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6thSemester CAD/CAM P08ME65

CAD/CAM - ME-56

I CHAPTER

INTRODUCTION

Role of Computers in Design & Manufacturing:

Computers are being used in all fields of our life, the application of Computers in industries

in various design & manufacturing activities has also increased. An increase in the demand for

quality of the product, shorter lead time, ease of documentation and other factors has led the

manufacturers to use computers in design & manufacturing activities.

The process of designing something is an iterative and lengthy procedure. The raw material

is converted into product with the help of process. Process may be machining, assembly and

finishing etc.

Computer aided design can be defined as the use of computers to assist in the creation,

modification, analysis or optimization of a design. The computer systems consist of the hard ware

and software to perform specialized design functions.

Computer aided manufacturing can be defined as the use of computer systems to plan,

manage and control operations of a manufacturing plant through either direct or indirectcomputer interface with plant's production resources.The role of computer in manufacturing may be broadly classified into two groups.

Computer Monitoring & Control: - These are direct applications in which computer is connected

directly to the manufacturing process for the purpose of monitoring or controlling the process.

There will be a direct computer interface with machines and associated equipments for the purpose

of collecting data from the process and also to control the process based on this observations.

Manufacturing Support Applications: - These are the indirect applications in which the computer is

used in support of the production operations in the plant, but there is no interface between the

computer and manufacturing process. Computer is used to provide plans, schedules, forecasts,

instructions and information by which the firm's production resources can be managed more

effectively.

Example:- CAPP, CAD, CADD, MRP, SFC, etc.

DEVADATTA V.R Asst. Professor M.E.D PESCE 1

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Human beings are presently required in the application either to provide input to the computer

programs or to interpret the computer output and implement the required action.

The product cycle in conventional manufacturing environment


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The figure shows the flow of information of the product from design to production in a

conventional manufacturing environment In the past fifteen years the interactive computer

graphics and CAD/CAM technology have been impacting the drafting, design, and manufacturing

tools significantly.

Fig: Product cycle in a Computerized Manufacturing Environment.

The figure shows the flow of information of the product from design to production with the

aid of computers. In this, processes in the design stage are standardized, time for each process is

reduced and more accurate information of the part is passed on to production. In production,

optimum process planning & scheduling of the parts for production can be obtained. Time taken for

material, tool & product movement can be reduced. Good quality of parts can be produced in less

time by using CNC machines & computer aided inspection. In engineering practice, CAD/CAM

has been utilized in different ways by different people. Some of the applications of this technology

are:

Production of drawings and design documents

Visualization tool for generating shaded images and animated displays

Engineering analysis of the geometric models (finite element analysis, kinematic analysis, etc.)

Process planning and generation of NC part programs.


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The CAD process is a subset of the design process. Similarly, the CAM process is a subset of the

manufacturing process.

Introduction to CAD

Fig: Application of computers to the design process

In each Engineering discipline, the approach is to determine the size of a preliminary design

manually and then to subject that design to some form of analysis. The analysis may involve

sophisticated engineering calculations or it may involve a very subjective judgment of the aestheticappeal possessed by the design. The analysis procedure identifies certain improvements that can be

made in the design. Computer is used in design of a component to facilitate the designer to obtain

optimum design of the component in very short time, and to document the design. The computer

systems consist of the hard ware and software to perform specialized design functions. These

functions are geometric modelling, engineering analysis, Design review & evaluation, Automated

drafting.

Geometric modeling:

It is concerned with the computer compatible mathematical description of the geometry of an

object. The mathematical description allows the image of the object to be displayed and

manipulated on a graphics terminal. The software that provides geometric modeling capabilitiesmust be designed for efficient use both by computer and the user. The designer at first generates

basic geometric elements like points, lines circles, then he transforms these elements like scaling


Recognition ofneed

Problem

definition

Synthesis Geometric

modelling

Analysis and

Optimization

Evaluation

Presentation

Engineering

Analysis

Design review

and Evaluation

Automated

Drafting

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rotation etc. later various elements are joined /assembled into the desired shape of the object.

During geometric modeling process, the computer converts the commands into a mathematical

model, stores it in the computer data files, and displays it as an image on the screen. The model can

be retrieved from data files for review, analysis or modification.

The object can be represented in different methods in geometric modeling.

1) Wire frame - 2D, 2 D, 3D

2) Surface model3) Solid model- 3D color model

Engineering Analysis:

In any engineering design project, some type of analysis is required to describe static or

dynamic behavior of the system being designed. The analysis may involve stress strain

calculations, heat transfer computations, or use differential equations to describe dynamic behavior

of the system being designed. The computer can be used in this analysis with the help of specific

software.

Example:- Analysis of Mass properties, Finite element analysis, etc.

Design review and evaluation:Accuracy of the design can be checked conveniently on the graphics terminal. Parts details

can be closely scrutinized & dimensioning errors can be reduced. Layering procedure helps in

checking rough cast part & machined part designs. Interference checking procedure helps to

analyze assembled structure. Kinematics packages helps to verify the movement of linkages &

hinged components. Example:- ADAMS

Automated drafting:

Automated drafting involves the creation of hard copy engineering drawings directly

from the CAD database using output devices. CAD systems increase productivity in the drafting

function by roughly five times over manual drafting. Many features available in drafting process

are automatic dimensioning, hatching, scaling, sectional views, enlarged views etc. CAD systems

are capable of generating six views of the part.

Another feature of the CAD database is that it can be used to develop a parts

classification and coding system. It involves the grouping of similar part designs into classes and

relating the similarities by means of a coding scheme. This helps the designer in many ways.

