15. Computer Aided Design (CAD)

15.1 Design

• Recall the basic process of design (or at least one of the many)


• The important phases of this diagram are,

The ongoing refinement of the overall design

The ongoing refinement of the detailed design

The analysis of the design

• This diagram (or at least the main concepts) form the basis for CAD systems. A complete CAD system will provide as much of the structure above as possible.

• Some of the tools provided in a CAD environment are,

Innovative and conceptual design

Qualitative design analysis

Structuring of part (e.g. assemblies)

Knowledge based/intelligent design tools

Engineering design information (standards lookup, or electronic catalogs)


Design interfaces, and tools

• Some applications are well suited to 2D CAD systems,

PCBs (Printed Circuit Board Design)

ICs (Integrated Circuit Design)

Mapping (road maps, topographical maps)

• Consider the example of a sheet metal layout


• 3D CAD systems are becoming widely used for Mechanical design in a number of businesses these days including,

Aircraft Design


Consumer electronics


• CAD systems provide advantages such as,


Minimizes design errors

Graphical display of hard to visualize information (e.g. 3D warping of plastic part)

Standardized drawings, and documents

Faster lead time

Customer perception is improved

Productivity improvement over time

Developing alternate concepts

Evaluation of alternate concepts

Analytical investigation of parts

Experimental investigation

Detailed drawings and specifications

Preliminary ‘construction’ of design prototype

Easy bridge to prototype construction

Easy to change designs


15.2 CAD History

• A very brief history of CAD development is listed

1940s: First digital computer developed

1950s: Commercial computers become available

1955: CRTs begin being used in military projects

1957: APT II (Automatic Programmed Tool) developed for generating NC control. Automated NC used in industry.

1959: Stromberk Carlson develops a system to interpret graphics on tape, then output them to a screen, or print on special paper

1963: Ivan Sutherland presents a paper on “Sketchpad” which allows interactive graphics

1965: Lockheed introduces a CAD/CAM system, and a FEM system. McDonnell introduces CADD

1966: Business world sees Wall Street Journal title “Electronic Sketching; Engineers Focus on Screen to Design Visually via Computer; Keyboard Enlarges, Rotates ‘Drawings’; Lockheed, GM Enthusiastic About Uses”

1971: David Prince writes first book on computer graphics

1975: ICAM (Integrated Computer Aided Manufacturing) project is begun by US Airforce

1976: Color raster graphics technology begins to develop.

1979: Development of IGES begins

1980: Introduction of PCs revolutionizes all markets

1980s: Solid Modeling on UNIX

1990s: Solid Modeling on low end systems

15.3 Basic CAD System Requirements

• A CAD System must,

Allow a user to input geometry, and other information

Provide methods for manipulating the geometry

Provide for display output for the user

Allow storage of the design in formats which can be used in other CAD and CAM packages

• There are other features of great value to a CAD system,

Don’t forget the manuals, They can help get through the tough times. If not, you may spend a lot of money calling help lines.

15.4 Editing and Creating

• Each CAD package allows us to manipulate the geometrical model using various interactive techniques.

• Editing Geometry depends upon the representation the geometry is stored with.

• If an elemental (remember: lines, circles, arcs, etc.) geometric model is used, then the methods are much different than a B-Rep model.

• The major editing methods used are for Elemental, Surfaces, and CSG.

15.4.1 2D Curves and Lines

• A number of functions must be provided to allow editing of 2D geometrical entities, such as lines, circles, arcs.

• Some of the basic editing functions are listed below,

Basic Entity Creation (lines/ circles/ arcs/ etc)

create using exact coordinates

two screen points for line ends, circle radius/center/diameter/etc

Line Trimming

trim lines back to intersection

extend lines to intersection

trim line to perpendicular point

cut a circle/arc on one side of an intersection

Point Creation

screen position

exact numerical coordinate

nearest tangent of line to an arc

nearest end of a line

midpoint of nearest line

center of nearest arc

nearest grid point

Arc Creation

intersection of circle with another line

Special Techniques

offset of a line

extend lines to intersection

trim line to perpendicular point

delete entities


• There are a number of ways (philosophies) for creating drawings using the basic elemental editing techniques. A few popular methods are listed below.

Construction Lines: A set of construction lines are set up, then segments of the lines are selected for the actual drawing


Trimming: The construction lines are all drawn, then the unwanted parts are trimmed off


Navigation: A line figure is built up using successive line segments.


Parametrization: Objects such as rectangles, circles, arcs, etc. are created using their dimensions, then positioned with traditional methods.

15.4.2 Surfaces

• Most surface modeling packages rely on the elemental definition of lines, and points.

• There are a number of basic philosophies for creating surfaces,

Swept profiles: a profile, and a path in space are used to sweep out a surface.

Rotated Profiles: a Profile is created then swept about an arbitrary axis

Extruded Profiles: a profile is created, then grown in one direction.

