andlogicaldefinitionsoftheapplication
models,suchaselectroniccircuits,mechanicalcomponents,automobile
bodies,andsoforth.Italsoincludesalphanumericinformationassociated
withthemodels,suchasbillsofmaterials,massproperties,andotherdata.
ThecontentsofthedatabasecanbereadilydisplayedontheCRTorplotted
outinhard-copyform.Section

54. 54
FIGUREModelofgraphicssoftwareconfiguration.
FUNCTIONSOFAGRAPHICSPACKAGE
Tofulfillitsroleinthesoftwareconfiguration,thegraphicspackage
mustperformavarietyofdifferentfunctions.thesefunctionscanbegrouped
into functionsets.Eachsetaccomplishesacertainkindofinteraction
betweentheuserandthesystem.Someofthecommonfunctionsetsare:
Generationofgraphicelements
Transformations
Displaycontrolandwindowingfunctions
Segmentingfunctions
Userinputfunctions
TRANSFORMATIONS
Manyoftheeditingfeaturesinvolvetransformationsofthegraphics
elementsorcellscomposedofelementsoreventheentiremodel.Inthis
section we discuss the mathematics ofthese transformations.Two-
dimensionaltransformationsareconsideredfirsttoillustrateconcepts.Then
wedealwiththreedimensions.
Two-dimensionaltransformations
To locatea pointin a two-axiscartesian system,thexand y
coordinatesarespecified.Thesecoordinatescanbetreatedtogetherasa
1x1matrix:(x,y).Forexample,thematrix(2,5)wouldbeinterpretedtobea
pointwhichis2unitsfrom theorigininthex-directionand5unitsfrom the
origininthey-direction.
Thismethod ofrepresentationcanbeconvenientlyextended to
definealineasa2x2matrixbygivingthexandycoordinatesofthetwoend
pointsoftheline.Thenotationwouldbe
L=

55. 55
Usingtherulesofmatrixalgebra,apointorline(orothergeometric
element represented in matrix notation) can be operated on by a
transformationmatrixtoyieldanewelement.
There are severalcommon transformations used in computer
graphics.Wewilldiscussthreetransformations:translation,scaling,and
rotation.
TRANSLATION.Translationinvolvesmovingtheelementfrom one
locationtoanother.Inthecaseofapoint,theoperationwouldbe
x'=x+m, y'=y+n
wherex',y'=coordinatesofthetranslatedpoint
x,y=coordinatesoftheoriginalpoint
m,n=movementsinthexandydirections,respectively
Inmatrixnotationthiscanberepresentedas
(x',y')=(x,y)+T
where
T=(m,n),thetranslationmatrix
AnygeometricelementcanbetranslatedinspacebyapplyingEq.to
eachpointthatdefinestheelement.Foraline,thetransformationmatrix
wouldbeappliedtoitstwoendpoints.
SCALING.Scalingofanelementisusedtoenlargeitorreduceits
size.Thescalingneednotnecessarilybedoneequallyinthexandy
directions.Forexample,acirclecouldbetransformedintoanellipsebyusing
unequalxandyscalingfactors.
Thepointsofanelementcanbescaledbythescalingmatrixas
follows:
(x',y')=(x,y)S
where

56. 56
thescalingmatrix
Thiswouldproduceanalterationinthesizeoftheelementbythe
factorminthex-directionandbythefactornintheydirection.Italsohasthe
effectofrepositioningtheelementwithrespecttothecartesiansystem
origin.IfthescalingfactorsarelessthanI,thesizeoftheelementisreduced
anditismovedclosertotheorigin.IfthescalingfactorsarelargerthanI,the
elementisenlargedandremovedfartherfromtheorigin.
ROTATION.Inthistransformation,thepointsofanobjectarerotated
abouttheoriginbyanangleO.Forapositiveangle,thisrotationisinthe
counterclockwisedirection.Thisaccomplishesrotationoftheobjectbythe
sameangle,butitalsomovestheobject.Inmatrixnotation,theprocedure
wouldbeasfollows:
(x',y')=(x,y)R
where
R= therotationmatrix
EXAMPLE6.1
As an illustration ofthese transformations in two dimensions,
considerthelinedefinedby
L=
Letussupposethatitisdesiredtotranslatethelineinspaceby2
unitsinthexdirectionand3unitsintheydirection.Thiswouldinvolve
adding2tothecurrentxvalueand3tothecurrentyvalueoftheendpoints
definingtheline.Thatis,

57. 57
FIGURE.ResultsoftranslationinExample6.1.
Thenewlinewouldhaveendpointsat(3,4)and(4,7).Theeffectof
thetransformationisillustratedinFigure6.3.
EXAMPLE
ForthesameoriginallineasinExample6.1,letusapplythescaling
factorof2totheline.Thescalingmatrixforthe2x2linedefinitionwould
thereforebe
T=
TheresultinglinewouldbedeterminedbyEq.asfollows:
ThenewlineispicturedinFigure.

