Process capability diagrams - Six Sigma

Once a process is stable, it is necessary to determine whether the outcomes of the process can meet customer expectations as described by tolerance limits in most product oriented applications and service level agreements in service oriented applications.

The importance of understanding process capability cannot be overstated. If we are to attempt at any level to design for manufacture we need to understand not only the requirements on the process (effectively our specifications) but also what the process is able to achieve (capability). Without both sides of the equation we are not able to make sensible decisions about how to manage our processes at an appropriate stage in the product lifecycle and we doom ourselves to fixing and ire-fighting when we actually try to make the product. A similar argument could be made for supplier selection.

Capability evaluation is the method by which we determine whether a process is up to the job of meeting the specifications set for it. It is important, before attempting to establish the capability of a process to ensure that the process is stable. The key issue is that if a process is not stable the capability will be constantly changing due to the transient effects of special causes and will hence be uncertain.

Process capability diagrams

Consider the four processes shown above with the specification limits. Clearly, process A is producing many components both above and below tolerance; process B is offset and is, as a result, producing components below the bottom tolerance limit; process C is producing almost all components within tolerance and process D is operating well within the tolerance limits.

Given the information provided in the above diagram we can act upon the process (resetting process B, for example, or attempting to reduce the spread of process A), without such information we would be making such decisions in the dark. Similarly, this information would be of use in selecting suppliers having these capabilities. If we do not understand the capabilities of processes at an early stage in the product lifecycle we give ourselves little chance of making appropriate decisions about which processes to use as they are and which to work on. If we find this out when we reach volume production we incur additional costs. Indices may be calculated to quantify capability, but these are not dealt with there.

The Taguchi Loss Function demonstrates the tangible business impact of lower process capability (i.e. higher process variation).

The detail on both process control and process capability are dealt with in chapter.


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