Table Of ContentSolid Modelling and CAD Systems
Ian Stroud Hildegarde Nagy
•
Solid Modelling
and CAD Systems
How to Survive a CAD System
123
Dr. IanStroud
Ecole PolytechniqueFederale de Lausanne
STI-IPR-LICP
Station9, BatimentME
ME-AI393
1015Lausanne
Switzerland
e-mail: ian.stroud@epfl.ch
ISBN 978-0-85729-258-2 e-ISBN978-0-85729-259-9
DOI 10.1007/978-0-85729-259-9
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Preface
Unless you are Homer, the sensible course to take in any narrative, whether it be
fictionorsomewhatfactual,istostartatthebeginning,proceedthroughthemiddle
andfinishattheend.Thisbookisnotlikethat.CADissomethingofaGordianknot
inwhichitisdifficulttofindanendfromwhichtostart.Thepurposeofthebookis
not to criticise, but to explain how CAD systems work, relate this to theoretical
aspects,explainshortcomingsandlimitations.Thebookisnotsupposedtobeabout
any one particular system, though some are mentioned. Many CAD systems have
similar functionality and, wherever possible, this functionality will be explained
ratherthanparticularimplementations.ThisisbecauseCADsystemsaredynamic
and,hopefully,willcontinuetochangeasnewrequirementsandideasappear.The
techniquesbehind,though,aremorestableand,inordertounderstandwhataCAD
system is doing, it is helpful tounderstand whatis going on behindthe interface.
CADsystemsareacombinationofknowledgefrom,atleast,thethreedomains:
computer science, mathematics and engineering. They are complicated andhence
itisdifficulttobeanexpertinallthreedomains.Forthisreasonitiscommonthat
software experts implement the software rather than the engineers for whom the
systems are intended. This means that there can be differences between the way
that a system works and what the user would expect.
The intended audience of this book is:
• CADusers—Bothstudentsandprofessionaluserswhowouldliketounderstand
more about the CAD tool that they are using. The examples and exercises are
intendedtogiveaninsightintowhatisgoingonbehindthefunctionssothatitis
easiertoavoidproblems,butalsotounderstandthoseproblemswhichdooccur.
• CADteachers—PeoplewhowouldliketoteachCADatahigherlevelthanjust
whichbuttonstopushtogetaresult.Theexercisesandexamplesinthebookare
intended as a supplement to normal teaching with example exercises.
• CAD system developers—The book tries to present some of the ‘other side’ of
CAD,alittleofwherethecurrentsystemphilosophyiswrongorwhereitcould
be improved.
vii
viii Preface
The way this book is organised is that the first chapter is intended to demon-
strate some simple object creation sequences and to pose some of the questions
that a user might put. These questions are a sort of introduction to the material in
some of the other chapters.
Chapter 2 explains some of the history, about different solid modelling
methods, before describing Boundary Representation (B-rep), the current main
method,inmoredetail.CADsystemsworkedwithwire-framemodellingandthen
constructive solid geometry (CSG) methods before settling for B-rep.
Chapter3describestwo-dimensionaldefinitionmethodsandChap.4 describes
themainmodelcreationoperationsthatareusedtocreatemodels.Alongwiththe
operations are brief descriptions of algorithms and special cases. There are sug-
gestionsforvariousexperimentstoillustratesomelimitationsandexplainwhatis
happening. Chapter 5 describes, briefly, another important area, the geometry of
parts.Chapter6definesnon-manifoldmodelsandexamplesoftheiruseinahybrid
modelling environment. Chapter 7 describes graphics input and output in CAD.
Together this group of chapters describes the core modelling functions of CAD
systems.
Thenextchapter,Chap.8,describesinformationinmodels.Thiscanbethought
of as an add-on to the basic model for improving the level of communication
between application areas. Chapter 9 describes current methods for exchanging
model data.
Chapter 10 describes some aspects of the complex topic offeatures. This is a
rather subjective topic and there is a wide variety of perception about what con-
stitutesafeature.Thechapterpresentssomeofthehistoryandsomeoftheusesof
features.
Chapter 11 presents some of the gaps in CAD. In general CAD systems are
often used as modelling systems to create detailed geometric models of products
whereas a lot of design precedes this phase. The chapter presents a little research
work in this area as an illustration of how CAD systems might (and should)
develop to provide a uniform design environment.
Chapter 12 presents some aspects of command files and their modern use as
history records in CAD is presented. The chapter also describes aspects of object
parametrisation. Often, products are collections of single models connected in
some way. Chapter 13 concerns the way that these collections, or assemblies, are
built up and connected. Chapter 14 describes different aspects of CAD in com-
munities.ThisconcernshowtoworkwithotherpeopleinCADaswellasanalysis
methods to aid the designer communicate with other partners in the production
chain.Finally,Chap.15outlinessomepossibleprojects.Theseprojectswereused
in teaching at the EPFL to reinforce different aspects of CAD.
Good luck.
