Tolerance limits and associated costs

When purchasing products, the customer and supplier specify the characteristics that will have to be respected upon delivery. These are usually functional characteristics that most often arise from the design. Aspects of responsibility always push to define rather tight boundaries, in order to protect themselves.

But tightening manufacturing tolerances can cause significant extra costs. In addition, the influence of the measurement process must remain negligible, this may require a more expensive measuring device. However, the investment can be justified, as we show in this article.

Today, measurement processes are increasingly subject to capability analyses, which allows us to know the uncertainty. When suppliers are able to assess desired characteristics with known uncertainty, they are in a position to discuss with the customer the design and limits of tolerance on which their contractual agreement is based.

Where the characteristic can be evaluated quantitatively, i.e. by a measure, the design has two ways to set tolerance limits:

1/ Extended measurement uncertainty, if acceptable, is accurately determined for the verification method, and incorporated into 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 questioning of the assessment of characteristics.

2/ Either the supplier or manufacturer is considered to be unfamila in the uncertainty of extended measurement, and tolerance is reduced - by a value assumed to be equal to measurement uncertainty.

The costs of tightening tolerances are rarely taken into account. The photo shows, using an example, the extra costs generated by overestimating the measurement uncertainty.

It would therefore be beneficial if the purchasing and sales departments had sufficient knowledge of the measurement processes to be used, and that they would take this into account in contractual decisions, which could lead to an expansion of tolerance limits.

Emmanuel MARIE - Managing Director Q-DAS France

Find out more: www.q-das.fr