The fundamental reasons for implementing CAD

1. To increase the productivity of the designer

2. To improve the quality of the design

3. To improve communications

4. To create database for manufacturingAdvantages of CAD:

1. Productivity improvement in design

2. Shorter lead times

3. Improvement in design analysis

4. Reduced design errors

5. Greater accuracy in design calculations

6. Standardization of design, drafting & documentation of procedures

7. Drawings are more understandable

8. Improved procedures for engineering changes

1. Productivity improvement in CAD increases as the following factors Complexity of the Engg. Drawing

Level of detail required in the drawing


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Degree of symmetry in the parts

Extensiveness of library of commonly used entities.

2. CAD is inherently faster than the traditional design process. It speeds up task of preparing

reports and lists. CAD system is able to produce a finished set of component drawing and the

associated reports in a relatively short time. Thus it helps to reduce the time involved in design,

analysis, and drafting.

3. Use of analysis software has made the design analysis an easy task. Lot of time is saved from

computerized analysis. Due to analysis capability, design can be created which are closer to

optimum. Alteration in preliminary designs are easier to make and analyze with CAD, more design

alternatives can be explored. This results in better design.

4. CAD provides an intrinsic capability for avoiding design, drafting & documentation errors that

occur quite naturally during manual data compilation. CAD performs time consuming repetitive

duties like multiple symbol placement, dimensioning etc., at high speeds with consistent and

accurate results. Use of part libraries makes for fast model development with accurate results.

5. There is a high level of dimensional control which cannot be accomplished manually (14 decimal

places). In case of 3-d curved space designs, parts can be labeled automatically throughout all

drawings The accuracy can be achieved in the form of accurate material & cost estimates and

tighter material procurement scheduling.

6. CAD provides a natural standard for design/drafting procedures because of single data base.

Drawing is standardized since the entire format built into the system program.

7. CAD system provided the facility to generate drawing in orthographic, oblique, isometric &perspective projections. Many CAD packages provide 3D, color & animation facilities so that the

Designer can visualize the design in different orientations.

8. CAD data storage is simple & compact any previous drawings can be easily retrieved from the

database for easy comparison with current design/drafting needs. They can be checked quickly

against new information for modification.

Introduction to CAM

Production activity can be divided into four main categories.

1. Continuous flow process:

It is the dedicated production of large amounts of bulk product.Example: - Continuous chemical plant, oil refineries etc.

2. Mass production of discrete parts:

Dedicated production of large quantities of one product (with little variation)

Example: - Automobiles, Appliances, Engine blocks etc.

3. Batch Production:

Production of medium lot sizes of the same product or component. The lots may be

produced once or repeated periodically.

Example: - Books, Clothing & certain machines, etc.

4. Job shop production:

Production of low quantities, often one of a kind specialized products.

Example:- Prototype, Aircraft, Ship, Machine tools etc.


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Computers are employed in all production activities for process planning, material requirementplanning, shop floor control, CNC M/Cs, Robots, etc. This will enhance production rate, improve

quality of the product & reduce wastage.

Advantages of CAM:

1. Shorter lead time

Time taken to produce the part is less due to higher speed and feed rates in CNC machines

hence manufacturing lead times would be reduced.

2. Greater operating flexibility

CAM enhances the flexibility in manufacturing method & in product lines.

3. Improved reliability

Because of improved manufacturing methods & control of the process, whole system wouldbe highly reliable.

4. Reduced material wastage

Use of CNC machinesreduces scrap level to minimum & no rework would be necessary5. Reduced maintenance

Most of the components of a CAM system do have integrated diagnostics & facilities, hence

they require less maintenance

6. Better management control

Since all the information are processed using computer, which helps in better control of

manufacturing process.

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II CHAPTER

1. Input Devices

Operator input devices are provided at the graphics workstation to facilitate

convenient communication between the user and the system. This allows the operator to select the

various preprogrammed input functions input functions. These functions permit the operator to

create or modify an Image on the screen or to enter alphanumeric data into the system. Most designwork stations have several different input devices. There are three categories of input devices.

1. Direction Keys on Key Board

2. Mouse

3. Light pen

4. Joystick

5. Tracker ball

6. Electronic tablet & pen

The control of cursor without direct physical contact of the screen by the user is done in 1,2,4,5

1.1 Four direction keys in the keyboard are used to move the cursor left. right, up & down

1.2 Joystick consists of a box with a vertical toggle stick that can be pushed in any direction tocause the cursor to move in that direction. It is best suited for video games.

1.3 The mouse contains a free floating ball on the underside when moved on a firm plane sulface

would follow the movement of the hand. The motion of the ball is resolved into x-y motion of the

cursor on the screen. Desired applications can be selected by clicking the buttons of the mouse.

1.4 In Tracker Ball, ball is rotated to move the cursor in the desired direction on the screen.

Movement of the ball is resolved in x-y motion of the cursor on the screen.


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1.5 The light pen is intrinsically a pointing or picking device that enables the user to select a

displayed graphics item on a screen by directly touching its surface in the vicinity of the item & by

depressing push button. It is a detector of light on the screen. In graphic applications it is used to

identify objects or locations on the screen for possible graphics handling.

1.6 Electronic tablet is a flat & electronically sensitive surface used with electronic stylus. It is a

low resolution digitizing board over which stylus can be moved by the user. The movement of the

pen tip would be communicated to the pen tip would be communicated to the screen. Tablet has

two areas:

1. Drafting Area

2. Menu Area

Designer selects any function from Menu area & draws it on the Drafting area using stylus/pen.

