CAD if often defined in a variety of ways and includes a large range of activities. Very broadly it can be said to be the integration of computer science (or software) techniques in engineering design. At one end when we talk of modeling, iIt encompasses the following:

• Use of computers (hardware & software) for designing products

• Numerical method, optimizations etc.

• 2D/3D drafting

• 3D modeling for visualization

• Modeling curves, surfaces, solids, mechanism, assemblies, etc.

The models thus developed are first visualized on display monitors using avariety of techniques including wire frame displa, shaded image display, hidden surface removed display and so on. Once the designer is satisfied, these models are then used for various types of analysis / applications. thus, at the other end it includes a number of analysis activities. These could be:

• Stress (or deflection) analysis, i.e. numerical methods meant for estimating the behaviour of an artifact with respect to these parameters. It includes tools like the Finite Element Method (FEM).

• Simulation of actual use

• Optimization

• Other applications like

  • CAD/CAM integration

  • Process planning

These are activities which normally use models developed using one or more of the techniques mentioned above. These activities are often included in other umbrellas like CAM or CAE. A term often used is CAx to include this broad set of activities. They all use CAD models and often the kind of application they have to be used ina determines the kind of amodel to be developed. Hence, in this course  I cover them under the umbrella of CAD. In this course we will strive to give an overview of modelling techniques followed by some applications, specifically CAM.

Thus there are three aspects to CAD.

• Modeling
• Display/ Visualization
• Applications
   

Modelling typically includes a set of activities like

• Defining objects
• Defining relation between objects
• Defining properties of objects
• Defining the orientations of the objects in suitable co-ordinate systems
• Modification of existing definition (editing)

The figure below explains what a typical CAD model would need to define, what kind of entities need to be defined and what relationships exist between them.

At the highest level we have the volume which is defined by (or "delimited by") a set of surfaces. These surfaces can be either planar or curved / warped. A planar surface can be bounded by a set of curves. A curved surface can be seen as a net of curves. These curves are typically a succession of curve segemnts which define the complete the curve. The curve segment is defined using a set of end points / control points which govern the nature of the curve. Thus a relation ship is defined between entities at each level.

Once such a relationship is defined, a geometric model of the artifact is available. In any design there might be manysuch artifacts. One then has to define properties of each of these artifacts and define a relationship between them. The properties and the relationships needed are dependant on the application the model is to be used for subsequently. But one common application that all models have to go through is visualization of the model (s).

DISPLAY / VISUALIZATION
 

Displaying the model requires the following:

• Mapping objects onto screen coordinates: Models are typically made in a model coordinate system. this could be the world coordinate system, or a coordinate system local to the object. these coordinate systems are typically three dimensional in nature. To display the object on a 2D screen, the object coordinates need to be mapped on to the 2D coordinate system of the screen. This requires two steps:
  • Viewing transformations: The coordinates of the object are transformed in a manner as if one is looking at the object through the screen. This coordinate system is referred to as the viewing coordinate system.
  • Projections: The object in the viewing coordinate system is then projected onto the two dimensional plane of the screen.
• Surface display or shading / rendering: In displaying the objects on the screen one often likes to get a shaded display of the object and get a good feel of the three dimensional shape of the object. This requires special techniques to render the surface based on its shape, lighting conditions and its texture.
• Hidden line removal when multiple surfaces are displayed: In order to get a proper feel of the three dimensional shape of an object, one often desires that the lines / surfaces which are not visible should not be displayed. this is referred to as hidden line / surface removal.

Once a model is visualized on the screen and approved by the conceptual designer, it has to go through a number of analysis. Some of the kinds of usage this model might have to go through are the following:

• Estimating stresses / strains / deflections in the objects under various static loading conditions
• Estimating the same under dynamic loading conditions
• Visualizing how a set of objects connected together would move when subject to external loading. This leads to a whole set of activities under simulation. These activities would vary depend upon the application the object is to be subject to.
• Optimizing the objects for
• Developing 2D engineering drawings of the object
• Developing a process plan of the object
• Manufacturing the object using NC / CNC machines and generating the programs for these machines so as to manufacture these objects.

Having given the overview of the kind of activties that can come under the umbrella of CAD the uses these CAD models can be put to, I know highlight what aspects of these would be covered in this course. Needless to say, all these activities would be well beyond the scope of one single course. Therefore this course, which is targeted to give an overview of CAD and its applications would include the following:

1. An overview of the hardware systems used in CAD
2. 2D and 3D transformations used to shift between coordinate systems
3. Projection transformation used to get the object in screen coordinate systems
4. Modeling of curves and surfaces
5. Modeling of solids