Lausanne, April 2008 Ian Stroud
Hildegarde Nagy
Acknowledgments
I would like to thank, again, all present and past colleagues for their support and
help throughout the years.
Many thanks to Claudia Rainfurth and Meinolf Gröpper of VDMA for
arranging the documentation of VDA-FS.
ThecommentsmadeinSect.14.1.1arenotonlyfrompersonalobservationsbut
also long discussions with many other people. While I would like to mention
especiallyFriedrichGlantschnig,PeterMüllerandProfessorSuk-HwanSuh,there
are many other people from the European STEP-NC project, the ISO TC184
SC1/WG7 and ISO TC184 SC4/T12 communities who are too numerous to
mention.Whilethesectionisapersonalpresentation,ithasbeenformedbasedon
their input and I would like to thank all concerned.
I would also like to thank Professor Paul Xirouchakis and all colleagues at
LICP of the EPFL for their support.
ix
Contents
1 Case Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Linear Extrusion Piece . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Modelling Method . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2 Rotational Piece. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.2.1 Modelling Method . . . . . . . . . . . . . . . . . . . . . . . . 17
1.2.2 Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.3 Hollowed Lofted Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.3.1 Modelling Method . . . . . . . . . . . . . . . . . . . . . . . . 25
1.3.2 Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1.4 Creating Assemblies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
1.4.1 Creating the Parts. . . . . . . . . . . . . . . . . . . . . . . . . 34
1.4.2 Creating the Assembly Structure. . . . . . . . . . . . . . . 34
1.4.3 Constraining the Elements. . . . . . . . . . . . . . . . . . . 38
1.4.4 Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
1.5 Determining a Modelling Sequence . . . . . . . . . . . . . . . . . . . 47
1.6 Design Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
1.6.1 A Bracket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
1.6.2 A Flange. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
1.6.3 A Car . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
1.7 CAD System Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
1.7.1 Geometric Kernels . . . . . . . . . . . . . . . . . . . . . . . . 53
1.7.2 The CAD User Interface . . . . . . . . . . . . . . . . . . . . 53
1.7.3 Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
1.7.4 Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
1.7.5 Discfiles and Databases . . . . . . . . . . . . . . . . . . . . 57
1.7.6 Standalone Systems and Multi-User Work. . . . . . . . 57
1.8 The Modelling Environment . . . . . . . . . . . . . . . . . . . . . . . . 59
1.9 Chapter Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
xi
xii Contents
1.10 User Exercises. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
1.10.1 The MBB Gehäuse Rohteil . . . . . . . . . . . . . . . . . . 61
1.10.2 The ANC-101 Object . . . . . . . . . . . . . . . . . . . . . . 63
1.10.3 Linear Extrusion Part Using Symmetry. . . . . . . . . . 63
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
2 How Objects Are Modelled. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
2.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
2.2 3D Wireframe Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.3 Drafting Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
2.4 Surface Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
2.5 Solid Modelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
2.6 Constructive Solid Geometry. . . . . . . . . . . . . . . . . . . . . . . . 70
2.7 Boundary Representation. . . . . . . . . . . . . . . . . . . . . . . . . . . 71
2.7.1 Finding Entities from Each Other. . . . . . . . . . . . . . 82
2.7.2 Finding Sets of Connected Entities. . . . . . . . . . . . . 84
2.7.3 Euler Operators . . . . . . . . . . . . . . . . . . . . . . . . . . 85
2.7.4 Stepwise Algorithms. . . . . . . . . . . . . . . . . . . . . . . 92
2.7.5 Complex Utilities . . . . . . . . . . . . . . . . . . . . . . . . . 97
2.8 Complex Geometry and Integration . . . . . . . . . . . . . . . . . . . 100
2.9 The Cylinder Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
2.10 Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
2.11 Chapter Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
2.12 Representation Exercises. . . . . . . . . . . . . . . . . . . . . . . . . . . 104
2.12.1 CSG Decompositions . . . . . . . . . . . . . . . . . . . . . . 104
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
3 2D Shape Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
3.1 BUILD’s 2.5D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
3.2 Two-Dimensional Elements. . . . . . . . . . . . . . . . . . . . . . . . . 109
3.2.1 Lines, Curves, Dimensions and Constraints . . . . . . . 109
3.2.2 Grids and Rulers. . . . . . . . . . . . . . . . . . . . . . . . . . 114
3.3 Shapes and Shape Modifications . . . . . . . . . . . . . . . . . . . . . 115
3.3.1 Corner Round-Off. . . . . . . . . . . . . . . . . . . . . . . . . 115
3.3.2 Corner Shaving (2D Chamfer) . . . . . . . . . . . . . . . . 116
3.3.3 Polygons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
3.3.4 Ellipses, Parabolae and Other Geometry . . . . . . . . . 116
3.3.5 Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
3.4 Constraint Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
3.5 Constraint Solving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
3.6 Tolerance Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
3.7 From 2D to Solid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
3.8 Inscribing on Faces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
3.9 Chapter Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
