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Chapter 1

CAD data representation Schemes

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Computer Aided Design

• the use of computers to support the design of products

• design or drawing?

• AutoCAD

• Silicon Graphics, Intergraph, HP

• 2-D and 3-D

• Output to Plotter...

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CAD allows:

• the display and editing of design drawings

• the storage of completed drawings for later retrieval

• Database of drawings

= Product specifications

• allows re-use of product specifications and designs

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CADD (Computer Aided Design and Drafting)

• Conventional CAD packages are based on computational geometry, which is based on the shapes and dimensions of items, and are flexible enough to DRAW anything from a house to a pin.

• DESIGN takes into account the strengths of materials and the conditions under which an item will be used, and also how it will be made.

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Geometric Modeling

• Definition: Geometric modeling is the technique we use to describe the shape of an object.

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CAD Computer- Aided Drafting and Design

• 2D - Two Dimensional

• 3D - Three Dimensional– Wire Frame Models– Surface Models– Solid Models– Parametric Models

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Different kinds of CAD Models

• Wireframe models– curved or straight lines

– represented by vertices and edges

• Surface models– represent an object in terms of points, edges and

faces

– frequently used to generate NC programs for the moulds and dies needed to make curved surfaces

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Solids models

• represent the outline, surface, volume and mass

• a more complete description of the properties of the object

• allows a more complete analysis

• two main approaches:– constructive solid geometry– boundary representation

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Product Data resources

• CAD data

• Product structure (BOM)

• manufacturing instructions incl.– routing data– NC program– inventory, history, forecast

• Product Data Management (PDM)

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Managing Product data

• CAD/CAM downloading• CAD-CAD transfer of data

– new CAD system installed

– supplier-customer cooperation

– upgrade from surface model to solids model

• Initial Graphics Exchange Specification• Standard for Exchange of Product model data

(STEP) - more generic framework

3-D Wire Frame Modeling

• Disadvantages:– Not able to fully define

real objects

(No volume and mass)

– Displays only edges of a model

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Surface Modeling

• Surface models provide a more complete description of the object, but is still not as good as the solid model.

Solid Modeling

Solid

B-Rep CSG

Boundary Representation:ExtrusionRevolvingSweepingBlending of 2-D profiles

Constructive Solid Geometry:Uses solid primitives (cylinder, cone, etc.) as building blocksConstruction of solid by combining primitives using Boolean operators of union, intersection, and difference

Operations in constructive solid geometry

Boolean union Boolean difference Boolean intersection

The merger of two objects into one. The subtraction of one object from another. The portion common to both objects.

Application of CSG

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Parametric Modeling

Parametric Modeling

Constraint-Based ModelingDimension-Driven Modeling

Uses parameters to specify dimensions of entities

Uses geometric constraints such as parallelism, concentricity, etc.

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Design

C o nc re te(R e a lism )

A b s tra ct

P e rson a l E xp ress ion(A rtis it ic )

A e s th e tic(In d u s tria l D e s ig n)

P ro du ct P ro ce ss

E n g in e eringD e s ig n C yc le

F u n ctio n a l(E n g in e e rin g D es ig n)

P ro d uc t/P ro ce ssD e ve lo p m e nt

(T e ch n ica l)

D e s ign

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More advanced Approaches

• Feature-based modeling– Uses features such as holes, ribs, pockets, slots– More advanced than CSG

• Assembly modeling– Modeling of assembled parts– Utilizes much of the same technology as solid modeling

• Product modeling– Includes not only the geometry of an object, but also

any information that is needed to design, develop, and produce the product. (e.g. stress-analysis)

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Drawing Exchange Format

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• What is CAM? Nominate then describe briefly the different applications of CAM?

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• Discuss the capabilities of current CAD/CAM systems?

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• Explain the methods of data transfer between dissimilar CAD systems; IGES and PDES?

• Explain the meaning of the different acronym?

• Differentiate among Wire frame, Surface and solid modeling

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• (c) Explain the B-rep and CSG methodologies of solid modeling? Then explain their advantages and disadvantages?

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• (d) Show how can you decompose a product model consists of a block having a blind step and a through slot into their lower primitives using both B-rep and CSG?

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