1.7 Digitizer consists of a large smooth board which resembles drawing board and an electronic

tracking device called puck/pen. The board surface is a position sensing surface

(electromagnetic/electronic). The designer can move the puck/pen to the desired position & then

clicking one of the button would initiate recording of x-y coordinates of the puck's position on the

board. It is the convenient method to input huge drawing from drawing sheet. A Digitizer is an

absolute measuring Device.

2. Output Devices:

It is also known as Hard copy devices. Once the output is finalized on display device,

it can be transformed into hardcopy using following devices1. Printers

2. Plotters


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3. Photographic devices.

2.1 Printers: - Printers are the fastest way of getting graphical output at low cost.

2.1.1 Dot matrix printer: - The print head consists of a vertical bank of pins (9,12,or24) which

moves horizontally over the paper, anyone of the pins can strike the paper through a ribbon to make

ink marks on, the paper. The cost is low but resolution is low (240dpi). But the cost of ink cartridgeis also high. Color print can also be obtained.

2.1.2 Laser printer: - A Laser beam scans the electro statically changed drum with a rotating 18-

sided mirror. This writes on the drum a number of points similar to pixels. For printing a positive

image, then toner powder is released. The toner powder sticks to the charged positions of the drum,

which is then transformed to a sheet of paper & bonded to it by heat. Resolution is around 600

-1200 dpi size of the sheet limited to A3 -A4. Printing rate is fast. Expensive & copy cost is high.

2.1.3 Thermal transfer: - It is similar to dot-matrix printer in operations. It uses special ribbon

placed between papers & printer head. The ribbon is a roll of thin polymer material. Spots of the

dye are transformed from the heat sensitive ribbon to the paper. The operation is noiseless,resolution is poor 400dpi. Color printers are also available. The cost of special ribbon is high.

2.2 Plotter:- The accuracy of plotters are very high and the plots can be made on all types of paper,

tracing paper & acetate film etc. There are different types of plotters used in CAD office

Drum Plotters

Flat bed plotters - Vertical and Horizontal

Electrostatic plotters

2.2.1 Drum plotter is similar to dot matrix printer but instead of print a pen is mounted on the

horizontal slide. As the drum rotates the paper is moved forward & the pen moves side ways to plot

the drawing. It is very fast, unlimited length of drawing can be plotted but width is limited. Less

expensive paper is used but it should have proper sprocket holes


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In flat bed plotters there are two types-horizontal bed plotters and vertical bed plotters.

2.2.2 In this plotter, parallel tracks are located on two sides of the flat surface a bridge is driven

along this tracks to provide x-direction motion. A writing head rides on another track which isattached to the bridge. The movement of the writing head relative to bridge produces y-direction

motion. Writing head carries pen or pencil, which can be raised or lowered to provide contact with

the paper as desired. Multi color plots can also be obtained. In horizontal bed platter Plotting

surface is horizontal and in Vertical bed plotter surface is near vertical orientation so as to conserve

floor space.

2.2.3 Electrostatic Plotters:-It consists of series of wire styli mounted on a bar which spans the

width of the charge sensitive paper. The styli have a density of up to 200/inch. The paper is

gradually moved past the bar and certain styli are activated to place dots on the paper. Each stylus

causes electro static charges at the required dot faster than pen plotters. Accuracy is better than Hard copyunits resolution is 100 to 500 dpi.

2.3 Photographic Devices: - Two types are Hard copy unit & computer output to microfilm units.

2.3.1 Hard copy units:- These are silver copiers that use light sensitive paper exposed through a

narrow CRT window inside the copier. The paper is exposed by moving it past the window and

coordinating the CRT beam to gradually transfer the image. The heated roller inside the copier is

used to develop the exposed paper. Still image of the design on the screen can be duplicated in a

matter of seconds. The copies are used as records of intermediate steps in the design process. It is

very fast but resolution is poor.

2.3.2 Computer output to microfilm (COM) units:COM units reproduce the drawing on microfilms. Speed of COM units is much faster than plotters.

Microfilms have storage benefit.

3 Display Devices:

Display device forms the most important element in a CAD/CAM system in which image is

created, modified and displayed on the screen.

The three types of display devices are

1. Cathode Ray Tube (CRT) display

2. Plasma panel display

3. Liquid crystal display

3.1 CRT Display:

In CRT display the heated cathode emits electrons which are accelerated. & focused on to a


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point on the display screen. The phosphor coated surface of the display screen gets illuminated

when electron beams hits the surface.

The two types of image - generation techniques used in CRT display are

I. Stoke Writing

2. Raster Scan Stroke Writing

The stroke writing system uses an electron beam which operates like a pencil to create a line image

on the CRT. Each line segment is drawn on the screen by directing the beam to move from onepoint on the screen to the next. Each point is defined by X & Y coordinates.

Two types of stroke writing Graphic Terminals are

I) Direct beam refresh tube (DBR)

2) Direct-view storage Tube (DVST)

3.1.1 The direct beam Refresh terminal utilizes the stroke writing approach to generate the image

on the CRT screen. Refresh means regeneration of the image many times per second in order to

avoid noticeable flicker of the image. In this technique electron beam is made to retrace the image

repeatedly. On densely filled screen it is difficult to avoid flickering.

3.1.2 DVST Terminals also use the stroke writing approach to generate the image on the CRT

screen. This tube has the ability to retain the image on the screen without retracing it. It has an

electron flood gun directed at the phosphor coated screen which keeps phosphor elements

illuminated once they have been energized by the stroke writing electron beam. Selective erase is

not possible & color is not possible.

