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CAD

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CAD/CAM

CAD

Computer-aided design/computer-aided manufacturing (CAD/CAM) refers to the integration of computers into the design and production process to improve productivity. The heart of the CAD/CAM system is the design terminal and related hardware, such as computer, printer, plotter, paper tape punch, a tape reader, and digitizer. The design is constantly monitored on the terminal until it is completed. A hard copy can be generated if necessary. A computer tape or other control medium containing the design data guides computer-controlled machine tools during the manufacturing, testing, and quality control.
The software for CAD/CAM is a collection of computer programs stored in the system to make the various hardware components perform specific tasks. Examples of software are programs developed to generate a NC tool path, to assemble a bill of materials, or to create nodes and elements on a finite element model. Some of these software packages are referred to as software modules and can be classified into four categories: (1) operating systems, (2) general-purpose programs, (3) application programs, and (4) user programs. Although there are other kinds of software, these are sufficient for an explanation of the complexities in developing a CAD/CAM system.
Operating systems are programs written for a specific computer or class of computers. For convenient and efficient operation, programs and data are available in the system’s memory. The operating system is especially concerned with the input/output (I/O) devices like displays, printers, and tape punches. In most cases the operating system is supplied with the computer.
Although it may be argued that there are no general-purpose programs as such, some are more general than others. An example is a graphics program written in a high-level language like FORTRAN that allows the generation of geometric entities such as lines, circles, and parabolas and a combination of these to make designs. These designs may range from printed circuits to drill jigs and fixtures.
Application programs are developed for a special or specific purpose. The first language for specialized application was Automatically Programmed Tools (APT) in 1956. APT was deve- loped to ease the job of NC programmers in developing input to NC machine tools, as illustrated in Fig. 7-1. Other examples of application programs, relative to CAD/CAM, are programs developed specifically for the generation of finite element mesh and flat pattern development or “unbending” of sheet metal parts. These programs are usually purchased with the system or from a software supplier.
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Fig. 7-1    Example of CAD where a lathe tool is called up to show a machining sequence

User programs in CAD/CAM are highly specialized packages for creating specific outputs. For example, a user program may automatically design a gear after the user inputs certain parameters like the number of teeth, pitch diameter, and so on. Another program may calculate optimum feeds and speeds, given cutter information, material, depth of cut, and so on. These programs are often developed by the user from a software module furnished by the supplier of general-purpose software. Not all CAD/CAM software packages have these user programs, even though considerable savings can be achieved with them.
1.    Computer Graphics
The computer graphics system accumulates and stores physically related data identifying the precise location, dimensions descriptive text, and other properties of every design element. The design-related data help the user-operator perform complex engineering analysis, generate bills of materials, produce reports, and detect design inconsistencies before the part reaches manufacturing.
With computer graphics two-dimensional drawings can be made into three-dimensional wire frames and solid models.
2.    Wire Frames
The simple wire frame plot is the least expensive form of geometrically displaying a model. It is useful to verify the basic properties of a shape and continuity of the model. However, when a complex model is developed, wire frame displays become inadequate. Solid models eliminate most of the problems of the wire frame.

3.    Solid Modeling
There are three basic techniques for generating solid models: constructive solid geometry (CSG), boundary representation (B-Rep), and analytical solid modeling.
In the CSG approach, various geometric patterns such as cylinders, spheres, and cones are combined by Boolean algebra to create designs.
In the B-Rep method, a profile of the part is defined and then swept, either linearly or radially, and the enclosed area represents the solid form.
4.    Analytical Method
This method is similar to the B-Rep but enhances the creation of finite element model during generation of the design. Commercial packages do not use strictly one method or another. As an example, CSG packages may use B-Rep techniques to generate initial patterns, while B-Rep or analytical packages may use Boolean algebra to subtract patterns, such as cylinders or cones, from a design to create a hole in the design.

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