Capability Analysis

A Capability Analysis is used to evaluate the capability of a manufacturing process to make good parts.

Once you have made a part, it can be measured. Once it is measured it either conforms to the part specifications or it doesn't. If it does not conform, we either change the molding machine settings, the mold, or the specification. If it does conform, that's great! However, how do we know all parts made in the future will conform. Common sense tells us that no two parts are exactly the same, but how close are they? How many parts do we have to measure to be confident that we can make compliant parts indefinitely? A Capability Analysis can give the knowledge and confidence to answer these questions.

For every feature on a part that can be measured, there exists a natural average value and natural spread of values (tolerance) for a given process. These numbers can be determined statistically by evaluating multiple production parts. If the part specifications fall within this natural tolerance, the process is said to be capable. This analysis can be used to predict how many parts (for example, parts / 1000) will not conform without actually measuring 1000 parts. This analysis works even if all parts measured do conform.

If a process is found not to be capable, we can choose to identify and remove the bad parts, or more preferably, change the process, tooling (mold), or specifications. For information on process tuning, see the DOE article.