Tolerance limits and associated costs

When a product is purchased, the customer and supplier specify the characteristics that must be met on delivery. These are generally functional characteristics, most often derived from the design. Aspects of liability always lead to the definition of rather tight limits, with the aim of self-protection.

But tightening manufacturing tolerances can lead to significant cost overruns. What's more, as the influence of the measurement process must remain negligible, this may require more expensive measuring equipment. However, the investment can be justified, as we show in this article.

Today, measurement processes are increasingly subject to capability analysis, which provides information on their uncertainty. When suppliers are able to evaluate the desired characteristics with a known uncertainty, they are in a position to discuss with the customer the design and tolerance limits on which their contractual agreement is based.

When the characteristic can be assessed quantitatively, i.e. by measurement, the design has two ways of setting tolerance limits:

1/ For the verification method used, the expanded measurement uncertainty, if acceptable, is accurately determined and incorporated into the tolerance limits, as required by ISO 14253. This is certainly the most suitable solution. If suppliers and customers are aware of the uncertainty of measurement processes, there should be no need to question the evaluation of characteristics.

2/ Or we assume that the supplier or manufacturer does not know the extended measurement uncertainty, and we reduce the tolerance - by a value assumed to be equal to the measurement uncertainty.

The costs involved in tightening tolerances are rarely taken into account. The photo shows, by way of an example, the additional costs generated by overestimating measurement uncertainty.

It would therefore be beneficial for the purchasing and sales departments to have sufficient knowledge of the measurement processes to be used, and to factor this into contractual decisions, which could lead to a widening of tolerance limits.

Emmanuel MARIE - General Manager Q-DAS France