3.1.3 Raster Scan

In raster scan, the complete screen is divided into a matrix of picture elements (pixels). It rangesfrom 256x256 to 1280x1024.


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Each pixel can be made to glow with different brightness. An electron beam creates the

image by sweeping along a horizontal line on the screen from left to right and energizing the pixelsin that line during the sweep. When the sweep of one line is completed, the electron beam moves to

the next line below. Like that sweeping of entire screen is repeated at a rate of 30-60 Hz. If the

beam intensity level only two i.e., ON/OFF i.e., the pixel will be either bright or dark. Then each

bit of memory contains the ON/OFF status of the corresponding pixel on the screen, this is called

frame buffer or refresh buffer, The Capacity of the frame buffer depends on screen resolution &

color capability. A color CRT uses Red Green and Blue electron (RGB) beam guns consisting of

and a triad of color dots on the phosphor screen to provide RGB colors. By combining these three

colors at different intensity levels, variety of colors can be created on the screen. For color displays

24 bits/pixel would be provide 224 different colors,

A color look table used in Raster scan display adapter cards contains a look up table which

sequentially codify the colors in terms of a number in each table which curtains the necessary color

mixing information as shown in fig. Whenever a particular color is to be displayed, the only

information to be stored in the frame buffer for each pixel is the color number & the lookup table;


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To the user color map is made available, & the application program relates the chosen color to the

proper pixel value. Ex: - If the user chooses color Blue for an image element, the corresponding

program sets the corresponding pixel to reflect the color Blue. Video RAM (VRAM) increases the

resolution of the display devices. Synchronous Graphic RAM (SGRAM) are new form of RAM

used in high end graphics.

Other types of terminals are plasma panel display & Liquid crystal display.

3.2 Plasma panel display:

It consists of two sheets of glass with thin, closely spaced gold electrodes attached to the inner

faces and covered with a dielectric material. The two sheets of glass are spaced a few thousandth of

a mm apart And the interfering space is filled with a neon based gas and sealed. When voltage is

applied between electrodes the gas within the panel is made to behave as if it were divided into tiny

cells. A cell is made to glow by placing a firing voltage across it by means of the electrodes. Thus

picture is generated, the plasma panel produces a very steady image and it is less bulky than other

CRT. It has relatively poor resolution.

3.3 Liquid crystal display (LCD):

They are used in laptops, they utilize less space & low power, resolution is good.

Each of the pixels is an LCD panel, is a multi-layer sandwich supported by a fluorescent backlight.


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At the two far ends of the LCD panel are non-alkaline, transparent glass substrates. The glass

substrates are attached to polarizer film that transmits or absorbs a specific component ofpolarized

light. In between the two glass substrates is layer of the nematic phase liquid crystals. There is also

a color filter containing the three primary colors (red, green and blue). Each of the polarized glass

is arranged at right angles to each other, so when electric current was passed through the LCD

panel, the liquid crystals are aligned with the first polarized glass encountered and will make a 90o

twist when approaching the other polarized glass at the end. When this happens, the light from thefluorescent backlight is able to pass through and thus giving us a lighted pixel on the monitor.

When there is no electric current, the liquid crystals will not twist and thus the light will not pass

through and a black pixel will be shown.

3.4 Light Emitting Diode (LED) display

Working principle is same as LCD monitor and only difference is instead of fluorescent backlight

LEDs are used. Resolution is high, images are sharp and clear.

4 Storage Devices:

Permanent storage of programs and of Data generated during various sessions of CAD/CAM

require a large amount of storage space. The various devices used are1. Floppy disks

2. Winchester disks (hard disks)

3. Magnetic tape cartridges

4. Compact disks

5. DVD

6. Blue Ray disks

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III CHAPTER

COMPUTER GRAPHICSThe graphic software is the collection of programs written to make it convenient for a user to

operate the computer graphics system. It includes programs to generate images on the CRT screen

to manipulate the images and to accomplish various types of interaction between the user and thesystem. In addition to this, analysis programs and manufacturing planning programs are also used

to implement certain specialized functions related to CAD-CAM. The graphics software for a

particular computer graphics system is very much a function of the type of hardware used in the

system. Example:- Types of display device & input device used in the system.

The six ground rules that should be considered in designing graphics software are;

1. Simplicity - It should be easy to use

2. Consistency - It should operate consistent & predictable way to the user.

3. Completeness - There should no inconvenient omissions in the set of graphics functions

4. Robustness - It should be tolerant of minor instances of misuse by the user

5. Performance - It should be efficient & speed of response should be fast & consistent

6. Economy - It should not be large or expensive as to make their use prohibitive

The software configuration of Graphics System: - In the operation of the graphics system by the

user a variety of activities takes place. This can be divided into three categories.

1. Interact with graphics terminal to create and alter images on the screen

2. Construct a model of something physical out of the image on the screen (Application Model)

3. Enter the model into computer memory or secondary storage.

These activities correspond to the general configuration of the software package. The graphic

software can be divided into three modules according to Foley & Van Dam is:

1. Graphics package

2. The Application Program

3. The Application database

Fig: Model of Graphics Software Configuration

Graphics package:- It is the software support between the user and the graphics terminal. It

manages the graphical interaction between the user and the system. It also serves as the interface

between the user and the application software. It consists of input subroutines and output subroutines. Theinput subroutines accept input commands and data from the user and forward them to the application

program. The output subroutines control the display terminal and convert the application models into 2D or3D pictures.