58. 58
EXAMPLE
Wewillagainuseoursamelineandrotatethelineabouttheoriginby
30
o
.Equation wouldbeusedtodeterminethetransformedlinewherethe
rotationmatrixwouldbe:
FigureResultsofscalinginExample.
R=
Thenewlinewouldbedefinedas:
Theeffectofapplyingtherotationmatrixtothelineisshownin
Figure.
Three-dimensionaltransformations
Transformations bymatrix methods can be extended to three-
dimensionalspace.Weconsiderthesamethreegeneralcategoriesdefined
intheprecedingsection.Thesamegeneralproceduresareappliedtouse
these transformations thatwere defined forthe three cases by Eqs.

59. 59
TRANSLATION.Thetranslationmatrixforapointdefinedinthreedimensions
wouldbe
T=(m.n,p)
FIGUREResultsofrotationinExample
andwouldbeappliedbyaddingtheincrementsm,n,andptothe
respectivecoordinatesofeachofthepointsdefiningthethree-dimensional
geometryelement.
SCALING.Thescalingtransformationisgivenby
S=
Forequalvaluesofm,n,andp,thescalingislinear.
ROTATION.Rotationinthreedimensionscanbedefinedforeachof
theaxes.
Rotationaboutthezaxisbyanangleisaccomplishedbythematrix

60. 60
Rz=
Rotationabouttheyaxisbytheangle6isaccomplishedsimilarly.
Ry=
Rotationaboutthexaxisbytheangleisdonewithananalogous
transformationmatrix.
Rx=
Concatenation
Theprevioussingletransformationscanbecombinedasasequence
of transformations.This is called concatenation,and the combined
transformationsarecalledconcatenatedtransformations.
Duringtheeditingprocesswhenagraphicmodelisbeingdeveloped.
theuseofconcatenatedtransformationsisquitecommon.Itwouldbe
unusualthatonlyasingletransformationwouldbeneededtoaccomplisha
desiredmanipulationoftheimage.Twoexamplesofwherecombinationsof
transformationswouldberequiredwouldbe:-
Rotationoftheelementaboutanarbitrarypointintheelement
Magnifyingtheelementbutmaintainingthelocationofoneofits
pointsinthesamelocation
In the firstcase,the sequence oftransformations would be'
translationtotheorigin,thenrotationabouttheorigin,thentranslationback
totheoriginallocation.Inthesecondcase,theelementwouldbescaled
(magnified)followedbyatranslationtolocatethedesiredpointasneeded:-
Theobjectiveofconcatenationistoaccomplishaseriesofimage

61. 61
manipulationsasasingle-transformation.Thisallowstheconcatenated
transformation to bedefined moreconciselyand thecomputation can
generallybeaccomplishedmoreefficiently.
Determining the concatenation of a sequence of single
transformationscanbefairlystraightforward ifthetransformationsare
expressedinmatrixform aswehavedone.Forexample.ifwewantedto
scaleapointbythefactorof2inatwodimensionalsystemandthenrotateit
abouttheoriginby45°,theconcatenationwouldsimplybetheproductofthe
two transformation matrices.Itis importantthatthe orderofmatrix
multiplicationbethesameastheorderinwhichthetransformationsareto
becarriedout.Concatenationofaseriesoftransformationsbecomesmore
complicatedwhenatranslationisinvolved,andwewillnotconsiderthiscase.
EXAMPLE
Letusconsidertheexamplecitedinthetextinwhichapointwasto
bescaledbyafactorof2androtatedby45°.Supposethatthepointunder
considerationwas(3,1).Thismightbeoneofseveralpoints.defininga
geometricelement.Forpurposesofillustrationletusfirstaccomplishthe
twotransformationssequentially.First,considerthescaling.
(x'.y')=(x,y)S
(x',y')=(3.1) =(6,2)
Next,therotationcanbeperformed.
(x",y")=(x',y')R
(x",y")=(6,2)
=(6,2) =(2.828.5.657)
Thesameresultcanbeaccomplishedbyconcatenatingthetwo
separatetransformationmatrices.Theproductofthetwomatriceswouldbe

