OVERVIEW OF CAD/CAM

What is computer Aided Design (CAD) ?   CAD or computer aided design defined in many methods and includes a  variety of activities.  It can be said to be the integration of computer   software techniques in engineering design. At  end when we talk of modelling, it emphasis on the following: 1.)Use of computer for  product design. 2.)Numerical methods, optimizations technique etc. 2-D/3-D drafting 3.)3-D modeoling for visualization of product. 4.)Modelling curve, surface, solid, mechanisms, assembly, etc. The model then developed is first visualize on  monitor using a variety of techniques including shaded images displays, hidden surfaces removed displays  etc. Once the product designer is convinced, then  model is then move to next step for many types of analysis softwares. thus, at the end it include various steps of analysis  processes. These could be following- 1.)Stress or deflection simulation, i.e. numerical method meant for estimation of the behavior of product with respect to given parameters. It includes software tools like the Finite Element Method (FEM). 2.)Simulation softwares of actual use 3.)Optimization softwares Other applications like 1.)CAD/CAM integrations 2.)Process planning methods These are processes which usually use model developed using CAD softwares. They all use CAD models and often the kind of application they have to be used in a determines the kind of model to be developed.  Thus there are these following aspects to CAD. 1.)Modelling of product. 2.)Display/ Visualization of product. 3.)Applications of product. MODELING   Modelling typically includes a set of processes like 1.)Defining object. 2.)Defining relations between objects. 3.)Defining properties of object. 4.)Defining the orientations of the object in suitable coordinate system 5.)Modification of existing definitions or editing. The figure below explains what a typical CAD models would need to be defined, what kind of entity need to be defined  and what relationships exist .   At the top level we have the product which is defined by set of surfaces. These surfaces can  be either planar or curved / warped. A planar surface can  bounded by set of curves. A curved surface can be seen as a net of curves. These curves are typically a succession of curves segments which defines the complete curves. The  curve segment is defined by using set of end points / control point which governed by nature of the curve. Thus a relationship is defined between entities at each level. Once a relationship is defined, a geometric models of the product is achieved. DISPLAY / VISUALIZATION  Displaying the model requires the following: Mapping object on screen coordinate system: Models are usually developed in a model coordinate systems. this could be the world coordinate systems, or a coordinate systems local to the objects. these coordinate system are normally 3-D in nature. To display the objects on a 2-D screen, the objects coordinates need to be mapped on the 2-D coordinate system of the screen. This requires two step: Viewing transformation: The coordinate of the objects are transformed in such a manner as if someone is looking at the objects through the screen. This coordinate system is referred to as the viewing coordinate system. Projection: The objects in  viewing coordinate systems is then projected on the two dimensional plane of the screen. Surfaces display or shading or rendering: In displaying the object on screen one often like to get  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 objects 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 design, 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: Estimation of stresses or strains or deflections in the objects under various static loading conditions Estimation of the same under dynamic loading condition. Visualization of 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. Optimization of the objects for Development of 2D engineering drawings of the object Development of 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 activities 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: An overview of the hardware systems used in CAD 2D and 3D transformations used to shift between coordinate systems Projection transformation used to get the object in screen coordinate systems Modelling of curve and surface Modelling of solid. 2D Transformations BASIC TRANSFORMATION Animation are produced by moving the 'camera' or the objects in a scene along animation paths. Changes in orientation, size and shape are accomplished with geometric transformations that alter the coordinate descriptions of the objects. The basic geometric transformations are translation, rotation, and scaling. Other transformations that are often applied to objects include reflection and shear. Use of transformations in CAD In mathematics, "Transformation" is the elementary term used for a variety of operation such as rotation, translation, scaling, reflection, shearing etc. CAD is used throughout the engineering process from conceptual design and layout, through detailed engineering and analysis of components to definition of manufacturing methods. Every