GEOMETRIC MODELINGSYSTEMS

CAD/CAMYOGI MULDANI HENDRAWAN

Background

Documentationi

CAD + CAE

EvaluationOprimaton

DesignAnalisys

Designn ModelAnalityc

Inquiry DesignSpecification Feasibility Study

Design Concept

ProcesPlanning

ProductionPlanning

Designing &Tooling

Preparation

Material Handling

NC, CNC, DNCprogramming

ProductionQualityControl Packaging Distribution

CAM

Marketing

Design Proces

Production Process

Analisis

Sintesis

Geometric Modelling

Background

Geometric Modeling Systems

The design process can be thought as the detailing of the shape as thedesigners idea evolves.

The geometric modeling system is a tool by which designer can manipulatethe shapes

Geometric modeling system came into being to overcome the problemencountered with the use of physical models in the design process

These systems provide an environment similar to the one in which thephysical model is created and naturally manipulated.

Geometric modeling systems are classified into wireframe, surface, solid,and nonmanifold modeling systems

Wireframe Model

This system represent a shape by its characteristic lines and endpoints.

The system use these lines and points to display the shapes and allowmanipulation of the shapes by modifying the line and point.

Connectivity information identities which points are the end points ofwhich curves and which curves are adjacent to each other and atwhich points

Wireframe Advantages

Its simplicity to construct a model. Does not require extensive training of users. Wireframe models form the basis for surface models. CPU time required to retrieve, edit, or update a wireframe models is

usually small compared to surface or solid models.

Wireframe Disadvantages

Wireframe models are usually ambiguousrepresentation of the real object and rely heavilyon human interpretation. See the Figure.

Its mathematical description does not includeinformation about the inside and outsideboundary surfaces of the objects beingmodeled object s impossible to calculateobjects mass properties and derive the toolpaths to machine its surfaces.

Therefore, wireframe modeling systems havetended to be replaced by surface modeling andsolid modeling system

Surface Modelling Systems

Shape design and representation of complex objects such as car, ship, andairplane bodies as well as castings cannot be achieved utilizing wireframemodeling (need another modeling system).

In surface modeling system, the mathematical description includes surfaceinformation in addition to the wireframe information.

Thus, a list of surface equations, a list of curve equations, and thecoordinates of end points are updated as the model is manipulated.

The mathematical description may include the information about surfaceconnectivity (i.e., information on how surfaces are joined and whichsurfaces are adjacent to each other at which curves, etc.)

This information is very helpful in some applications programs, e.g. NC toolpath program.

Surface Modeling Systems

Typically, three methods to create a surface in surface modeling system:1. By interpolating the input points.2. By interpolating the curve nets specified.3. By translating or revolving a specified curve.

Surface modeling systems are used to create models with complex surfacesfor two purposes:1. The visual model is used to evaluate the model aesthetically,2. The mathematical description is used to generate the NC tool paths to

machine its surfaces.

Surface Advantages

More complete and less ambiguous representation than wireframe. Can be used in volume and mass properties calculation, finite

element modeling, NC path generation, and interference detections

Surface Disadvatages

Surface modeling does not lend itself to drafting background. Requires more training and mathematical background on the users

part. Surface models are generally more complex and thus require more

terminal and CPU time and computer storage than wireframe models. They are still ambiguous in some application as is the case when

application, determining which of an objects surfaces define itsvolume.

Surface models are sometimes difficult to maneuver to create andmay require unnecessary manipulations of wireframe entities

Solid Modeling

This systems are used to model a shave having a closed volume, calledsolid.

A solid model is a more complete representation than its surface model. Itis unique in the topological information it stores which potentially permitsfunctional automation and integration.

The mathematical description of a shape created by a solid modelingsystem contains information both the topological and geometrical data(see Figure 1-5).

Any information related to the volume of the solid can be derived, and thusapplication programs can be written to do operations at the level ofvolume instead at the level of surface.

Defining an object with a solid model is easiest of the available threemodeling techniques (wireframe, surface, and solid).

Solid Advantages

Give a more complete and unambiguous model than both wireframeand surface models.

Solid models contain both topological and geometrical data. Can be used in volume and mass properties calculation, finite

element modeling, NC path generation, and interference detections.

Solid Disadvantages

Requires more training and mathematical background on the userspart.

Solid models generally require more terminal and CPU time andcomputer storage than surface models.

Creating a solid model requires a large amount of input data inproportion to the amount of data stored in mathematical description Nonmanifold modeling system is developed

Non-Manifold Modeling System

Solid modeling system allows a user to create only a shape having a closedvolume or the manifold model in the mathematical terminology.

Solid modeling system disallows nonmanifold conditions as: two surfaces touching at a single point, two surfaces touching along an open or closed curve, two distinct enclosed volumes sharing a face, edge, or vertex as a

common boundary, a wire edge originating from a point on a surface, and faces forming a

cellular structure.

Nonmanifold modeling system allow a mixture of wireframe, surfaces andsolids

Non-Manifold Example

Why we Create a nonmanifold model ?

Disallowance of nonmanifold models has been considered to be one of thegood features of solid modeling system because solidmanifold modelsyield only for manufacturable models.

It is a different story when we want to use a geometric modeling system forthe entire design process.

A designer who wants to design a plastic container, for example, mightstart from a model such as the one shown in above figure, withoutconsidering its thickness.

Similarly, the geometric model in above figure, would be a natural initialdesign for a structure composed of a solid block holding a plate with acord.

Thus, an abstract model of mixed dimensions is very useful in supportingthe designers reasoning design process.

Why we Create a nonmanifold model ?

A model of mixed dimensions might include some dangling edges, laminarplates, and solids.

Unfortunately, conventional/manifold geometric modeling system(wireframe, surfaces, or solid modelers), do not support the representationof the nonmanifold models.

Most abstract models, as an intermediate result in the design process, haverepresentations composed of mixture of one-, two-, and three-dimensionalelements we need a nonmanifold modeling system.

An abstract model is also useful in providing a basis for analysis. Variousanalytic tools can be applied to verify the design at each stage in the designprocess.

Nonmanifold model is indispensable in capturing the evolution of a designfrom incomplete lower level prescriptions to a true solid.

Homework

Drawing box which is dimension 100 x 50 x 75 mm with 3 modelingsystem (wireframe, surface, and solid)

Convert all file to STEP. Open file STEP by notepad. Compare a result. Search example of non-manifold model & draw it.

Deadline = October 7th, 2014. 09:00 a.m Send by email to yogi@polman-bandung.ac.id

References

1. Ibrahim Zeid, CAD/CAM: Theory and Practice, McGraw-Hill, Int.Edition, 1991.

2. Chris McMahon and Jimmie Browne, CAD/CAM: Principles,Practice and manufacturing Management, Addison-Wesley, 2ndEdition, 1998.

3. Kunwoo Lee, Principles of CAD/CAM/CAE Systems, Addison-Wesley, 1st Edition, 1999.