62. 62
SR=
=
Now,applying this concatenated transformation matrix to the
originalpoint,wehave
(x",y")=(3,1)
=(2.828,5.657)
WIRE-FRAMEVERSUSSOLIDMODELING
Theimportanceofthree-dimensionalgeometry
EarlyCADsystemswerebasicallyautomateddraftingboardsystems
which displayed a two-dimensionalrepresentation ofthe objectbeing
designed.Operators(e.g.,thedesignerordrafter)couldusethesegraphics
systemstodevelopthelinedrawingthewaytheywanteditandthenobtaina
veryhighqualitypaperplotofthedrawing.Byusingthesesystems,the
draftingprocesscouldbeaccomplishedinlesstime,andtheproductivityof
thedesignerscouldbeimproved.
However,there was a fundamentalshortcoming ofthese early
systems.Althoughtheywereabletoreproducehigh-qualityengineering
drawingsefficientlyandquickly,thesesystemsstoredintheirdatafilesa
two-dimensionalrecordofthedrawings.Thedrawingswereusuallyofthree-
dimensionalobjectsanditwaslefttothehumanbeingswhoreadthese
drawingstointerpretthethree-dimensionalshapefrom thetwo-dimensional
representation.TheearlyCADsystemswerenotcapableofinterpretingthe
three-dimensionalityoftheobject.Itwaslefttotheuserofthesystem to
makecertainthatthetwo-dimensionalrepresentationwascorrect(e.g.,
hiddenlinesremovedordashed,etc.),asstoredinthedatafiles.
Morerecentcomputer-aideddesignsystemspossessthecapability
todefineobjectsinthreedimensions.Thisisapowerfulfeaturebecauseit

63. 63
allowsthedesignertodevelopafullthree-dimensionalmodelofanobjectin
thecomputerratherthanatwo-dimensionalillustration.Thecomputercan
thengeneratetheorthogonalviews,perspectivedrawings,andclose-upsof
detailsintheobject.
Theimportanceofthisthree-dimensionalcapabilityininteractive
computergraphicsshouldnotbeunderestimated.
Wire-Framemodels
Mostcurrentdaygraphicssystemsuseaform ofmodelingcalled
wire-framemodeling.Intheconstructionofthewire-framemodeltheedges
oftheobjectsareshownaslines.Forobjectsinwhichtherearecurved
surfaces,contourlinescanbeadded;asshowninFigure,toindicatethe
contour.Theimageassumestheappearanceofaframeconstructedoutof
wire-hencethename“wireframe”model.
FIGUREOrthographicviewsofthree-dimensionalobjectwithouthidden-line
removal.

64. 64
FIGUREPerspectiveviewofthree-dimensionalobjectofFigurewithout
hiddenlineremoval.
There are limitations to the models which use the wire-frame
approachtoform theimage.Theselimitationsare,ofcourse,especially
pronouncedinthecaseofthree-dimensionalobjects.Inmanycases,wire-
framemodelsarequiteadequatefortwo-dimensionalrepresentation.The
mostconspicuouslimitationisthatallofthelinesthatdefinetheedges(and
contouredsurfaces)ofthemodelareshownintheimage.Manythree-
dimensionalwire-framesystemsinusetodaydonotpossessanautomatic
hidden-lineremovalfeature.Consequently,thelinesthatindicatetheedges
attherearofthemodelshowrightthroughtheforegroundsurfaces.Thiscan
causetheimagetobesomewhatconfusingtotheviewer,andinsomecases
theimagemightbeinterpretableinseveraldifferentways.Thisinterpretation
problem canbealleviatedtosomeextentthroughhumaninterventionin
removingthehiddenbackgroundlinesintheimage.
Therearealsolimitationswiththewire-framemodelsintheway
manyCADsystemsdefinethemodelintheirdatabases.Forexample,there
mightbeambiguityinthecaseofasurfacedefinitionastowhichsideofthe
surfaceissolid.Thistypeoflimitationpreventsthecomputersystem from
achievingacomprehensiveandunambiguousdefinitionoftheobject.

65. 65
FIGUREWireframemodelofF/A-18fighteraircraft,showingprimarycontrol
curves.
Solidmodels
Animprovementoverwire-framemodels,bothintermsofrealism to
theuseranddefinitiontothecomputer,isthesolidmodelingapproach.In
thisapproach,themodelsaredisplayedassolidobjectstotheviewer,with
verylittleriskofmisinterpretation.Whencolorisaddedtotheimage,the
resultingpicturebecomesstrikinglyrealistic.Itisanticipatedthatgraphics
systemswiththiscapabilitywillfindawiderangeofapplicationsoutside
computer-aideddesignandmanufacturing.Theseapplicationswillinclude'
colorillustrationsinmagazinesandtechnicalpublications,animationin
moviefilms,andtrainingsimulators(e.g.,aircraftpilottraining).
Therearetwofactorswhichpromotefuturewidespreaduseofsolid
modelers(i.e.,graphicssystemswiththecapabilityforsolidmodeling).The
firstistheincreasingawarenessamongusersofthelimitationsofwire-
framesystems.Aspowerfulastoday'swire-frame-basedCADsystemshave
become,solidmodelsystemsrepresentadramaticimprovementingraphics
technology.Thesecondreasonisthecontinuingdevelopmentofcomputer
hardwareandsoftwarewhichmakesolidmodelingpossible.Solidmodelers
requireagreatdealofcomputationalpower,intermsofbothspeedand

