5 geometric modeling

Introduction
“Geometric modeling is as important to CAD as
governing equilibrium equations to classical
i i fi ld h i d th lengineering fields as mechanics and thermal
fluids.”
intelligent decision on the types of entities necessaryintelligent decision on the types of entities necessary
to use in a particular model to meet certain geometric
requirements such as slopes and/or curvatures.
interpretation of unexpected results
evaluations of CAD/CAM systems
innovative use of the tools in particular applicationsinnovative use of the tools in particular applications.
creation of new attributes, or modify the obtained
models to benefit new engineering applications.
understanding of terminology

The Role of Geometric Modeling in aThe Role of Geometric Modeling in a
CAD System
Computer
Display
Mathematical
representation
(Database)
User
Interface Drawing
Generation
Geometric
Generation
A l i d
Determine Determine
Modeling
Technique
Analysis and
Manufacturing

General RequirementsGeneral Requirements
• Complete part representation including topologicalComplete part representation including topological
and geometrical data
– Geometry: shape and dimensions
– Topology: the connectivity and associativity of the
object entities; it determines the relational
information between object entitiesinformation between object entities
• Able to transfer data directly from CAD to CAE and
CAM.
• Support various engineering applications, such as
mass properties, mechanism analysis, FEA/FEM and
tool path creation for CNC and so ontool path creation for CNC, and so on.

Comments on Geometric Modeling
• Geometric modeling is only a means not the goal in
engineering.
E i i l i d d t t th• Engineering analysis needs product geometry; the
degree of detail depends on the analysis procedure
that utilizes the geometry.g y
• There is no model that is sufficient to study all
behavioral aspects of an engineering component or a
systemsystem.
• Attributes facilitate analysis and grow with
applicationspp

Basic Geometric Modeling TechniquesBasic Geometric Modeling Techniques
• 2-D Projection (Drawings)
• Wireframe Modeling
• Surface Modeling
A l ti l S fAnalytical Surface
Free-form, Curved, & Sculptured Surface
• Solid ModelingSolid Modeling
Constructive Solid Geometry (CSG)
Boundary Representation (B-Rep)
…
Feature Based Modeling
Parametric ModelingParametric Modeling

2-D Projection
Problems:
• Training is necessary to understand the drawing• Training is necessary to understand the drawing
• Mistakes often occur
• Does not support subsequent applications such as
fi i l l i (FEA) NCfinite element analysis (FEA) or NC part
programming

Wireframe Modeling
• Developed in 1960s and referred as “a stick figure” or “an edge
representation”
g
• The word “wireframe” is related to the fact that one may imagine
a wire that is bent to follow the object edges to generate a
model.
• Model consists entirely of points, lines, arcs and circles, conics,
and curves.
• In 3D wireframe model, an object is not recorded as a solid., j
Instead the vertices that define the boundary of the object, or the
intersections of the edges of the object boundary are recorded
as a collection of points and their connectivity.
front
fro nt
or
front
ambiguous

Example of Wireframe Modelingp g

S f M d liSurface Modeling
• A surface model is a set of faces• A surface model is a set of faces.
• A surface model consists of wireframe entities that form
the basis to create surface entitiesthe basis to create surface entities.
• In general, a wireframe model can be extracted from a
surface model by deleting or blanking all surface entitiessurface model by deleting or blanking all surface entities
• Shape design and representation of complex objects such
as car, ship, and airplane bodies as well as castingsp p g
• Used to be separated, shape model are now incorporated
into solid models (e g Pro/E)into solid models (e.g. Pro/E)

Examples of Surface Models
A l ti l S f
Free-form, Curved, or
Sculptured SurfaceAnalytical Surfaces Sculptured Surface

E l f S f M d lExamples of Surface Models
• Surface models define only the geometry, no topology.y g y, p gy
• Shading is possible
Shading - by interpreting the polygons’Shading - by interpreting the polygons
• Direction (normal)
• Spatial order

Solid Modelingg
• The solid modeling technique is based upon the "half-space"
concept.p
• The boundary of the model separates the interior and exterior of the
modeled object.
• The object is defined by the volume space contained within the
defined boundary of the object.
• In general speaking, a closed boundary is needed to define a solid
object.
i f ti ll l t lid d bi t ti• informationally complete, valid, and unambiguous representation
(Spatial addressability)
– points in space to be classified relative to the object, if it is
inside outside or on the objectinside, outside, or on the object
• store both geometric and topological information; can verify
whether two objects occupy the same space.
• improves the quality of design improves visualization and hasimproves the quality of design, improves visualization, and has
potential for functional automation and integration.