Skins (Splines): a direct creation of points, then the splines that connect them

Polygon Approximation: polygons are defined which join up to define a surface

Sections: sections are defined for different points along a path, which then allow generation of complex transition geometries.


• Once surfaces have been created, they may be operated on by boolean operations.

• This method is often used as a preliminary stage to CSG editing.

15.4.3 CSG

• This is by far the simplest method

• Solid Primitives are progressively cut and joined to form new shapes.

• Primitives may come from,

Traditional Sources: Blocks, Spheres, Wedges, etc.

Surfaces: A Volume is assigned to a surface model

Previous operations

others ?

• CSG editing requires storage of the results of operations. This is because a part may be used many times to cut another part, for example a chamfered hole for a sunken screw.

• The fundamental CSG operations are,

Union: both parts joined as one

Intersection: Only where two parts overlap

Subtraction: only where parts do not overlap, One of the parts is typically discarded.


15.5 User Interpretation of Geometric Models

• Every CAD system uses a graphical display for user interpretation of the final part.

• The display methods discussed in the computer graphics section are all used in CAD packages. (Please refer to that section for examples, and explanation of how each display method affects the user).

• There are many techniques possible with computer graphics that make on screen designs easier to understand.


Placed manually, but updates when dimensions change.

Annotation: the user may add comments to drawings

text with a leader pointing to something

text alone


Drawing information

Graphics effects

Fill Styles: hatching and other patterns selected

Line Styles: such as hidden, shadow, phantom, etc.

Color: helps differentiate when there are many lines

15.6 User Directed Changes to the Geometric Model

• This feature is of the greatest importance to a user

• Some onscreen input selection features are,

Boxes: a good example of this method is the zoom boxes

Types: all objects of a certain type are selected

Last: last object created

All: all visible objects

Names: in some systems parts are named, and these can be used for reference.

Markers: symbolic markers can be placed on the screen to allow easy differentiation between object (like in Ideas)

Layers: drawings can be layered up, which allows easy separation of distinctive parts. One example is a factory layout, including separate layers for machines, plumbing, electrical, HVAC, etc.

• Objects often require manipulation on the screen, and certain parameters defined by a number of techniques,


select two points (they define a translation vector)

angle and distance

select a line (a direction vector)

move a point to a point




about a pivot point by 1 angle (2 angles for 3D)

about an axis by an angle


mirror over a line/plane


a magnification about a point by a factor


a simple copy

a copy with an offset for each copy

a copy with a rotation for each copy

15.6.1 Modern Hardware for CAD Systems

• The CRT is taken for granted now, but previously CAD systems underwent many metamorphosis.

• The array of current and previous hardware input devices are,




button/dial boxes

light pen

touch screen

track balls

joy sticks

punched cards



3D scanners


• Future hardware input devices include,

virtual reality gloves


scanned input and recognition

vision systems

• Previous output devices include,

Text printers

Graphics printers



Rapid prototyping

Virtual reality vision systems

15.7 Selecting a CAD System

• While this apparently seems easy, it is a very complex decision

• There are a number of factors which affect how the system is received,

Current computer use by employees

Perceived role of computer by employees

Cost of computer system and software

Available training for staff

Maintenance requirements for computer

Required number of users

Design complexity

Availability of CAM Facilities

Successful implementation in similar facilities

Management philosophy

Redundancy of design

• Major mistakes are,

Assuming more expensive is better

Assuming it will be well received because it will make work easier

Assuming that high tech means easier to use

Failing to get the potential users interested and involved in the decision to buy/selection/implementation

Neglecting the break in period

Not thoroughly examining the existing manual/computerized system which ALREADY WORKS

Forgetting that accountants want numbers plus a rate of return.

15.7.1 An Example Plan for Selecting a CAD system

1. Examine the existing situation. Involve staff to find out what they perceive as problems, and possible solutions. This establishes allies required for whatever decision you choose. (Expect some who will resist, but they can become allies if handled properly).

2. Identify key people with an interest in the system, and get them involved with selection.

3. Devise a definite list of requirements, to support existing functions, and problems which exist, and possible solutions.

4. Get the accountant on your side by consulting them about costs, budgets, etc.

5. Gather information about existing systems by visiting trade shows, reading magazines, talking to others using systems.

6. Talk to Salesmen and companies of interest.

7. Get the salesmen to present to the CAD selection group

8. Narrow the possible vendors to about three.

9. Talk to their other customers about their system problems, and advantages, support, etc.

10. Pick a package using the CAD selection group and management.

11. Prepare budget, using help from accounting, and include a large portion of the budget for training, and maintenance.

12. Schedule training and implementation dates. Ensure that implementation is gradual, and does not overlap with the busiest times

13. Propose the budget and schedule to management, and request approval.

14. Give a general announcement to all concerned, and those partially concerned. A General meeting will help. The more information the better.

15. Follow schedule, and evaluate after each stage of implementation.

15.7.2 A Checklist of CAD/CAM System Features

• A list below is suggested for hardware, but in light of recent advances in consumer computing, most of the previous concerns, such as special plotter papers, are no longer problems.

