【文章內(nèi)容簡介】 Functional considerations during design involve, among other things, weight, strength, thermal properties, kinematics, and dynamics. The performance of a design can be evaluated by paring its performance measurements with the required specifications. As important as satisfactory performance is, there are other areas part should be designed economically. This implies that the finished part should be designed as close to the specifications as possible. If the function of a supporting member of a structure requires the member to withstand 10,000 psi of pressive stress, then designing the member to withstand 30,000psi is unnecessary and will probably be more expensive than the required design. Engineering design, therefore should address functionality and economics, Functionality is determined by a part’s geometry, material properties, and environment. The economic factors include materials, processing costs, and marketing details. As much as 70% of the production costs of a manufactured part are determined during the engineering design process. This means that by the time the part has left the designer’s hands the large majority of its production costs have been established. They are defined implicitly by the materials, dimensions, tolerances, surface finishes, and other parameters which determine processing costs. Therefore, only 30% of the part’s cost is subject to moneysaving efforts during the manufacturing planning stage. This 70/30 ratio emphasizes the important of the design can be a major help in assuring proper function and reasonable production costs. Computeraided design (CAD) is a term which means many things to many people. To some, it means puteraided drafting or drawing. To others, it means puteaided analysis. And to still others, it suggests totally automated design where the engineer need specify only the function of a part and the puter arrives at a satisfactory or even optimal design. CAD is all of the above, aiming mostly for the fast description through techniques in artificial intelligence. CAD, however, still exists separately in each of the stages mentioned. Probably the most mon and simplest CAD systems are limited to automated drafting capabilities. More advanced systems can perform analyses and even help guide the engineer to the equations necessary to determine the effectiveness of the design.Because of the availability of a wide variety of CAD systems with different characteristics supplied by different vendors, proper munication and exchange of data between these systems have bee a significant problem. The need for a single neutral format for better patibility is presently filled largely by the Initial Graphics Exchange Specificaton (IGES). Vendors need only provide translators for their own systems to preprocess the data into the neutral format into their system. IGES is used for translation in two directions (into or out of a system) and is also used widely for translation of 3D line and surface data. A more recent development is a solidmodelbased standard called Product Data Exchange Specification (PDES), which is based on IGES. Although IGES is adequate fro most requirement, PDES has less memory size, requires less time for execution, and is less error prone. Because of the existence of various standards in other countries as well, it is expected that in the near future these standards will culminate in an international standard, called the Standard for the Exchange of Product Model Data (STEP).Hardware and Software Hardware varies a great deal between CAD systems. Mainframes have long been used for CAD systems. The rapid increases in processing speeds have made micro and miniputers viable CAD stations. Networks generally are used so that designs can be stored centrally and data can be shared among designers. Many input devices are used. Common I/O devices include keyboard, mouse, trackball, digitizing pad, joystick, keypad, and light pens. Output devices are typically screen, plotter, printer, and hard drives and disks for storage.There is a wide variety of CAD software available for mainframes engineering stations and microputers. The packages vary to some degree in information in the form of drawing exchange files. These files can be imported into other software to assist in programming the machine tool.Elements of CAD SystemsThe design process in a CAD system consists of four stages, as described below.Geometric modeling. In geometric modeling, a physical object or any of its parts is described mathematically or analytically. The designer first constructs a geometric model by giving mands that create or modify lines, surfaces, solids, dimensions, and text that together are and accurate and plete two or threedimensional representation of the object. The models can be presented in three different ways. In the line representation (wireframe。 Figure ), all edges are visible as solid lines. This image can be ambiguous, particularly for plex shapes. However, various colors are generally used for different part of the object, thus making the object easier visualize.In the surface model, all visible surface are shown in the model, and in the solid model, all surfaces are shown, but the data describe the interior volume (Figure ,f)Design analysis and optimization. After the design’s geometric features have may consist of analyzing stresses, strains, deflection, vibrations. This phase temperature distribution, or tolerance. Various sophisticated software packages capable of puting these quantities accurately and rapidly are available.Design review and evaluation. An important design stage is review and evaluation to check for any interference between various ponents in order to avoid difficulties during assembly or use of the part, and whether moving members, such as linkages, are going to operate as intended. Software is now available with animation capabilities to identify potential with moving members and other