Why Solid Modeling?Why Solid Modeling?
Solid Modeling SupportSolid Modeling Support
• Using volume information
– weight or volume calculation, centroids, moments of
i ti l l tiinertia calculation,
– stress analysis (finite elements analysis), heat
conduction calculations, dynamic analysis,
– system dynamics analysis
U i l d b d i f ti• Using volume and boundary information
– generation of CNC codes, and robotic and assembly
simulation

Foundation of SOLID Modeling
in
g
(Element of Primitives): Half Space
out
• Unbounded geometric entities
• Each one of them divides the representation space into
out
infinite portions, one filled with material and the other empty
• Surfaces can be considered half-space boundaries and half
fspaces can be considered directed surfaces
• An object is defined by the volume space contained within
the defined boundary of the objectthe defined boundary of the object
Introduces the direction into the modeling, thus enables the
topological information be stored in a geometric model

in
Half Spaces
• By specifying different boundary
surface, we can have any half-spaces;
out
• The most commonly used half-spaces are
– planar, cylindrical, spherical, conical, and
toroidal.
• By combining half-spaces (using Boolean
ti ) i b ildi bl k f hioperations) in a building block fashion,
various solids can be constructed.

Half Spaces
• Planar half-space
• Cylindrical half-space
}0:),,{( <= zzyxH
}:),,{( 222
RyxzyxH <+=y p }),,{( yy
Block: {( , , ) :0 , 0 , 0 }x y z x W y H and z D< < < < < <
Cylinder: }0,:),,{( 222
HzandRyxzyx <<<+

Example of Half SpacesExample of Half Spaces

Different Solid Modeling TechniquesDifferent Solid Modeling Techniques
• Constructive Solid Geometry (CSG)
• Sweeping
• Boundary Representation (B-Rep)
• Feature-Based Modeling - uses feature-based primitives
to conduct a design
• Primitive Instancing - uses large numbers (200 - 300) of
“primitives" to build object - used for programming NC
machine tools (past)machine tools (past)
• Cell Decomposition, Spatial Enumeration, Octree
(connected cubes of varying size) – used for irregular
objects, image processing, medical applications (CT)

Information complete, unambiguous, accurate solid model

Constructive Solid Geometry (CSG)
• Pre-defined geometric primitivesPre-defined geometric primitives
• Boolean operations
• CSG tree structure (building process/approach)

Geometric Primitives - CSG
• A collection of pre-defined (low level) geometric primitives
• Sweeping of a 2D cross section in the form of extrusion• Sweeping of a 2D cross-section in the form of extrusion
and revolving are used to define the 3D shape (for
uncommon shapes).
Low Level
G t i
Dimensions
Geometric
Primitives
Location
Orientation
Defined
GeometricGeometric
Features

Boolean OperationsBoolean Operations
in CSG
• Union, U
• Intersection, n
• Difference or Subtraction -• Difference, or Subtraction, -
U n- -

Alternati e Paths of ModelingAlternative Paths of Modeling

Sweeping
Sweeping can be carried out in two different forms:
• Extrusion - to produce an object model from a 2D cross-section
shape the direction of extrusion and a given depth Advancedshape, the direction of extrusion, and a given depth. Advanced
applications include curved extrusion guideline and varying
cross-sections.
• Revolving - to produce a rotation part either in solid or in shell• Revolving - to produce a rotation part, either in solid or in shell
shape. Revolving a 2D cross-section that is specified by a closed
curve around the axis of symmetry forms the model of an axially
symmetric object.symmetric object.
Sweeping is most convenient for
solids with translational or
rotational symmetry Sweepingrotational symmetry. Sweeping
also has the capability to
guarantee a closed object.
Advanced: spatial sweeping; &
varying cross-section

Boundary Representation (B-rep)Boundary Representation (B-rep)
• The boundary representation
method represents a solid as ap
collection of boundary surfaces.
The database records both of the
surface geometry and the
topological relations among thesetopological relations among these
surfaces.
• Boundary representation does not
guarantee that a group of boundary
f ( ft l ) fsurfaces (often polygons) form a
closed solid. The data are also not
in the ideal form for model
calculations.
• This representation is used mainly
for graphical displays.
• Many CAD systems have a hybrid
d t t t i b th CSG ddata structure, using both CSG and
B-rep at the same time (i.e. Pro/E).

B d R t ti (B )
• Object List ---- giving object name, a list of all its boundary
Boundary Representation (B-rep)
Object List giving object name, a list of all its boundary
surfaces, and the relation to other objects of the model.
• Surface List ---- giving surface name, a list of all its component
polygons, and the relation to other surfaces of the object.p yg , j
• Polygon List ---- giving polygon name, a list of all boundary
segments that form this polygon, and the relation to other
polygons of the surface.
• Boundary List ---- giving boundary name, a list of all line
segments that for this boundary, and the relation to other
boundary lines of the polygon.
• Line List ---- giving line name, the name of its two end points,
and the relation to other lines of the boundary line.
• Point List ---- giving point name, the X, Y and Z coordinates of
the point and, and the relation to other end point of the line.