aspect of modeling in CAD is dependent on the transformation to view model from different directions we need to perform rotation operation. To move an object to a different location translation operation is done. Similarly Scaling operation is done to re size the object. Coordinate Systems In CAD three types of coordinate systems are needed in order to input, store and display model geometry and graphics. These are the Model Coordinate System (MCS), the World Coordinate System (WCS) and the Screen Coordinate System (SCS). Model Coordinate System  The MCS is defined as the reference space of the model with respect to which all the model geometrical data is stored. The origin of MCS can be arbitrary chosen by the user. World Coordinate System As discussed above every object have its own MCS relative to which its geometrical data is stored. Incase of multiple objects in the same working space then there is need of a World Coordinate System which relates each MCS to each other with respect to the orientation of the WCS. It can be seen by the picture shown below. Screen Coordinate System In contrast to the MCS and WCS the Screen Coordinate System is defined as a two dimensional device-dependent coordinate system whose origin is usually located at the lower left corner of the graphics display as shown in the picture below. A transformation operation from MCS coordinates to SCS coordinates is performed by the software before displaying the model views and graphics. Viewing Transformations As discussed that the objects are modeled in WCS, before these object descriptions can be projected  to the view plane, they must be transferred to viewing coordinate system. The view plane or the projection plane, is set up perpendicular to the viewing zv axis. The World coordinate positions in the scene are transformed to viewing  coordinates, then viewing coordinates are projected onto the view plane. The transformation sequence to align WCS with Viewing Coordinate System is. 1. Translate the view reference point to the origin of the world coordinate system. 2. Apply rotations to align xv, yv, and zv with the world xw, yw and zw axes, respectively. 3D Projections OBLIQUE PROJECTIONS oblique projection illustrates the general 3-D shape of the objects. However only face of the objects parallel to the planes of projection are shown at there true sizes and shapes, that is angle and length are preserved for these faces only. Infact ,the oblique projections of these faces is equivalent to an orthographic front view of model. TYPES OF OBLIQUE PROJECTION- 1.Cavalier OBLIQUE PROJECTION. 2.Cabinet OBLIQUE PROJECTION. Cavalier OBLIQUE PROJECTION-A Cavalier OBLIQUE PROJECTION is obtained when the angles between oblique projector and the planes of projection is 45 degrees. In a Cavalier OBLIQUE PROJECTION the foreshortening factor for all three principal directions are equal. The resulting figure appear too thick. A Cavalier OBLIQUE PROJECTION is used to correct this deficiencies. Cabinet OBLIQUE PROJECTION - An Cabinet OBLIQUE PROJECTION for which the foreshortening factors for edges perpendicular to the planes of projection is one half is called a Cabinet OBLIQUE PROJECTION . •

STANDARD PERSPECTIVE PROJECTIONS A perspective transformations are the transformations from one of three space into another three space. In contrast to  parallel transformations , in perspective transformation parallel lines converges, object size is reduced with increasing distances from the center of projections, and non-uniform foreshortening of line in the objects as a function of orientations and the distance of the objects from the center of projections occurs. All of these effects laid the depth perceptions of the human visual systems., but the shape of the objects is not preserved. Perspective drawing is characterized by perspective foreshortening and vanishing point .Perspective foreshortening is the illusions that objects and length appear smaller as there distances from the center of projection increases. The illusions that certain set of parallel lines appear to meet at a point is another feature of perspective drawing. These point is called vanishing point .Principal vanishing point is formed by  apparent intersections of lines parallel to one of the three x,y or z axes. The number of principal vanishing points are determined by the number of principal axis interested by the view planes. Perspective Anomaly 1.Perspective foreshortening- The farther the object is from the center of projection ,the smaller it seems. 2.vanishing Point- Projections of lines that are not parallel to the view planes (i.e. lines that are not perpendicular to the view planes normal) appear to meet at some point on the view planes. This point are called the vanishing points. A vanishing point correspond to every set of parallel line. Vanishing point corresponds to the three principle direction are referred to as "Principle Vanishing Point (PVP)". We can thus have at most three PVP. If one or more of these are at infinity (that is parallel line in that direction continue to appears parallel on the projection planes), we get 1 or 2 PVP perspective projections.