Application Program:- It controls the storage data into and retrieves data out of the application data


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base. This is driven by the user through the graphics package. The application program is

implemented by the user to construct the model of a physical entity whose image is to be viewed on

the graphics screen. Application programs are written for particular problem areas. Like Engg.

Design would include architecture, construction, mechanical components, electronics, chemical

engineering & aerospace engineering etc. other than design would be graphical display of data,

mathematical analysis, Art work etc. in each case, the application software is developed to deal

with images and conventions which are appropriate for the field.Application Database:- The database contains mathematical, numerical, and logical definitions of

the application models, such as electronic circuits, mechanical components, automobile bodies and

so for forth. It also includes alphanumeric information associated with the models, like bill also

contains most part of the graphics software, like system commands, function menus, and plotter

output routines. The database resides in main memory and in secondary storage. The contents of

the database can be displayed on the screen or plotted out in hard copy form.

Functions of a Graphics Package:

The graphics package performs variety of different functions. These functions are

grouped into function sets. Each set accomplishes a certain kind of interaction between the user and

the system. Some of the common function sets are;

1. Generation of graphic elements2. Transformations

3. Display control & wind owing functions

4. segmenting functions

5. User input functions.

1. Generation of Graphic elements:

The user can construct the application model out of a collection of elements available on the

system. A graphic element is basic image entity such as dot, line segment, circle etc. other elements

are alphanumeric characters and special symbols. In 3-D modeling primitives such as cube,

cylinder, sphere etc. are used as building blocks to construct the application model.

2. Transformations:

Transformations are used to change the image on the display screen and to reposition the item in

the database. Transformations are applied to the graphic elements in order to aid the user in

construction an application model. Transformations are translation, scaling, rotation mirroring etc..

3. Display Control & Windowing functions:

This function set provides the user with the ability to view the image from the desired angle and at

the desired magnification. It makes use of various transformations to display the application model

the way user wants it. A window can be placed wherever desired on the object being modeled to

have closer look. Hidden line removal is another feature in display control. In this, image is divided

into its visible & invisible lines and the invisible lines (hidden lines) are removed from the picture

automatically.

4. Segmenting functions:This provides users within capability to selectively replace, delete, or otherwise modify portions of

the image. The segment may define a single element or logical grouping of elements that can be

modified as a unit.

5. User input functions:

This permits the user to enter commands are data to the system. The entry is accomplished by

means of input devices. The user input function will be written specifically for the particular input

devices used on the system. The input functions should be written to maximize the benefits of the

interactive features of ICG. The main aim in computer graphics is to simplify the user interface.

Constructing the Geometry:

Use of graphic elements.

A graphic element in computer graphics is basic image entities like point, line, circle and so on. A

model is constructed using graphic elements. These elements are called by the user during the

construction process and added, one by one, to create the model.


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I. Each new element will be called before it is added to the model; the user can specify its size, its

position, and its orientation. This is necessary to form the model to the proper shape & scale.

Various transformations can be utilized to achieve this.

II. Graphic elements can be subtracted as well as added in other words, model can be formed out of

negative elements as well as positive elements.

III. It has the capacity to group several elements together into units, known as cells. A cell refers to

a combination of elements which can be called to use any where in the model. Ex:- Bolt. The boltcan be formed as a cell and added anywhere to the model.

Editing the Geometry:

A CAD system provides editing capabilities to make corrections and adjustments in the

geometric model. While developing the model, the user must be able to delete, move, copy and

rotate components of the model. The editing procedure involves selecting the desired portion ot the

model (Segmenting function) and executing the appropriate command (one of transformation

function). A common method of selecting the segment is by forming rectangle on the screen around

is by forming rectangle on the screen around the model segment. In case of light pen, pen is placed

over the component to be selected. In electronic tablet & pen, the method is to stroke a line across

the portion of the model which is to be altered. The computer indicates the user by placing a mark

on the segment, making the segment brighter than the rest of the image, and making the segmentblink. The common editing features in CAD are Move, Copy, Rotate, Mirror, Trim, Scale, Erase,

Cell etc.

Transformations

The editing features involve transformations of graphic elements. The mathematics of

transformations are given below

2D transformations:

A line can be defined as a 2x2 matrix L =

By using matrix algebra rules a graphic element can be operated on by a transformation matrix to

yield a new element. There are several transformations used in computer graphics viz. translation,

scaling, rotation, mirror, etc.

Translation: Moving a graphic element across the screen is called translation. This isaccomplished by adding to the co-ordinates of each corner point of the graphicelement to the distance through which it is to be translated

X1 = X + TXY1 = Y + TY

New translated line L1 = +

Fig: Translated SquareScaling: Enlarging or reducing the size of the graphic element is called scaling. Thiscan be accomplished by multiplying the co-ordinates of the graphic element by an


x1

y1

x2

y2

x1

y1

x2

y2

+

Tx Ty

Tx Ty

x1

y1

x2

y2

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enlargement or reductionfactor called scalingfactor S.