Body, Face, Polygon
(Edge Loop), Edge, Vertex
Euler Operators
MBFV: Make body face
(Edge Loop), Edge, Vertex
MBFV: Make body, face,
vertex
MME: Make multi edges
MEF: Make edge, face
KEML: Kill edge, make loop
KFMLG: Kill face, make loop,
genus

Feature-based, Parametric Models – Pro/E
• Feature-based, Parametric Solid Modeling system represents the recent
advance of computer geometric modeling. It is used as the foundation of
Pro/ENGINEER, etc.
F t b d t i lid d li li i t d th di t f• Feature-based, parametric solid modeling eliminated the direct use of
common geometric primitives such as cone, cylinder, sphere, etc, since
these primitives only represent low-level geometric entities. In designing
and manufacturing mechanical parts, one would always refer to
h i l f tmechanical features.
• The modeling approach uses sweeping to form the main shape of the part,
and build-in mechanical features to specify the detailed geometry of the
model. These features include holes (through, blind, sink), rounds,( g , , ), ,
chamfers, slots, etc. Operations to solid model, such as cut and shell
(change a solid model into a hollow shell) are also supported.
• To create the 2D cross-section for sweeping, a 2D sketch needs to be
generated in the 2D Sketcher A user can sketch the rough shape of thegenerated in the 2D Sketcher. A user can sketch the rough shape of the
closed shape. The system will automatically assign a dimension value of the
sketched feature. The dimensions of the sketched feature can be changed
at any time by simply entering the desired value, or kept as a variable,
allowing even more convenient change of its value The user has to provideallowing even more convenient change of its value. The user has to provide
all necessary dimensions to pass the section of cross-section generation.
Problems of under- or over- dimensioning can be identified.

P /E M d l G iPro/E Model Generation
Introducing Datumg
Primary Shape Definition
• Drawing Rough 2D Cross-section in A 2D Sketcher
D fi i th P i G t• Defining the Precise Geometry
• Building Solid Objects
– Extrusion to Form Depthp
– Revolving to Form Rotational Features
– Sweeps and Blends
Adding Detailed GeometryAdding Detailed Geometry
• Making Holes and Cuts
• Adding Rounds, Chamfers, Slots, and Shells

Summary - Geometric Modeling
• Geometric Modeling is a fundamental CAD technique.
• The capability of various CAD tools in geometric modeling is
usually used as a crucial factor in tool selectionusually used as a crucial factor in tool selection.
• Wireframe models consist entirely of points, lines, and
curves.
Since wireframe models do not have “body knowledge”Since wireframe models do not have body knowledge ,
topological data are not needed in construction.
• Surface models store topological information of their
corresponding objectscorresponding objects.
Both surface models and solid models support shading.
Surface models is still ambiguous and thus cannot support a
full range of engineering activities such as stress analysisfull range of engineering activities such as stress analysis.
• Solid models have complete, valid and unambiguous spatial
addressability.
In general a wireframe model can be extracted from aIn general, a wireframe model can be extracted from a
surface or a solid model.

Wireframe Modeling
• Advantages
Simple to construct
Does not require as much as computer time and memory asDoes not require as much as computer time and memory as
does surface or solid modeling (manufacturing display)
As a natural extension of drafting, it does not require
extensive training of users.g
Form the basis for surface modeling as most surface
algorithms require wireframe entities (such as points, lines
and curves))
• Disadvantages
The input time is substantial and increases rapidly with the
complexity of the object
Both topological and geometric data need to be user-input;
while solid modeling requires only the input of geometric data.
Unless the object is two-and-a-half dimensional, volume and
Cmass properties, NC tool path generation, cross-sectioning,
and interference cannot be calculated.

Surface Modelingg
• Advantages:
Less ambiguousg
Provide hidden line and surface algorithms to add
realism to the displayed geometry
Support shadingSupport shading
Support volume and mass calculation, finite element
modeling, NC path generation, cross sectioning, and
interference detection. (when complete)( p )
• Disadvantages
Require more training and mathematical background of
the usersthe users
Require more CPU time and memory
Still ambiguous; no topological information
A k d t t tAwkward to construct

CSG and Brep
1. CSG uses Euler operators in modeling.
2. CSG needs low storage due to the simple tree structure and primitives.
3. CSG primitives are constructed from the half-space concept.
4. Directed surfaces, Euler operations and Euler’s law fundamentally
distinguish the B-rep from wireframe modeling.distinguish the B rep from wireframe modeling.
5. Traditionally, CSG cannot model sculptured objects and thus is limited
in modeling capability. (This is no longer true for Adv. CAD systems,
such as Pro/E)such as Pro/E)
6. It is easier to convert a CSG model to a wireframe model than to
convert a B-rep model to a wireframe model.
7 B b th CSG d B f di ti (h lf f7. Because both CSG and B-rep use face direction (half-space or surface
normal), they can have a full “body knowledge.”
8. Generally speaking, most high-end CAD tools have the B-rep (or
hybrid) method while most low-end tools rely heavily on the CSG
method.

New Challenges toNew Challenges to
Geometric Modeling
• Modeling Porous Medium
• Modeling Non-homogeneousg g
Materials
– varying density
h i iti– changing composition
– multiple phases (solid, liquid)
– …
• Biomedical Applications (geometry, materials, motion and
mechanics)
Medical Images (surgical operation simulator training and planning)– Medical Images (surgical operation simulator, training and planning)
– Computer models from CT scans (quantify motion in actual knees)

5 geometric modeling

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