66. 66
memory,inordertooperate.Theadventofpowerful,low-costminicomputers
hassuppliedtheneededcapacitytomeetthisrequirement.Developmentsin
softwarewillprovideapplicationprogramswhichtakeadvantageofthe
opportunitiesofferedbysolidmodelers.Amongthepossibilitiesaremore
highlyautomatedmodelbuildinganddesignsystems,morecompletethree-
dimensionalengineering analysis ofthe models,including interference
checking,automatedmanufacturingplanning,andmorerealisticproduction
simulationmodels.
Twobasicapproachestotheproblem ofsolidmodelinghavebeen
developed:
1.Constructive solid geometry(CSG orC-rep),also called the
building-blockapproach
2.Boundaryrepresentation(B-rep)
TheCSG systemsallow theusertobuildthemodeloutofsolid
graphicprimitives,suchasrectangularblocks,cubes,spheres,cylinders,and
pyramids.Thisbuilding-blockapproachissimilartothemethodsdescribed
inSection6.4exceptthatasolidthree-dimensionalrepresentationofthe
objectisproduced.Themostcommonmethodofstructuringthesolidmodel
inthegraphicsdatabaseistouseBooleanoperations,describedinthe
precedingsectionandpicturedinFigure.
Theboundaryrepresentationapproachrequirestheusertodrawthe
outlineorboundaryoftheobjectontheCRTscreenusinganelectronictablet
andpenoranalogousprocedure.Theuserwouldsketchthevariousviewsof
the object (front,side,and top,more views if needed),drawing
interconnectinglinesamongtheviewstoestablishtheirrelationship.Various
transformationsandotherspecializededitingproceduresareusedtorefine
themodeltothedesiredshape.ThegeneralschemeisillustratedinFigure.
The two approaches have their relative advantages and
disadvantages.TheC-repsystemsusuallyhaveasignificantprocedural
advantageintheinitialformulationofthemodel.Itisrelativelyeasyto
constructaprecisesolidmodeloutofregularsolidprimitivesbyadding,

67. 67
subtracting,andintersectingthecomponents.Thebuilding-blockapproach
alsoresultsinamorecompactfileofthemodelinthedatabase.
FIGUREInputviewsofthetypesrequiredforboundaryrepresentation(B-rep)
.
Ontheotherhand,B-repsystemshavetheirrelativeadvantages.One
ofthem becomesevidentwhenunusualshapesareencounteredthatwould
notbeincludedwithintheavailablerepertoireoftheCSGsystems.Thiskind
ofsituationisexemplifiedbyaircraftfuselageandwingshapesandby
automobilebodystyling.Suchshapeswouldbequitedifficulttodevelopwith
thebuilding-blockapproach,buttheboundaryrepresentationmethodisvery
feasibleforthissortofproblem.
Anotherpointofcomparisonbetweenthetwoapproachesisthe
differenceinthewaythemodelisstoredinthedatabaseforthetwo
systems.TheCSGapproachstoresthemodelbyacombinationofdataand
logicalprocedures.
(theBooleanmodel).Thisgenerallyrequireslessstoragebutmore

68. 68
computationtoreproducethemodelanditsimage.Bycontrast,theB-rep
system storesanexplicitdefinitionofthemodelboundaries.Thisrequires
morestoragespacebutdoesnotnecessitatenearlythesamecomputation
efforttoreconstructtheimage.ArelatedbenefitoftheB-repsystemsisthat
itis relativelysimple to convertback and forth between a boundary
representationandacorrespondingwire-framemodel.Thereasonisthatthe
model'sboundarydefinitionissimilartothewire-framedefinition,which
facilitatesconversionofoneformtotheother.ThismakesthenewersolidB-
repsystemscompatiblewithexistingCADsystemsoutinthefield.
Because ofthe relative benefits and weaknesses ofthe two
approaches,hybridsystemshavebeendevelopedwhichcombinetheCSG
andB-repapproaches.Withthesesystems,usershavethecapabilityto
constructthe geometric modelbyeitherapproach,whicheveris more
appropriatetotheparticularproblem.
VectorGeneration
-Theprocessof‘turningon’thepixels.
TwoV.G.Algorithm(linegrassing)
1. DDA(DigitalDiffernetialAnalysers)
2. Bresenham’sAlgoritm.
DDAAlgorithm
-Basedondyofdx
-FloatingpointArithmetic,slower
-Moreaccurate.
1. Readtheendpointsco-ordinates(x1,y1)&(x2,y2)foraline
2. dx=x2-x