S=

Fig: Scaled Triangle

Rotation: Rotation of involves rotation of graphic element about the origin in counter clockwise

direction and movement of graphic element to new position. This can be accomplished by

multiplying the co-ordinates of the graphic element by a rotation factor R

R =

Fig: Rotated Point

Mirror: Moving the graphic element to create replica of the image along x- axis and y- axis is called

mirroring. To obtain this, co-ordinates of the graphic element is multiplied by mirrormatrix Mx or My

Mx = My =


x1

y1

x2

y2

+

x

New scaled line =

New rotated line =

x

x1

y1

x2

y2

+

x1

y1

x2

y2

+x

1y

1

x2

y2

+

1 0

0 -1

+

-1 0

0 1

+

New mirrored line along X axis =

New mirrored line along Y axis =

x

x

1 0

0 -1

+

-1 0

0 1

+

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Fig: Mirrored line in X direction and Y- direction

Sequences of transformations can be combined into a single transformation using theconcatenation process

Homogeneous transformations are 4 x 4 matrix

Problems on transformations:-

Wire frame and Solid Modeling:

The geometric models can be broadly grouped into two categories

a. 2-D

b. 3-D

2-D Models depict top view, front view & side view of component in true size. It is very difficult to

represent complex objects. This is useful in manufacturing applications such as simple turning job

(axi-symmetric), sheet metal punching or flame or Laser cutting- Ex.-AutoCAD


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3-D Model has the ability to provide all the information required for manufacturing applications.

i.e., most of the sides of the model can be seen in one view. Different types of projection methods

used in3-D model are Isometric projection, perspective & oblique projection. Ex.: Solid works,

CATIA, Pro-E, Ideas, Iron CAD etc.

The classification of 3-D Models: 1. Wire frame Model. 2. Surface Model 3. Solid Model.

1. Wire frame Model:- In the construction of the wire frame models the edges of the objects are

shown as mesh of lines. The image assumes the appearance of a frame constructed out of wire.Wire frame models are quite adequate for 2-D Model. For complex 3D Model it is not

recommended. Construction of wire frame model is a lengthy process ie. It requires both

topological and Geometrical data are needed to construct a model

Advantages:

1. It is simple to construct2. It is an extension of traditional method of drafting hence it does not require extensive

training of users

3. It forms the basis for Surface models

4. The CPU time required to retrieve and edit a wire frame model is comparatively lesser than

surface & solid model.

5. Only vertex data are stored hence less space is used in data base

Disadvantages:

1. These are ambiguous representations of real objects and rely heavily on human

interpretation.

2. Hidden line removal facility helps the user in this aspect

3. It lacks visual coherence & information to determine object profile.Ex:- Hole or Curve.

Application: - Used in a manual finite element modeling and tolerance analysis.


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Ex: - AutoCAD, Versa CAD, Micro station, Analysis packages etc.

2. Surface Model:- A surface model of an object is a more complete and less ambiguous

representation than wire frame model. It has associated geometric entities which make it more

suitable for engg. & design applications. A surface model consists of wire frame entities that form

the basis to create surfaces connecting the object edges. This model defines only the geometry of

their corresponding objects. They store no information regarding the topology of these objects.Visualization of a surface is aided by the addition of lines cattle mesh, which criss cross the

surface, it can be seen as a network of inter connected patches.

Advantages:

1. They are less ambiguous

2. They provide hidden line & surface algorithm to add realism to the model

3. Shading algorithm provide better realization of the model

Disadvantages:

1. It requires more training and mathematical background on the user's part

2. Surface models are more complex, hence they require more time of CPU & more space in database.

3. They are still ambiguous in some complex application models

4. Some times it requires unnecessary manipulations of wire frame entities

Ex:- Surface with holesApplication:- Surface models can be used in volume & mass property calculations, finite element modeling,

NC path generation & interference detection.

Surface entities:- Plane Surface, Ruled Surface, Surface of resolution, Tabulated Surface, Bezier Surface, B-

Spline surface, Coons patch, Fillet surface, Off set surface.

3 Solid Model:- A solid model of an object is a more complete representation than its surface

model. The way in which it stores the topological information permits functional automation and

integration. A Solid model consists of both topological & geometrical data of its corresponding

object. In many cases it is easier to build model than wire frame model, when color is added to the

image, the resulting picture becomes strikingly realistic. For the user, geometry is visible &

topology information is stored in the database is not visible. Solid model of an object may beconstructed in many ways. The Solid models are complete & unambiguous.

Advantages:


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1. It is a completely unambiguous & realistic model

2. The complete 3D definition of the part shape is stored in data base. This is utilized in

automating & integrating design & manufacturing functions.

3. It improves the quality of design

4. Improves visualization

Disadvantages:

1. It cannot create automatically other models from solid definition2. It cannot use data created in other models to create a solid

3. More computation time, huge data base is required.

Applications:- Interference Analysis, FEM, NC Programming, CAPP & Machine vision etc.

Solid entries;-:- Block, Cylinder, Cone, Sphere. Wedge, Torus

There are nine schemes to create Solid models of real objects

1 Half Spaces

2. Boundary representation (B-Rep)3. Constructive solid Geometry (CSG)

4. Sweeping

5. Analytic Solid Modeling

6. Cell decomposition

7. Special enumeration

8. Octree encoding

9. Primitive instancing.

The most popular and widely used three approaches are

1. B-Rep - Boundary representation

2. CSG - Constructive Solid Geometry3. Sweeping

B-Rep:- In B-Rep approach user has to draw the outline or boundary of the object on the CRT

screen using an electronic tablet/pen, or using analogous procedure. Various views of the object &

inter connecting lines among the views to establish their relationship are to be drawn. Specialized

editing features are used to refine the model to the desired shape. B-Rep model is based on

topology information of an object A Boundary model of an object is comprised of faces edges &

vertices of the object linked together in such a way as to ensure the topological consistency of the

model.

Advantages:

1. It is best suited for the construction of solid models of unusual shapes.Ex:- Automobile body styling. Aircraft fuselage & Wing shapes etc.

2. It is very simple to convert B-rep models to wire frame models. Since boundary definition is


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similar to wire frame definition.

3. Algorithms based on B-Rep are reliable and competitive with those based on CSG.

B- Rep Technique

Disadvantages:

1. It requires large amount of data storage space because it stores the explicit definition of the

model boundaries.

