Lecture Autodesk inventor Solid modeling data structures

Lecture Autodesk inventor Solid modeling data structures. The main contents of the chapter consist of the following Constructive solid geometry (CSG), boundary representation (Brep), hybrid of CSG and Brep.

Solid Modeling Data Structures

Types of Solid Modelers

Constructive solid geometry (CSG)

Boundary representation (B-rep)

Hybrid of CSG and B-rep

Categorization based upon way data is stored in database

Constructive Solid Modeling (CSG)

Uses Boolean operations to create complex solids from primitives

Primitives, Boolean operations stored in database Can make changes to primitives

Tree structure

Range of shapes limited (only primitives and

Booleans)

Since primitives stored (not surface boundaries), must recalculate model each time it is shaded

Calculation of NC tool paths for machining is a

CSG Tree Structure

Tree structure

represents model

history

Shows which

primitives (leaves)

and operations

(nodes) were used,

and their order

Boundary Representation Modeling

Stores boundaries of solid in database (e.g.,

vertices, edges, faces)

Similar to surface modeling

More difficult to edit once stored

Edge information stored in a systematic way

(CCW from outside)

Shading, hidden line removal can be

displayed at will

B-rep Data Structure

Euler’s Formula: V – E + F = 2

V = # of vertices

E = # of edges

F = # of faces

CSG and B-rep Comparison

Union of two

box primitives

CSG information

B-rep information

Hybrid Systems

Store model using both CSG and B-rep

methods

 Uses CSG as primary, B-rep as secondary for display

Large file sizes

Most commercial solid modelers employ

a combination of both CSG and B-rep

Modeling Kernels

Engine used by software to perform basic

functions (Boolean operations, blending, mass properties, exporting geometry, chamfer/fillet) Common kernels:

 ACIS

 Parasolid

Easier to translate between modelers that use same kernel

3D Modeling Software

Faceted versus Exact Representation

Uses of Solid Models

Accurate representation of 3D objects Aid to visualization, communication

Can extract 2D drawing information

Used to control CAM methods

Interference identification

Mass property determination

Analysis and simulation (finite element, kinematics, ergonomics, etc)

Rapid Prototyping

Shortcomings of Solid Models

Difficult to modify a model’s features

Does not capture the design intent

Design Intent is the purpose or function of a feature in a part (or of a part in an assembly)

Solid Modeling Data Structures

Khallas!