【正文】 ical processing sequence to fabricate an individual part consists of: (1) a basic process, (2) secondary processes, (3) operations to enhance physical properties, and (4) finishing operations. A basic process determines the starting geometry of the work parts. Metal casting, plastic molding, and rolling of sheet metal are examples of basic processes. The starting geometry must often be refined by secondary processes, operations that transform the starting geometry (or close to final geometry). The secondary geometry processes that might be used are closely correlated to the basic process that provides the starting geometry. When sand casting is the basic processes, machining operations are generally the second processes. When a rolling mill produces sheet metal, stamping operations such as punching and bending are the secondary processes. When plastic injection molding is the basic process, secondary operations are often unnecessary, because most of the geometric features that would otherwise require machining can be created by the molding operation. Plastic molding and other operation that require no subsequent secondary processing are called net shape processes. Operations that require some but not much secondary processing (usually machining) are referred to as near net shape processes. Some impression die forgings are in this category. These parts can often be shaped in the forging operation (basic processes) so that minimal machining (secondary processing) is required.Once the geometry has been established, the next step for some parts is to improve their mechanical and physical properties. Operations to enhance properties do not alter the geometry of the part。 for example, plastic molding rarely require finishing. When finishing is required, it is usually the final step in the processing sequence.Processing Planning for Assemblies The type of assembly method used for a given product depends on factors such as: (1) the anticipated production quantities。 and (3) assembly processes used, for example, mechanical assembly versus welding. For a product that is to be made in relatively small quantities, assembly is usually performed on manual assembly lines. For simple products of a dozen or so ponents, to be made in large quantities, automated assembly systems are appropriate. In any case, there is a precedence order in which the work must be acplished. The precedence requirements are sometimes portrayed graphically on a precedence diagram.Process planning for assembly involves development of assembly instructions, but in more detail .For low production quantities, the entire assembly is pleted at a single station. For high production on an assembly line, process planning consists of allocating work elements to the individual stations of the line, a procedure called line balancing. The assembly line routes the work unit to individual stations in the proper order as determined by the line balance solution. As in process planning for individual ponents, any tools and fixtures required to acplish an assembly task must be determined, designed, built, and the workstation arrangement must be laid out.Make or Buy DecisionAn important question that arises in process planning is whether a given part should be produced in the pany’s own factory or purchased from an outside vendor, and the answer to this question is known as the make or buy decision. If the pany does not possess the technological equipment or expertise in the particular manufacturing processes required to make the part, then the answer is obvious: The part must be purchased because there is no internal alternative. However, in many cases, the part could either be made internally using existing equipment, or it could be purchased externally from a vendor that process similar manufacturing capability.In our discussion of the make or buy decision, it should be recognized at the outset that nearly all manufactures buy their raw materials from supplies. A machine shop purchases its starting bar stock from a metals distributor and its sand castings from a foundry. A plastic molding plant buys its molding pound from a chemical pany. A stamping press factory purchases sheet metal either fro a distributor or direct from a rolling mill. Very few panies are vertically integrated in their production operations all the way from raw materials, it seems reasonable to consider purchasing at least some of the parts that would otherwise be produced in its own plant. It is probably appropriate to ask the make or buy question for every ponent that is used by the pany.There are a number of factors that enter into the make or buy decision. One would think that cost is the most important factor in determining whether to produce the part or purchase it. If an outside vendor is more proficient than the pany’s own plant in themanufacturing processes used to make the part, then the internal production cost is likely to be greater than the purchase price even after the vendor has included a profit. However, if the decision to purchase results in idle equipment and labor in the pany’s own plant, then the apparent advantage of purchasing the part may be lost. Consider the following example make or Buy Decision. The quoted price for a certain part is $ per unit for 100 units. The part can be produced in the pany’s own plant for $. The ponents of making the part are as follows: Direct labor cost = per unit Equipment fixed cost = per unit Automated process planning leads to more logical and consistent process plans than when process is done pletely manually. Standard plans tend to result in lower manufacturing costs and higher product quality.The systematic approach and the availability of standard process plans in the data files permit more work to be acplished by the process planners..Reduced lead time for process planning. .Improved legibility. .Incorporation of other application programs. 1