69. 69
dy=y2–y
3. Ifabs(dx)>abs(dy)then
step=abs(dx)
otherwise
Step=abs(dy)
4. xinc=dx/step
yinc=dy/step
x=x1
yx=y1
5. Putpixel(x,y,colour0
6. x=x+xinc
y=y+yinc
Putpixel(x,y,colour)
7. Repeatstep6untilx=x2
Drawlinefrom(1,2)to(4,6)usingDDAAlgorithm.
1. x1=1 y1=2
x2=4 y2=5
2. dx=3 dy=4
3. Step=dt=4
4. Xinc=dx =3=0.75

70. 70
Step 4
5. Plot(112)
6. x=x+xinc x=1 y=2
y=y+yinc x=1.75 y=3
x=2.5 y=4
x=3.25 y=5
x=4 y=6
7. Stop
[Roundedtohighervalue]
-Eliminatingstaircasingoraliasingisknownasantaliasing.
Bresenham’slineDrawingAlgorithm.
-UsesIntegerarithmetic.
-FasterthanDDAbecauseofIntegerArithmatic.
-Separatealgorithmsfor|m|<|&|m|>|
m=y2–y1
x2–x1
for|m|<|
1. Read(x1,y1)and(x2,t2)astheendpointsco-ordinates.
2. dx=|x2–x1|
dy=|y2–y1|

71. 71
P=2dy–dx (P decisionparameter)
3. Ateachxk,alongtheline,statingatk>o,------------followstest.
IfPk<O,thennextpointtoplotis(xk+1,yk)and
Pk+1=Pk+2dy
Otherwiseifbknextpointtoplotis(xk+1,3yk+1)and
Pk+1=Pk+2dy–2dx
4. Repeatstep3dxtimes.
5. Stop.
Q. Scanconvertthelineendpoints(10,5)and(15,9)usingBresenham
Algorithm.
n=y2–y1 4
x2–x1 5 <1
dx=x2–x1=15–10=5
dy=y2–y1=9–5=4 (10,5)
Po=2dy–dx=2x4–5=3
SinceP>O,x1=x0+1 = 10+1=11
Y1=x0+1 = y+1=6 (11,6)
P1 = Pk+2dy–2dx
= 3+2x4–2x5
= 1
SinceP1>0, x2=12 (12,7)

72. 72
Y2=7
P2 = 1+2x4–2x5=-1
SinceP2<O
X3=13
Y3=7 (13,7)
P3 = -1+2x4=7
P3>0 (14,8)
X4=14
Y4=8
P4>0
X5=15 (15,9)
Y5=9
Stop
Forslope|m|>|
1. Read(x1,y1)and(x2,y2)astheendpointsco-ordinates.
2. dx=|x2–x1|
dy=|y2–y1| (P=decisionpercents)
P=2dx–dy
3. Ateachxkalongtheline,startingatk=0,portionfollowingtest.
IfPk<0,thennextpointtoplotis(xk,yk+1)and

73. 73
Pk+1=Pk+2dx
Otherwise,nextpointtoplotis(xk+1,yk+1)and
Pk+1=Pk+2dx–2dy
4. Repeat‘step3’dytimesory1=y2
5. Stop
MODULEII
NUMERICALCONTROL
INTRODUCTION
Numericalcontroldefined
Numericalcontrolcan be defined as a form ofprogrammable
automationinwhichtheprocessiscontrolledbynumbers,letters,and
symbol.InNC,thenumbersform aprogram ofinstructionsdesignedfora
particularwork partorjob.When the job changes,the program of
instructionsischanged.Thiscapabilitytochangetheprogram foreachnew
jobiswhatgivesNCitsflexibility.Itismucheasiertowritenewprograms
thantomakemajorchangesintheproductionequipment.
NC technologyhasbeenappliedtoawidevarietyofoperations,
includingdrafting,assembly,inspection,sheetmetalpressworking,andspot
welding.However,numericalcontrolfindsitsprincipalapplicationsinmetal
machiningprocesses.Themachinedworkpartsaredesignedinvarious
sizesandshapes,andmostmachinedpartsproducedinindustrytodayare
madeinsmalltomedium-sizebatches.Toproduceeachpart,asequenceof
drilling operations may be required,ora series ofturning ormilling
operations.ThesuitabilityofNCforthesekindsofjobsisthereasonforthe
tremendousgrowthofnumericalcontrolinthemetal-workingindustryover
thelast25years.

