Department of Engineering   University of Cambridge

Materials & Process Selection: Selecting a Process

What is Process Selection?

The problem of determining the best way of performing a task is one that is found in many walks of life. Whatever the type of problem, the aim is usually similar - to select the best way of fulfilling a set of requirements from the options available.

The field of engineering design is no exception. For many reasons, such as the pressure to reduce cost and time to market, systems are needed to help with the choices that arise on the route from an initial idea for a product to its realisation in the marketplace. The designer needs a systematic and robust way of evaluating the available options and identifying (without bias) which might be the best.

In brief, manufacturing process selection is the task of choosing a method for transforming a set of materials into a given shape using one or more processes. The 'best' process is normally considered to be the most economic, subject to it meeting the technical constraints. The aids to selection tend to fall into one of three main categories:

  1. Methodologies - are general approaches to tackling a large variety of problems. They are not aimed at ways of selecting a particular process, but at creating frameworks within which certain classes of problem can be addressed. The intention is to guide the creation of procedures, ensuring that all the relevant aspects of selection are addressed.
  2. Procedures - can be considered as algorithms for tackling a set of related process selection problems. These will contain all the necessary steps and the appropriate tools for selecting a process in a given situation. The way in which the procedure is actually used to carry out a selection will depend upon its implementation. Implementations are usually software based, although they can be as simple as a series of instructions and charts.
  3. Tools - used in selection procedures usually help to answer single specific question. Examples include charts of properties, linear regression analyses, databases and all types of modelling. At the implementation stage it becomes possible to utilise more generic tools such as fuzzy logic and expert systems.
The choice of approach depends, to a large extent, on the degree of detail required of the answers. At the preliminary stages of design generic selection tools that have a broad scope, but use low precision data, are usually most appropriate. As the design proceeds, and the problem is specified in increasing detail, more focused selection tools are required. At the most detailed levels of design, the selection field becomes so constrained that design optimisation becomes more relevant.

Currently, selecting a manufacturing process relies heavily on in-house experience. Whilst this knowledge and expertise will never be completely superseded by design tools, there are several major inherent problems with relying solely upon it:

  1. There will be a bias towards existing technologies and, as a result, new processes may be overlooked.
  2. Existing knowledge may not always be helpful when new processing conditions arise. Using selection tools, such as modelling, suitable processing conditions can be predicted more rapidly and accurately.
  3. Expertise can be difficult to rationalise or quantify. This makes the design process difficult to document and does not provide a route to reliable and repeatable decision making.
Process selection, then, is intended to provide a systematic and robust way of evaluating the available processing options and identifying (without bias) which might be the best. There are many approaches that can be taken, which will depend on the level of detail required; some of these are discussed below.

A more in-depth discussion of some of the selection issues that arise in engineering design, and in particular process selection issues, can be found in the introductory chapters of a PhD thesis by AM Lovatt (Cambridge University Engineering Department, 1998).

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