Introduction To Geometric Modeling

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Introduction To Geometric Modeling

What is CAD ?

In the fast moving world of technology and development of technical aspect made it

necessary for the students to keep themselves updated the latest developments in

Mechanical engineering. Computer Aided Drafting (CAD)is one of the recently

developed and highly advanced technologies.

2-dimensional and 3-dimensional drawings are the ways to express the efforts of

Mechanical design Engineer. Traditional Methods involve drawing manually using

drafters and drawing boards, which is comparatively time consuming and tedious

work. Advanced soft wares are fast, accurate and user friendly.Autocad , ,Ideas,Pro-

Engineer are are some of the very advanced tools for CAD.

Before starting any work on CAD software it is necessary to have some basic

knowledge of engineering drawing.

Applications:

Mechanical, Civil, Aeronautic Engineers in order to visualize ,design or plot the actual

diagrams of various engineering components as per their requirements extensively

use AUTOCAD .

Some of the typical examples are listed below:

- Drawing of Mechanical structure

- Drawing of building plans

- Drawing of Structured framework

- Drawing of various machine components

- Drawing of sectional views of engineering components

- Architectural drawing

- Mechanical Design and assemblies

- Space modeling

- Aerospace designs

- Machine Design


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Basic Shapes and Objects

There are two different types of basic shapes viz. : Two dimensional and

there dimensional. In two dimensional, line curve arc the basic shapes which forms

complete 2D drawings. For 3-dimensional drawings cone, cylinder, cube and wedge

are basic objects.

Understanding Co-ordinate System:

There are two types of co-ordinate systems:

(i) Cartesian co-ordinate system

(ii) Polar co-ordinate system

Cartesian co-ordinate system :

This system defines a point in the space using three co-ordinates viz. X, Y and

Z. X-dimensional used to describe length, Y-dimensional to describe width and Z-

dimensional used for describing height.


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2-Dimensional Drawings: 2-dimensional drawings are defined by length and

width dimensional only. There drawings are represented on one single plane. Only x

and y co-ordinates are required for defining any point on this plane. 2-dimensional

are described in (x, y) format. (Refer fig.3)

3-Dimensional Drawings: 3-dimensional drawings are defined by length, width

and Height. These drawings are represented using x, y and z co-ordinates.any point

is defined using x, y and z co-ordinates. These 3-dimensionas are described in (x, y,

z) format.(Refer Fig.4).

Polar co-ordinate system :

This system defines a point in the space using its distance and angle of direction

from reference point and fixed reference direction. Generally East is as a zeroreference direction. North is 90 o, West is 180 o and south 270 o with respect to East.

Other directions are specified by measuring angle from zero reference direction.


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INTRODUCTION

Strengths and weaknesses of conventional 2D drawing:

Two dimensional model is basically a model which is made in typical 2D ordinates.

Strengths:

Better Drawings and Visualization

Storage and retrieve of drawing is easier

Increases accuracy, Uniformity and quality of drawings

Increases Productivity

Standard component library and parametric program can be used for production

of CAD work

No scrap work

Inbuilt dimensioning facilities

Effective time Utilization

Hard can be generated by printing the drawing on paper

The size can be altered as per requirements

Unit of measurement can be selected

Multi colored display makes user more comfortable to understand intricate details

in more effective manner.

Drawings can be sent/received via email in seconds.

Time & cost of implementing the CAD system

Time & cost of training people to use the CAD.


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Time & cost of migrating legacy drawings into CAD format

Weaknesses :

Such models can be used for limited applications only, as most of the real life

products are 3D in nature.

2D objects are useful only in information sharing and is not efficiently used for

other engineering applications except in special conditions.

Advantages of 3D over 2D :

When using 2D cad drawings during the manufacturing stage ,numerous

problems arise .Viewing 3D models helps indetify errors easily. These erros can

be found while simulating the matching and mating of parts .Through the use of

3D CAD the assembly procees of any given product can be simulated, visualized

and analyzed before the design goes into production.3d CAD models are essential

beforehand in determining the volume of material needed to mold specific parts

as well. The use of 3d cad files also ensures that a design has sufficient room for

other parts within design.

3D CAD is simply more accurate than 2d cad drawings, the end result is better

product,optimized in design ,weight with better performance,no faults in

design,in less time for less money.

Most 3d soft ware will allow you to rotate your models so you can get a look at

any side you want.

You will be able toturn it over to look at it from any angle-a good reason to go

3D.

Volumetric calculations are fast and easy with 3D model which late helps you

with things like cost analysis.

More stylish /complex products can be designed using 3D surfacing.

Clearance/inteference checks can be made easily

3D CAD data can be imported directly in to CAM software, further reducing .

3D models are also invaluable when it comes to rapid prototyping and

manufacturing.

Need of a Geometric Model:


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Computer aided design attempts to eliminate the need of developing a prototype for

testing and optimizing the design. Instead it assists in evaluating the design using a

model with geometric parameters created on the computer. Such a model is called a

Geometric Model.

The different fields for which a geometric model is used are listed below with

following characteristics:

1] Modeling or design (CAD):

Model must be easy to generate

Facilitate minimum calculations at the user level

Encompass different options of generating the model with advanced and easy

modifying tools and techniques.