74. 74
BASICCOMPONENTSOFANNCSYSTEM
Anoperationalnumericalcontrolsystem consistsofthefollowing
threebasiccomponents:
1.Programofinstructions
2.Controllerunit,alsocalledamachinecontrolunit(MCU)
3.Machinetoolorothercontrolledprocess
Thegeneralrelationshipamongthethreecomponentsisillustrated
inFigure.Theprogram ofinstructionsservesastheinputtothecontroller
unit,whichinturncommandsthemachinetoolorotherprocesstobe
controlled.Wewilldiscussthethreecomponentsinthesectionsbelow.
Programofinstructions
The program ofinstructionsisthe detailed step-by-step setof
directionswhichtellthemachinetoolwhattodo.Itiscodedinnumericalor
symbolicform onsometypeofinputmedium thatcanbeinterpretedbythe
controllerunit.Themostcommoninputmedium todayisl-in.-widepunched
tape.Overtheyears,otherformsofinputmediahavebeenused,including
punchedcards,magnetictape,andeven35-mmmotionpicturefilm.
TherearetwoothermethodsofinputtotheNCsystemwhichshould
bementioned.Thefirstisbymanualentryofinstructionaldatatothe
controllerunit.Thismethodiscalledmanualdatainput,abbreviatedMDI,and
isappropriateonlyforrelativelys1fupleJobswheretheorderwillnotbe
repeated.Thesecondothermethodofinputisbymeans

75. 75
FIGUREThreebasiccomponentsofanumericalcontrolsystem:(a)program
ofinstruction;(b)controllerunit;(c)machinetool.
ofadirectlinkwithacomputer.Thisiscalleddirectnumerical
control,orDNC,.
Theprogram ofinstructionsispreparedbysomeonecalledapart
programmer.The programmer's job is to provide a setofdetailed
instructionsbywhichthesequenceofprocessingstepsistobeperformed.
Fora machining operation,the processing steps involve the relative
movementbetweenthecuttingtoolandtheworkpiece.
Controllerunit
ThesecondbasiccomponentoftheNCsystem isthecontrollerunit.
Thisconsistsoftheelectronicsandhardwarethatreadandinterpretthe
program ofinstructions and convertitinto mechanicalactions ofthe
machinetool.ThetypicalelementsofaconventionalNC controllerunit
includethetapereader,adatabuffersignalout-putchannelstothemachine
tool,feedbackchannelsfrom themachinetool,andthesequencecontrolsto
coordinatetheoveralloperationoftheforegoingelements.Itshouldbe
noted that nearly allmodern NC systems today are sold with a
microcomputerasthecontrollerunit.ThistypeofNCiscalledcomputer
numericalcontrol(CNC).
Thetapereaderisanelectromechanicaldeviceforwindingand
readingthepunchedtapecontainingtheprogram ofinstructions.Thedata
containedonthetapearereadintothedatabuffer.Thepurposeofthis
deviceistostoretheinputinstructionsinlogicalblocksofinformation.A
blockofinformationusuallyrepresentsonecompletestepinthesequence
ofprocessingelements.Forexample,oneblockmaybethedatarequiredto
movethemachinetabletoacertainpositionanddrillaholeatthatlocation.
Thesignaloutputchannelsareconnectedtotheservomotorsand
othercontrolsinthemachinetool.Throughthesechannels,theinstructions
aresenttothemachinetoolfromthecontrollerunit.Tomakecertainthatthe
instructionshavebeenproperlyexecutedbythemachine,feedbackdataare