• Hardware



Personal Computer

Unix Workstation


Network ready

Backup capabilities

Disk space

CPU performance

Uninterruptable power supply (UPS)


Screen resolution (768*1024 or 1024*1280 are suggested)

screen size (14” is absolute minimum)

dual monitors

I/O Devices

Drawing output


laser printer (color?)

ink jet printer

Input Devices



track ball/roller ball

• System Software

Operating System


MS-Dos/Windows 3.1,95,NT/etc.


Other ? (VMS, ???)

• CAD Software

Geometrical model


Exact or faceted with planar polygons

Mass properties



Object Organization

Named Objects


Part libraries

Drawing Output

Drafting module

Analysis Module

Finite Elements

Plastic Flow




Surface formats: IGES, DXF, CDL

Solid Formats: PDES/STEP, SAT

Files for systems such as NASTRAN

Can be linked to a user written program


Hidden line

Shaded Image

Ray Tracing

Real Time Rotations

15.8 Design

• Design interfaces have been continuously improving over the years,

ASCII Text Files

Keyboard Entry, with printed output

Keyboard Entry with graphic terminal output

Icon and Menu Driven with on-screen graphics

Fully windowed interfaces

• As computers become cheaper, and more powerful, the only interfaces of real importance are the Graphical User Interfaces (GUI).

• An example of novel technology is the visual scanner available for 3D input.

15.8.1 Graphical User Interfaces

• The current demands on user interfaces are,

on-line help

adaptive dialog/response


ability to interrupt processes

consistent modules

a logical display layout

deal with many processes simultaneously

• The common trend is to adopt a user interface which often have,


A pointer device (such as a mouse)

Full color

Support for multiple windows, which run programs simultaneously

Popup menus

Windows can be moved, scaled, moved forward/back, etc.

• The history behind these machines are,

Development of Mouse based graphical interface at Xerox Palo-Alto Research park (70s)

Personal Computers began providing graphical programs for system management, games, etc (Early 80s)

MacIntosh, Sun, Apollo, Silicon Graphics, and others introduced mouse driven, fully windowed computers (Mid 80s)

MacIntosh Competitor IBM PC gets OS/2 and Microsoft Windows (Late 80s). Marking massive movement to Windowed environment by all players in scientific computing.

X-Windows becomes a new, and widely accepted standard on workstations (Late 80s)

Microsoft introduces Windows, bringing windowed interfaces to the last major computer platform.

• Some Concepts in GUIs are,

button: An item which is shown within a window. When a user points at it, and presses a mouse button, it initiates an action.

icon: A small graphical symbol on the screen which can be opened to expose a window

menu: A pop up menu which stays hidden until called up by mouse. This simplifies problems of crowded screens.

mouse: a very popular input device for graphics programs. The use can point and choose an item. Contemporary alternatives are track-balls, joy-stick, dial boxes, tablets, etc.

scrollbars: At this side of some graphical, and text windows are bars which can be used to move the window around, to see previous text, or hidden areas of a graphics screen.

slider: A bar chart type of input, where the user can use the mouse to pull the slider along, and change an input value

window: A panel for keyboard and mouse I/O, which can be layered on a screen with other windows, like paper on a desk. The user often selects to work in a specific window by pointing the mouse into it. A Window may be closed, to become an icon

• Popular window systems are (not a complete list),

OS/2: IBMs attempt to take control of the operating system used on the IBM PCs, and bring full capability to PC architecture.

Windows 3.1: Microsoft’s answer to the MacIntosh interface

Windows 95: Microsoft’s answer to Windows 3.1: adds a true multitasking environment.

Windows NT: Microsoft’s answer to Windows 95: adds more capable network and file security issues.

MacIntosh Interface: The proprietary windowed operating system, considered one of the forerunners in user friendly systems.

Sunview: The original windowed systems used on Sun computers

X-Windows: A defacto standard for newly developed windowed operating systems.

Openwindows: Sun’s new windowed operating system which is a superset of X-Windows

Motif: A competitor to Openwindows, also based on the X-Windows standards

• The Implications of X-Windows will be very important in future computer purposes. Some of the X-Windows Features are,

intended for networking, including display of programs across a network. The implication of this is that I may sit at a Sun computer in my office, and run Ideas across the network from the SGI lab.

Shared definitions makes software very portable between machines. (The quantity of public domain software is huge).

The user interface is very similar when going between different X-Windows based machines.

Easy to Customize for an individual user

The differences between systems like Motif and Openwindows are mainly based on definitions of things like buttons, fonts, etc.

When using X-Windows, a program (called the X server) runs which controls all the windowed graphics. Programs that use X are written to let it set up buttons, get input, call functions, etc.


• Windows NT is not yet as capable as X-Windows, but if the trend continues it will become more similar over time.

• Automatic GUI generators are available on commercial systems. One example is given for a system which allows window layout, then automatic program generation.

15.9 Problems

Problem 15.1 Create a time-line of CAD technologies.