Chapter12 - Case studies: process selection 12.1...

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Case studies: process selection 12.1 Introduction and synopsis The previous chapter described a systematic procedure for process selection. The inputs are design requirements; the output is a shortlist of processes capable of meeting them. The case studies of this chapter illustrate the method. The first four make use of hard-copy charts; the last two show how computer-based selection works. More details for each are then sought, starting with the texts listed under Further reading for Chapter 11, and progressing to the specialized data sources described in Chapter 13. The final choice evolves from this subset, taking into account local factors, often specific to a particular company, geographical area or country. The case studies follow a standard pattern. First, we list the design requirements: size, minimum section, surface area, shape, complexity, precision and finish, and the material and the processing constraints that it creates (melting point and hardness). Then we plot these requirements onto the process charts, identifying search areas. The processes which overlap the search areas are capable of making the component to its design specification: they are the candidates. If no one process meets all the design requirements, then processes have to be ‘stacked’: casting followed by machining (to meet the tolerance specification on one surface, for instance); or powder methods followed by grinding. Computer-based methods allow the potential candidates to be ranked, using economic criteria. More details for the most promising are then sought, starting with the texts listed under Further reading for Chapter 11, and progressing to the specialized data sources described in Chapter 13. The final choice evolves from this subset, taking into account local factors, often specific to a particular company, geographical area or country. 12.2 Forming a fan Fans for vacuum cleaners are designed to be cheap, quiet and efficient, probably in that order. Case study 6.6 identified a number of candidate materials, among them, aluminium alloys and nylon. Both materials are cheap. The key to minimizing process costs is to form the fan to its final shape in a single operation - that is, to achieve net-shape forming - leaving only the central hub to be machined to fit the shaft with which it mates. This means the selection of a process which can meet the specifications on precision and tolerance, avoiding the need for machining or finishing of the disk or blades. The design requirements The pumping rate of a fan is determined by its radius and rate of revolution: it is this which determines its size. The designer calculates the need for a fan of radius 60mm, with 20 blades of
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282 Materials Selection in Mechanical Design Table 12.1 Design constraints for the fan Constmint Value Materials Complexity Min. Section Surface area Volume Weight Mean precision Roughness Nylons T,, = 550-573 K H = 1.50-270MPa p = 1080 kg/m3 H = 1.50-1500MPa p = 2070 kg/m3 Al-alloy~ T, = 860-933 K 2 to 3 I .S-6 mm 0.01 -0.04 m2 0.03-0.5 kg f0.S mm il pm 1.5 10-5-2.4 10-4m3 average thickness 3 mm. The surface area, approximately
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This note was uploaded on 04/26/2008 for the course M&AE 212 taught by Professor Miller during the Spring '07 term at Cornell University (Engineering School).

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Chapter12 - Case studies: process selection 12.1...

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