76. 76
sentbacktothecontrollerviathefeedbackchannels.Themostimportant
functionofthisreturnloopistoassurethatthetableandworkparthave
beenproperlylocatedwithrespecttothetool.
Sequencecontrolscoordinatetheactivitiesoftheotherelementsof
thecontrollerunit.Thetapereaderisactuatedtoreaddataintothebuffer
from thetape,signalsaresenttoandfrom themachinetool,andsoon.
Thesetypesofoperationsmustbesynchronizedandthisisthefunctionof
thesequencecontrols.
AnotherelementNCsystem,whichmaybephysicallypartofthe
controllerunitorpartofthemachinetool,isthecontrolpanel.Thecontrol
panelorcontrolconsolecontainsthedialsandswitchesbywhichthe
machineoperatorrunstheNCsystem.Itmayalsocontaindatadisplaysto
provideinformationtotheoperator.AlthoughtheNCsystem isanautomatic
system,thehumanoperatorisstillneededtoturnthemachineonandoff,to
changetools(someNCsystemshaveautomatictoolchangers),toloadand
unloadthemachine,andtoperform variousotherduties.Tobeableto
dischargetheseduties,theoperatormustbeabletocontrolthesystem,and
thisisdonethroughthecontrolpanel.
Machinetoolorothercontrolledprocess
ThethirdbasiccomponentofanNCsystem isthemachinetoolor
othercontrolledprocess.ItisthepartoftheNCsystem whichperforms
usefulwork.InthemostcommonexampleofanNCsystem,onedesignedto
perform machiningoperations,themachinetoolconsistsoftheworkable
andspindleaswellasthemotorsandcontrolsnecessarytodrivethem.It
alsoincludesthecuttingtools,workfixtures,andotherauxiliaryequipment
neededinthemachiningoperation.
NCmachinesrangeincomplexityfrom simpletape-controlleddrill
pressestohighlysophisticatedandversatilemachiningcenters.TheNC
machiningcenterwasfirstintroducedinthelate1950s.Itisamultifunction
machinewhichincorporatesseveraltimesavingfeaturesintoasinglepiece
ofautomatedproductionequipment.First,amachiningcenteriscapableof

77. 77
performingavarietyofdifferentoperations:drilling,tapping,reaming,milling,
andboring.Second,ithasthecapacitytochangetoolsautomaticallyunder
tapecommand.Avarietyofmachiningoperationsmeansthatavarietyof
cuttingtoolsarerequired.Thetoolsarekeptinatooldrum orotherholding
device.Whenthetapecallsaparticulartool,thedrum rotatestopositionthe
toolforinsertionintothespindle.Theautomatictoolchangerthengrasps
thetoolandplacesitintothespindlechuck.AthirdcapabilityoftheNC
machiningcenterisworkpiecepositioning.Themachinetablecanorientthe
jobsothatitcanbemachinedonseveralsurfaces,asrequired.Finally,a
fourthfeaturepossessedbysomemachiningcentersisthepresenceoftwo
tables orpallets on which the work piece can be fixtured.While the
machiningsequenceisbeingperformedononeworkpart,theoperatorcan
beunloadingthepreviouslycompletedpiece,andloadingthenextone.This
improvesmachinetoolutilizationbecausethemachinedoesnothaveto
standidleduringloadingandunloadingoftheworkparts.
THENCPROCEDURE
Toutilizenumericalcontrolinmanufacturing,thefollowingsteps
mustbeaccomplished.
1.ProcessPlanning.Theengineeringdrawingoftheworkpartmust
beinterpretedintermsofthemanufacturingprocessestobeused.thisstep
isreferredtoasprocessplanninganditisconcernedwiththepreparationof
aroutesheet.Theroutesheetisalistingofthesequenceofoperations
whichmustbeperformedontheworkpart.Itiscalledaroutesheetbecause
italsoliststhemachinesthroughwhichthepartmustberoutedinorderto
accomplishthesequenceofoperations.Weassumethatsomeofthe
operationswillbeperformedononeormoreNCmachines.
2.Partprogramming.Apartprogrammerplanstheprocessforthe
portionsofthejob to beaccomplished byNC.Partprogrammersare
knowledgeableaboutthemachiningprocessandtheyhavebeentrainedto
program fornumericalcontrol.They are responsible forplanning the
sequenceofmachiningstepstobeperformedbyNCandtodocumentthese

78. 78
inaspecialformat.TherearetwowaystoprogramforNC:
Manualpartprogramming
Computer-assistedpartprogramming
Inmanualprogramming,themachininginstructionsarepreparedon
aform calledapartprogram manuscript.Themanuscriptisalistingofthe
relativecutter/workpiecepositionswhichmustbefollowedtomachinethe
part.In computer-assisted part programming,much of the tedious
computationalworkrequiredinmanualpartprogrammingistransferredto
the computer.This is especially appropriate forcomplex work piece
geometriesandjobswithmanymachiningsteps.Useofthecomputerin
thesesituationsresultsinsignificantsavingsinpartprogrammingtime.
3.Tapepreparation.A punchedtapeispreparedfrom thepart
programmer’sNCprocessplan.Inmanualpartprogramming,thepunched
tapeisprepareddirectlyfrom thepartprogram manuscriptonatypewriter
likedeviceequippedwithtapepunchingcapability.Incomputer-assistedpart
programming,the computer interprets the list of part programming
instructions,performsthenecessarycalculationsto convertthisinto a
detailedsetofmachinetoolmotioncommands,andthencontrolsatape
punchdevicetopreparethetapeforthespecificNCmachine.
4.Tapeverification.Afterthepunchedtapehasbeenprepared,a
methodisusuallyprovidedforcheckingtheaccuracyofthetape.Some
timesthetapeischeckedbyrunningitthroughacomputerprogram which
plotsthevarioustoolmovements(ortablemovements)onpaper.Inthisway,
majorerrorsinthetapecanbediscovered.The"acidtest"ofthetape
involvestryingitoutonthemachinetooltomakethepart.Afoam orplastic
materialissometimesusedforthistryout.Programmingerrorsarenot
uncommon,anditmayrequireaboutthreeattemptsbeforethetapeis
correctandreadytouse.
5.Production.ThefinalstepintheNCproceduretousetheNCtape
in production.Thisinvolvesordering theraw workpartsspecifying and
preparingthetoolingandanyspecialfixturingthatmayberequired,and

