ME5372/7372 Introduction to CAD/CAM

Session 2,(August 28, 2008)

Instructor : Rajeev Dwivedi

www.engr.smu.edu/~rdwivedi/ME5372.html

Department of Mechanical Engineering

Southern Methodist University

Dallas TX

Computer modeling of objects (in context of CAD/CAM)

computer based well defined representation of a physical object that can be used/processed/analyzed for various production/manufacturing applications.

Computer modeling of objects (in context of CAD/CAM)

Computer modeling of objects (in context of CAD/CAM)

Computer modeling of objects (in context of CAD/CAM)

Computer modeling of objects (in context of CAD/CAM)

Computer modeling of objects (in context of CAD/CAM)

Computer Model of object

The computer model of the object can be - approximate representation of the solid or accurate representation.

The representation essentially is driven by functionality and resources.

Approximate representation of objects and key issues –

- LOSSY

Approximate representation of objects and key issues –- The object is represented by subdivision/discretization such as

mesh and other geometric primitives.

Approximate representation of objects and key issues –

-It is sometimes easier to apply complex mathematical operations such as FEM to smaller well defined set of elements-OR-fabricate a complex/large solid by subdividing it (e.g. Slicing)

DIVIDE AND CONQUER

Approximate representation of objects and key issues –

-Usage of point clouds is inevitable in the geometries generated by reverse engineering.

Approximate representation of objects and key issues –

-Amount of data used to represent geometry is extremely large, complex and expensive.

Approximate representation of objects and key issues –Traingulated representation (STL) : the solid is represented in

context of surface that is approximated by triangles.

Approximate representation of objects and key issues –voxel (volume element) : representation of a solid by a regular

rectangular grid such that the voxel element essentially reflects (1) existance of the solid at (2) a given set of spatial coordinates.

Approximate representation of objects and key issues –Mesh:The solid/surface is expressed by set of points (nodes) that

are connected to generate building blocks (elements) such as triangles, quadrilaterals, tetrahedral, polyhedral. The edges of these elements may be straight or curvilinear as governed by the function.

Approximate representation of objects and key issues –

summary

1. LOSSY2. The object is represented by subdivision/discretization such

as mesh and other geometric primitives.3. It is sometimes easier to apply complex mathematical

operations such as FEM, slicing etc.4. Usage of point clouds is inevitable in the geometries

generated by reverse engineering.5. Amount if data used to represent geometry is extremely large,

complex and expensive.6. Example – STL, voxel, mesh

Accurate representation of objects and key issues –-- Not possible to represent every geometry; however, is capable

of representing most of real life geometries.

-- Extensive mathematical representation that is driven by complex data structure and expensive algorithm.

--Easy translation from one platform to another…?

--Example –BREP, CSG etc.

BRep (Boundary Representation) –

The object is represented in the context of limiting elements or the boundary. Various surfaces are interconnected to represent a solid.

BRep (Boundary Representation) –

The solid comprises by the limiting faces. The faces are limited by edges and edges are bounded by vertices.

Two primary components of the BRep -

(1) the geometric elements (vertices, edges, faces) and

(2) topological elements i.e. how the geometric elements relate to each other to define a solid in unambiguous fashion.

CSG(Constructive solid geometry)

-A solid is created by performing a set of logical operations on a set of solid primitives.

Solid Primitives for CSG

Logical Operations

CSG(Constructive solid geometry)

-A solid is created by performing a set of logical operations on a set of solid primitives.

CSG(Constructive solid geometry)

The three major component of solid modeling based on CSG include –

(1) solid primitives,

(2) relative location of primitives

(3) The tree of set of Boolean operations.

HOMEWORK Problem Based on this principle

Solid Modelers in commercial software

Due to its inherent attributes BRep method underlies most of the commercial solid modelers -ACIS (Spatial Corp)-Parasolid (UGS, Siemens PLM)-ProE (proprietary)-etc.

Some common attributes of popular CAD software (apply to ProE)

A set of predefined or user defined features are used to generate a solid (Similar to CSG but CSG is not used in literal sense ).

The solid is created in context of features rather than the underlying geometry.

The hierarchy of features is bounded by a logical order also called parent-child relationship.

parent-child relationship allows changes in a part to propagate thorough a well defined relationship/dependence.

ProE

-- is a 3 dimensional parametric feature modeling software-- product of Parametric Technology Corporation (PTC)-- is based on the Propriety ProE kernel

-- and what else??( we will explore)

Other Issues

1. Maintain a copy of the assignment on a flash media2. The computers in the lab are reset and all the documents

wiped periodically3. Feel free to use the unix server space to save the files

however you should be confident with file handling on unix servers.

4. No delay in the home assignment submission5. New assignments will be handed out on Thursday and the

home assignments are due week after.6. Best way to reach the instructor – rdwivedi@engr.smu.edu.

*Always mention ME5372…. in the subject7. Send a mail to the address so that instructor can make a

mailgroup to send important messages.