Better graphics to visualize the design graphically

2] Analysis and Optimization (CAE): Mass Property calculation

Volumetric/Area calculation

Stress analysis/CFD analysis

Assembly mating and interference checking

Motion analysis of mechanisms and Robots

Tolerance check analysis

Crash analysis for virtual crash testing

3] Computer aided Design and Drafting(CADD):

Generating production drawings

Visualizing information and drawings

Generating exploded views, cut sections for technical illustrations

4] Computer Aided manufacturing(CAM):

Process Planning and Scheduling

Part programming

Robot programming

Actual production control

5] Bill of Material (BOM)Generation:

Material requirement

Manufacturing resource planning

6]Inspection and Quality Control:

Inspection machines /Inspection Robots


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Comparison of design and Part Specification

Types of Geometric Modeling:

3D modeling is done in three different ways:

1) Wire frame Modeling

2) Surface Modeling

3) Solid Modeling

1)Wire-Frame Modelling :

It is the oldest and simplest method of geometric modelling which can be used to

store model mathematically in the computer memory. It contains information about

the locations of all points and edges in space co-ordinates. Various wire frame

entities are points,lines,planer arcs, circle ,curves etc.Each vertex is defined by a pair

of vertices and faces are defined as three or more edges.Thus wireframe is the

collection of edges,there is no skin between area defining edges.This is the lowest

level of modelling and has serious limitations. But in some applications such as tool

path simulation it is very convenient to use wire frame models.

Advantages:

It is the simplest method and uses a less memory space.

It forms basis for Surface and Solid Modelling.

Manipulations in the model can be done quickly and easily.

Disadvantages:

One of the serious limitations is the ambiguity of orientation

and viewing plane. Due to this it is very difficult to interpret the

object from a particular viewing

Plane and creates confusion.

Lack of information to determine object profile as circles or

curved surfaces.

Physical properties such as mass, surface area, volume, centre

of gravity are not possible to calculate.

Wire frame model has no knowledge of surface faces ,therefore

it will not detect any interference between two mating

components. This is a serious drawback in Robot arm

manipulations.

A lot of data is requird to indetify such model.

Can not model complex curved surfaces.


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2) Surface Modeling:

Surface modeling is the next step of Wire frame Modelling.A surface model

represents skin of an object,these skins have no thickness or material type.Surface

model define the surface features,as well as the edges, of objects.It is used to create

complex shapes of automobile bodies,ship hulls and also air craft bodies. A surface

model is defined in terms of faces in addition to wire frame entities. Various analytic

and synthetic surface entities are available.Advantages:

Eliminates ambiguity and non-uniqueness present in wire frame

models by hiding lines not seen.

Renders the model for better visualization and presentation,

objects appear more realistic.

Provides the surface geometry for computer aided manufacturing

(CAM),NC machines.

Surface properties such as roughness. Color and reflectivity can be

assigned and demonstrated.

Disadvantages:

Provides no information about inside of an object.

Curved surfaces need a fine mesh to be accurate.

Provides wrong results if mesh is too coarse.


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Complicated computation, depending on the number of surfaces.

3) Solid Modelling:

In the solid modeling, the solid definitions includes vertices(nodes),edges, surfaces

,weight and volume. This model consisting of the complete description of the solid in

a certain form is most ideal representation, as all the information required at every

stage of product life cycle can be obtained with this technique. This is the most

widely used model method and a number of different techniques are widely famous

viz. Constructive Solid Geometry(CSG) and Boundary Representation(B-rep).

Advantages :

Has all the advantages of surface models (uniqueness, non-

amiguous,realistic,surface profile) plus volumetric information.

Allows the designer to create multiple options for a design.

2D standard drawings ,assembly drawing and exploded drawings are

generated from the model.

Can be used in newly manufacturing (CAM) and design for manufacturing

(DFM),computer aided manufacturing(CAM).

Volumetric and mass properties of an object can be easily obtained

.Corresponding mass properties are obtained if density is included.

Disadvantages :


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More intensive computation than Wireframe and surface modelling.

Requires more powerful computers (faster with more memory) .

Difference between Wire Frame , Surface and Solid Modelling


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ParametersWire frame

Modelling

Surface

ModellingSolid Modelling

1. Computer memory

required

Less Moderate Large

2. Entities Used Point, line, circle,arc,,

ellipse synthetic curves

such as Benzier,

Hermite,B-spline etc

Plane

,revolved,ruled,tab

ulated,free from

surfaces

Solid primitives such as

cone,cube,wedge,

3. Input data required More Moderate Less

4. Automatic

Orthographic,

perspective ,isometric

view generation

Impossible Impossible Easily possible

5. NC code generation Not possible Automatic Possible Automatic Possible

6. Interferencebetween mating parts

Not possible to detect Can detect Can detect

7. Rendering and

shadow effect

Not Possible Possible Possible

8.Cross-Sectioning Not Possible done

manually

Not Possible Possible done automatically

9. Elimination of

hidden lines

Done manually May be possible Possible

10. Calculation of

physical properties

such as volume ,

surface area , centre

of gravity,M.I. etc.

Not Possible Possible to

calculate some

properties

Possible to calculate all properties

required for analysis

11. Design parameter

optimization

Not Possible Not Possible Possible

12.Generation of

assembly and detailed

drawings from model

and vice versa

Not Possible Not Possible Possible

13. CAD/CAM/CAE

modules

Drafting Drafting.Design Drafting.Design,Manifacturing

,analysis,

assembly,mechanism,optimization

Documents

Introduction To Geometric Modeling