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
section, surface area, shape, complexity, precision and finish, and the
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