外文原文：
CAD/CAM
Computer technology has risen in an exponential curve. Only a generation ago computers were not really needed for NC control. Except for relatively few five-axis machines. NC was point to point, which was easily accomplished with manual programming.
With the advent of two-dimensional profiling, hower, a need arose for numerous calculations to generate the curved profile. At that point the computer was looked upon as a useful tool in manufacturing engineering. After some development, it was found that punched or magnetic tape could be dispensed with entirely in favor of computer programs that controlled the machines directly. Then came the minicomputers, which when operating with the central processor brought about the first real concept of the automatic factory.
All of these developments were important steps in the advance of technology. The computer is no longer viewed as a remote piece of hardware used primarily in accounting and engineering departments, but instead as an important tool to be used increasingly in CAD and CAM.
Today CAD and CAM have become closely linked for complete processing from design to production. The desin engineer can now create the desired design on a CRT screen that is linked to a computerized graphics plotter. The same system can be used to determine the viability of the design by analyzing stresses and deflections that will take place under loading conditions. The functioning of mechanical linkages may also be checked. After the design is complete, the production group can draw on the information developed in CAD for computer-aided manufacture. Thus the unified
CAD-CAM team has been developed to bring the product from the design concept through manufacture.
CAD Functions
Initially CAD systems were primarily automated drafting stations in which computer plotters produced engineering drawings. The systems were later linked to graphic display terminals, which allowed the user to communicate with the computer in pictures instead of raw columns of numbers. Now advanced systems have added analytical capabilities. CAD functions may be grouped into four major categories: geometric modeling, engineering analysis, kinematics, and automated drafting.
Geometric Modeling Traditionally, engineers do the sketching and drafts-men the drafting. Engineers then discover needed changes or improvements and draftsmen draft what is required. Moving a bracket 2 in, to the left causes an entree chain reaction throughout a complete set of drawings.
Now the computer has opened up a new way to meet design engineering needs with speed and reliability in products ranging from high-performance aircraft to complex, integrated circuits. The designer can sketch directly on the screen of a graphics terminal. Lines and contours are defined by pressing keys and positioning a light pen, and the computer displays what is expressed.
Curve fitting, or reducing the design to a set of control equations is done at the terminal, eliminating coding, card punching, punching, and repeated computer batch runs;then CAD converts the preliminary design to a dimensional drawing with auxiliary views.
If the drawing reveals a problem of compatibility with a subassembly, the pieces can be moved around with the light pen. The computer then converts this pictorial
representation to a mathematical model stored in the computer data base for later use. The model may be used or other CAD fuctions or it may be recalled and refined by the engineer at any point in the design process.
Geometric modeling is often considered to be the most important feature of a CAD system because so many other design functions in the system depend heavily on the model. The geometric model may be used to create a finite element model of a structure for stress analysis. The model may also serve as an input for automated drafting to produce engineering drawings. If CAD/CAM systems are interfaced, the geometric model can be used to create NC tapes for fabricating the part.
Most modeling is done with a wire frame that represents the part shape with interconnected line elements .
Analysis After the geometric modeling is done, the CAD system allows the designer to move directly to analysis. With simple keyboard instructions, the computer is asked to calculate weight, volume, surface area, moment to inertia, or center of gravity of a part. However, the most powerful method of analyzing a part is by the finite element method. In this technique the structure is broken down into a network of simple element that the computer uses to determine stresses, deflections, and other structural characteristics. Such analysis requires the tremendous computational power of a mainframe computer.
Kinematics Many CAD system have kinematic features for plotting or animating the motion of simple, hinged parts such as doors or cranks. Such analysis can ensure that moving components do not impact on other parts of the structure. The design of an automobile hood linkage, for example, requires only a few minutes of interaction with a computer, whereas hundreds of hours of hours can be required for
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