2. It is a lengthy process. The model is defined by faces. edges and vertices which tend to

increase for complex models.

3. It is not suitable for applications like NC tool path generation

C.S.G:- The CSG system allows the user to build the model out of solid graphic primitives. These

primitives are combined using Boolean operations to form the object model. It is relatively easy

approach, results in more compact file of the model in the data base. The CSG approach stores the

model by a combination of data and logical procedures. This requires less storage but more

computation to reproduce the model.

Advantages:

1. It is easy to construct model out of primitives and Boolean operations

2. It is concise & requires minimum storage for solid definitions

3. Model construction is even through different from conventional drafting & it is easy tounderstand

Disadvantages:

1. It takes more computation time to reproduce the model

2. Slow in generating wire frames. Since CSG must be converted into B-Rep to generate wire

frame model.

3. It is impossible to generate sculptured surfaces or unusual shapes.

Sweep Representation:

The Sweep representation are useful in creating solid models of2% D solids models of2% D


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solids are solids of uniform thickness in a given direction & axisymmetric solids. Sweeping in used

in general as a means of entering object descriptions into B-Rep or CSG based modelers Sweeping

is based on the notion of a moving a point. curve or a surface along a path. There are three types of

sweeps- Linear, Non Linear & Hybrid.

Advantages:

1. It is useful in complex mechanical parts such as screws, springs & other components

2. Nonlinear sweeps are useful in creating non-rigid objects.Disadvantages:

Limited only to 2 D objects.

Applications:

1. Simulations of material removal due to machining operations2. Interference detection of moving bodies.

Commercial solid modelling packages use all the three modelling approaches. Ex:- CATIA,

Solidworks, Unigraphics, Pro-E, Auto CAD Inventor, Iron CAD etc.

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Defining the graphic elements

The user has a variety of different ways to call a Particular graphic element and position it on the

geometric model. There are several ways to define point, line, circle & other graphical elements.

These components are maintained in the database in mathematical form and referenced to 3-D co-

ordinate system. Ex:- A polygon would be defined as an ordered set of points representing the

comers of the polygon. The mathematical definition can be converted into its corresponding edges

and surfaces for filling in the database and display on the CRT screen

Methods of defining elements in Computer Graphics

Points

1. Pointing to the location on the screen by cursor control

2. Entering the co ordinates via keyboard

3. Entering the offset distance from previous point

4. Locating points at fixed intervals along an element etc.

Lines:

1. Using two previously defme point

2. Using one point & angle of the line with horizontal

3. Using a point and making the line normal/tangents to curve

4. Making line tangent to two curves.

5. Using a point and making the line paralle and perpendicular to another point etc.Arcs & Circles:

1. Specifying center & the radius

2. Specifying center and a point on the circle

3. Making the curve pass through three defined points

4. Making the curve tangent to three lines

Conics:

1. Specifying five points on the element

2. Specifying three points and a tangency condition.


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CAD/CAM Integration:

Modem CAD Systems provide extensive capabilities to Graphic packages like.

1. Automatic cross hatching the surface

2. Capability to write text on the drawings3. Semi automatic dimensioning

4. Automatic generation of bill of materials & Analysis of application model.

These are the features and manufacturing planning procedures for the product development. There

is a significant overlap in the database required for design and that of manufacturing. This makes

the integration of CAD/CAM successful. A CAD system develops a data base (material, Geometry,

No. of parts etc.) needed to manufacture the product. In an integrated CAD/CAM system, a direct

link is established between product design & manufacturing. The Aim of CAD/CAM not only to

automate certain phases of design & certain phases of manufacturing but also to automate the

transition from design to manufacturing.

A manufacturing database is an integrated CAD/CAM database, which includes all the datagenerated during design (material, geometry, parts list etc.) and additional data required for

Manufacturing.


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Desirable Modeling Facilities:

Modelling facilities desired in any CAD System are as follows

1. The Geometric modeling features

2. The Editing features

3. The display control facilities

4. The Drafting features5. The Programming facility

6. The analyses features

7. The connecting features

1. Geometric Modeling features:

It include facilities like,

1. Cartesian & polar coordinates, absolute & incremental dimensions, various types of units,

grid, snap, object snap, layer etc.

2. Interactive & dynamic dragging of points, lines arcs, circle, splines etc.

3. Wire frame modelling, 3D lines, 3D faces, ruled surfaces, linear & rotational sweeps

4. Revolve options for generating helical & spiral shapes5. Solid modeling with different approaches like CSG, B-Rep & Sweep.

6. Skinning around regular & arbitrary surfaces/profiles

7. Sculptured surfaces like Bezier, Coons etc.

8. Assembly of Solids, Surface filleting & trimming.

2. Editing features:

To edit the model created it should include facilities like

1. Transformations like Move, Dopy, Mirror Rotate, Scale etc.

2. Extra features like Stretch, Delete, Erase Undo, Delete etc.

3. Symbols or cells (set of geometric entities like-Bolt) Scale & exploded view of

symbols/cells.

4. Display Control Facilities:

a) Window, Zoom, Pan, Hidden line/surface removal, shading

b) Animation

c) Clipping (discarding part ofthe Geometry outside the viewing window)

d) Perspective views, orthographic views, Isometric views, sectioning etc.

4. Drafting Features:

This refers to the way the model is utilized for the purpose of transferring the information in

hard copy form for other applications like printouts of the part to shop floor. The facilities required

are various types of lines text handling capability & different methods of text presentation,Dimensioning & associated dimensioning etc. Diffe7rent types of views like isometric, perspective,

orthographic view with sectioning details etc.