79. 79
settingupTheNCmachinetoolforthejob.Themachinetooloperator's
functionduringproductionistoloadtherawworkpartinthemachineand
establishthestartingpositionofthecuttingtoolrelativetotheworkpiece.
TheNCsystem thentakesoverandmachinesthepartaccordingtothe
instructionsontape.Whenthepartiscompleted,theoperatorremovesit
fromthemachineandloadsthenextpart.
NCCOORDINATESYSTEMS
Inorderforthepartprogrammertoplanthesequenceofpositions
andmovementsofthecuttingtoolrelativetotheworkpiece,itisnecessary
toestablishastandardaxissystem bywhichtherelativepositionscanbe
specified.UsinganNCdrillpressasanexample,thedrillspindleisinafixed
verticalposition,andthetableismovedandcontrolledrelativetothespindle.
However,tomakethingseasierfortheprogrammer,weadopttheviewpoint
thattheworkpieceisstationarywhilethedrillbitismovedrelativetoit.
Accordingly,thecoordinatesystem ofaxesisestablishedwithrespecttothe
machinetable.
Twoaxes,xandy,aredefinedintheplaneofthetable,asshownin
Figure.Thezaxisisperpendiculartothisplaneandmovementinthez
directioniscontrolledbytheverticalmotionofthespindle.Thepositiveand
negativedirectionsofmotionoftoolrelativetotablealongtheseaxesareas
showninFigure7.A.NCdrillpressesareclassifiedaseithertwo-axisor
three-axismachines,dependingonwhetherornottheyhavethecapabilityto
controlthezaxis.
A numericalcontrolmilling machine and similarmachine tools
(boringmill.forexample)useanaxissystemsimilartothatofthedrillpress.
However,inadditiontothethreelinearaxes,thesemachinesmaypossess
thecapacitytocontrol

80. 80
FIGURENCmachinetoolaxissystemformillinganddrillingoperations.
FIGURENCmachinetoolaxissystemforturningoperation.
oneormorerotationalaxes.ThreerotationalaxesaredefinedinNC:
thea,b,andcaxes.Theseaxesspecifyanglesaboutthex,y,andzaxes,
respectively.Todistinguishpositivefromnegativeangularmotions,the"right
-handrule"canbeused.Usingtherighthandwiththethumbpointinginthe
positivelinearaxisdirection(x,y,orz),thefingersofthehandarecurledto
pointinthepositiverotationaldirection.

81. 81
Forturningoperations,twoaxesarenormallyallthatarerequiredto
commandthemovementofthetoolrelativetotherotatingworkpiece.Thez
axisistheaxisofrotationoftheworkpart,andxaxisdefinestheradial
locationofthecuttingtool.ThisarrangementisillustratedinFigure.
Thepurposeofthecoordinatesystem istoprovideameansof
locatingthetoolinrelationtotheworkpiece.DependingontheNCmachine,
the partprogrammermayhave severaldifferentoptions available for
specifyingthislocation.
Fixedzeroandfloatingzero
Theprogrammermustdeterminethepositionofthetoolrelativeto
theorigin(zeropoint)ofthecoordinatesystem.NCmachineshaveeitherof
twomethodsforspecifyingthezeropoint.Thefirstpossibilityisforthe
machinetohaveafixedzero.Inthiscase,theoriginisalwayslocatedatthe
samepositiononthemachine.Usually,thatpositionisthesouthwestcomer
(lowerleft-handcomer)ofthetableandalltoollocationswillbedefinedby
positivexandycoordinates.
ThesecondandmorecommonfeatureonmodernNCmachines
allowsthemachineoperatortosetthezeropointatanypositiononthe
machinetable.Thisfeatureiscalledfloatingzero.Thepartprogrammeris
theonewhodecideswherethezeropointshouldbelocated.Thedecisionis
basedonpartprogrammingconvenience.Forexample,theworkpartmaybe
symmetricalandthezeropointshouldbeestablishedatthecenterof
symmetry.