Constructing process capability maps from manufacturing data

The procedure shown in Figure 2.13 can be used by a company to construct their own process capability maps. It is necessary only when a new or specialized manufacturing process is to be used, which is not contained in Appendix IV, and when data from the

Tolerance band

Figure 2.12 Modelling tolerance risk using a 'squared' relationship

Tolerance band

Figure 2.12 Modelling tolerance risk using a 'squared' relationship machine tool supplier is not available. Of course, this activity may become prohibitive when the costs involved and/or time in performing such studies are high.

The data used to generate the maps is taken from a simple statistical analysis of the manufacturing process and is based on an assumption that the result will follow a Normal distribution. A number of component characteristics (for example, a length or diameter) are measured and the achievable tolerance at different conformance levels is calculated. This is repeated at different characteristic sizes to build up a relationship between the characteristic dimension and achievable tolerance for the manufacture process. Both the material and geometry of the component to be manufactured are considered to be ideal, that is, the material properties are in specification, and there are no geometric features that create excessive variability or which are on the limit of processing feasibility. Standard practices should be used when manufacturing the test components and it is recommended that a number of different operators contribute to the results.

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