SPECIFICATION LIMITS AND COSTS

Why unnecessarily tight tolerances cause additional costs

22 October 2012: Edgar Dietrich

An unnecessarily close distance between two specification limits can cause a considerable amount of additional costs. The measurement process should hardly contribute to the increase of costs which might lead to the fact that a higher quality and more expensive measuring instrument is required. However, the investment can be worth it as the following article shows.

When purchasing products customers and suppliers specify characteristics the supplier has to meet upon delivery. Generally, these characteristics are functional characteristics with a defined range in which the characteristic has to lie. This is why we talk about specification limits or tolerance limits. Such characteristics can be limited to both sides or even to one side in specific cases. There are two types of one-sided characteristics. The first type describes characteristics whose physical limit is zero. Typical examples are shape dimensions and positional dimensions. However, in case of characteristics like torque or coating thickness, a minimum limit differing from zero is generally demanded and must be met.

In principle, it is always a question of whether the specified limit makes sense or not. In most cases the specifications are defined based on the respective design of the product. After completion, this design is responsible for the proper functioning of the product according to specifications. As a consequence, you always tend to demand narrow limits for liability reasons. This requirement serves your own protection – a quite comprehensible fact. These tolerances are referred to as “anxiety tolerances” in practice...


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