Budynas-Nisbett:Shigley's I.Basics 1.Introduction to ©The McGraw-Hil Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design Introduction to Mechanical Engineering Design 13 First,observe that nothing can be said in an absolute sense concerning costs. Materials and labor usually show an increasing cost from year to year.But the costs of processing the materials can be expected to exhibit a decreasing trend because of the use of automated machine tools and robots.The cost of manufacturing a single product will vary from city to city and from one plant to another because of over- head,labor,taxes,and freight differentials and the inevitable slight manufacturing variations. Standard Sizes The use of standard or stock sizes is a first principle of cost reduction.An engineer who specifies an AISI 1020 bar of hot-rolled steel 53 mm square has added cost to the prod- uct,provided that a bar 50 or 60 mm square,both of which are preferred sizes,would do equally well.The 53-mm size can be obtained by special order or by rolling or machining a 60-mm square,but these approaches add cost to the product.To ensure that standard or preferred sizes are specified,designers must have access to stock lists of the materials they employ. A further word of caution regarding the selection of preferred sizes is necessary. Although a great many sizes are usually listed in catalogs,they are not all readily avail- able.Some sizes are used so infrequently that they are not stocked.A rush order for such sizes may mean more on expense and delay.Thus you should also have access to a list such as those in Table A-17 for preferred inch and millimeter sizes. There are many purchased parts,such as motors,pumps,bearings,and fasteners, that are specified by designers.In the case of these,too,you should make a special effort to specify parts that are readily available.Parts that are made and sold in large quantities usually cost somewhat less than the odd sizes.The cost of rolling bearings, for example,depends more on the quantity of production by the bearing manufacturer than on the size of the bearing. Large Tolerances Among the effects of design specifications on costs,tolerances are perhaps most sig- nificant.Tolerances,manufacturing processes,and surface finish are interrelated and influence the producibility of the end product in many ways.Close tolerances may necessitate additional steps in processing and inspection or even render a part com- pletely impractical to produce economically.Tolerances cover dimensional variation and surface-roughness range and also the variation in mechanical properties resulting from heat treatment and other processing operations. Since parts having large tolerances can often be produced by machines with higher production rates,costs will be significantly smaller.Also,fewer such parts will be rejected in the inspection process,and they are usually easier to assemble.A plot of cost versus tolerance/machining process is shown in Fig.1-2,and illustrates the drastic increase in manufacturing cost as tolerance diminishes with finer machining processing. Breakeven Points Sometimes it happens that,when two or more design approaches are compared for cost, the choice between the two depends on a set of conditions such as the quantity of pro- duction,the speed of the assembly lines,or some other condition.There then occurs a point corresponding to equal cost,which is called the breakeven point.Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design © The McGraw−Hill 19 Companies, 2008 First, observe that nothing can be said in an absolute sense concerning costs. Materials and labor usually show an increasing cost from year to year. But the costs of processing the materials can be expected to exhibit a decreasing trend because of the use of automated machine tools and robots. The cost of manufacturing a single product will vary from city to city and from one plant to another because of over￾head, labor, taxes, and freight differentials and the inevitable slight manufacturing variations. Standard Sizes The use of standard or stock sizes is a first principle of cost reduction. An engineer who specifies an AISI 1020 bar of hot-rolled steel 53 mm square has added cost to the prod￾uct, provided that a bar 50 or 60 mm square, both of which are preferred sizes, would do equally well. The 53-mm size can be obtained by special order or by rolling or machining a 60-mm square, but these approaches add cost to the product. To ensure that standard or preferred sizes are specified, designers must have access to stock lists of the materials they employ. A further word of caution regarding the selection of preferred sizes is necessary. Although a great many sizes are usually listed in catalogs, they are not all readily avail￾able. Some sizes are used so infrequently that they are not stocked. A rush order for such sizes may mean more on expense and delay. Thus you should also have access to a list such as those in Table A–17 for preferred inch and millimeter sizes. There are many purchased parts, such as motors, pumps, bearings, and fasteners, that are specified by designers. In the case of these, too, you should make a special effort to specify parts that are readily available. Parts that are made and sold in large quantities usually cost somewhat less than the odd sizes. The cost of rolling bearings, for example, depends more on the quantity of production by the bearing manufacturer than on the size of the bearing. Large Tolerances Among the effects of design specifications on costs, tolerances are perhaps most sig￾nificant. Tolerances, manufacturing processes, and surface finish are interrelated and influence the producibility of the end product in many ways. Close tolerances may necessitate additional steps in processing and inspection or even render a part com￾pletely impractical to produce economically. Tolerances cover dimensional variation and surface-roughness range and also the variation in mechanical properties resulting from heat treatment and other processing operations. Since parts having large tolerances can often be produced by machines with higher production rates, costs will be significantly smaller. Also, fewer such parts will be rejected in the inspection process, and they are usually easier to assemble. A plot of cost versus tolerance/machining process is shown in Fig. 1–2, and illustrates the drastic increase in manufacturing cost as tolerance diminishes with finer machining processing. Breakeven Points Sometimes it happens that, when two or more design approaches are compared for cost, the choice between the two depends on a set of conditions such as the quantity of pro￾duction, the speed of the assembly lines, or some other condition. There then occurs a point corresponding to equal cost, which is called the breakeven point. Introduction to Mechanical Engineering Design 13