5. Programming Facility:

Programming ability (MACRO Programming) within a CAD system is useful. It allows the

designer to program specifically for an application. Once programmed it requires least amount of

data for modelling. Ex;- Auto LISP in Auto CAD, GRIP in unigraphics, GLUE in CAM-X

6. Analysis features required are calculations of area, perimeter, volume mass C.G., MOl etc. finite

element analysis where model created in CAD would be passed to FEA through intermediate

processor known as finite element modeler, which converts geometric data into finite element mesh& calculates all the data required for the analysis & then transmits it through FEA program. Ex:-

GFEM for Uni-graphics other features are assembly & interference checking, Animation and also


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explode an assembly facility. .

7. Connecting features:

The data stored by the modelling system should be documented & should have good

connectivity with other allied modules. A neutral data interface standard would be needed with

each modeling system so that geometric data may be transferred through this for further processing.

Introduction to exchange of modeling data:

There is a need to establish standards in CAD that help in linking different hardware

and software systems from different vendors to be intergraded to serve the requirements of the

industry. CAD/CAM software is available from a number of vendors hence it is necessary that

there should be a means by which different systems are able to interchange information to avoid the

duplication effort involved in the geometric model creation i.e., data base formats used by all the

software should be the same. The data base formats are identified on the basis of the modelling

requirements and hence it is not possible to have identical format for all the systems. However, it is

possible to identify a certain format for drawing exchange and make it a standard so that the

various systems can convert their internal format & vice versa. Numbers of formats available are

IGES, DXF, STEP, DMIS, etc.

Various standards in Graphics Programming

Initial Graphics Exchange Specification IGES:

IGES is the most comprehensive standard and designed to transmit the entire product

definition including the manufacturing & other associated information. In IGES the records are

present with 80 column fields. 1-72 provides data 73-80 for sequence number. IGES file consists of

five sections they are

1. Start section(S):-I_ is setup manually by the person initiating the IGES file and contains

information that may assist the user at the destination. Ex:- Listing of the drawing.2. Global Section

(G):-It has 24 fields, it contains information about details of the product, the person originating the

product, name of the company, drafting standard and other information required for its postprocessing.3. Directory Section (D):-It has 20 fields of 8 characters. This provides an index for the

file and stores attribute information like color, line type etc. which may be present directly or


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through a Pointer Parameter 4. Data Section (P):- This contains the data associated with the entities.

Like coordinate values, text, spline data points etc. each entry has a pointer in columns 66 to 72 to

the directory entry for entity. 5. Termination Section (T):- This marks the end of the data file, and

contains subtotals of records for data transmission checking purposed.

Standard for the Exchange of Product Model Data (STEP)

It incorporates a formal model for the data exchange, which uses data modelling languagecalled "Express". STEP models are divided into two main elements. Application model & resource

model. Application model refers to drafting & modelling Resource model refers to description of

raw geometry or of the topology.

An overview of STEP

Description methods:- A modelling language EXPRESS is developed such that the protocols be

properly defined. Part Number 11-13. Implementation methods:- This specifies how the STEP

information be physically represented for the exchange format. Parts No. 21-26 Conformance and

tools:- This provides the specifications for conformance testing of the processors used for STEP

information. They provide information n methods for testing of software-.product conformance to

the STEP standard. Parts No, 31-35 Integrated-generic resources:- This contains geometric &

topological representation, material, visual representation & other information of the model PartsNo. 41-46 Application information models:- This specifies the information models to be used for

specific application areas such as drafting, FEA, kinematics etc. Parts No.-1Ol-upwards

Application protocols:- These are the main protocols to be used as subsets of STEP information

model for exchange of data between specific application systems. (Between two FEM systems or

between CAD & Process planning system parts No. 20l-upwards) Application interpreted

constructs:- These relate to the specific resources useful for defining the genetic structures useful

for applications. This is useful in expressing identical semantics in more than in one application

protocol. Parts-50l-above Ex: edge based wire frame, shell based wire frame etc.

Data Exchange Format (DXF)

It is a neutral file format developed by Auto Desk for use with Auto CAD software. This file

allows information to be transferred readily between Auto desk products, or exchanged with other

CAD systems. It is an ASCII text file with extension of DXF. The DXF file has.

Header Section:- This contains general information about the drawing.

Classes section:- This has information for application defined classes like Blocks, Entities &

objects sections of the data base.

Tables section:- This contains definitions of the symbol tables used in Auto CAD Ex:- Line type

table, layer table, view table, DCS table etc.

Blocks section:- This contains block (Symbol) definition & drawing entities that make up each

block reference in the drawing.

Entities section:- This has the graphical objects (entities) in the drawing including block references.Object Section:- This has non graphical objects in the drawing. Ex:- dictionaries of multilane styles

& groups.

DXF file is composed of many groups each occupies two lines in the DXF file. First is for group

code second is for group value

Dimensional Measurement Interface Specification (DMIS)

It is a new standard in communication for manufacturing established by CAM-I. This

establishes bi-directional communication of inspection data between computer systems &

Inspection equipment.

The data base in the form of geometric instructions and manufacturing information present

in the system can be used to convert it into CNC part programs. From the same database it is alsopossible to generate the inspection programs for the CMM. The part produced by CNC would be

checked on CMM. After CMM checks the part, data about the part is sent back to the computer.


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Thus, part geometry as designed. DMIS provides a complete vocabulary for passing inspection

program to the dimensional measuring equipment and to pass results back to the computer.

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