Capability of the Measuring Process

The capability of the measuring process is examined based on a comparison of the attainable (feature-dependent) measuring uncertainty, including all related influences, and the equally feature-related tolerance. A similar procedure is described in the company standards mentioned above.

General procedures for determining the measuring uncertainty and assessing the capability of the measuring process are provided in the German VDA Directive 5. Another VDI/VDE regulation containing similar information especially for coordinate measuring machines is in preparation. The capability of the measuring process can be ensured only if the measuring uncertainty is substantially lower than the corresponding dimensional tolerance. A ratio of 1:10 is generally required to ensure the capability of the measuring process. However, in cases involving dimensions with extremely close tolerances, it may occasionally be necessary to make concessions. In any case, it should be ensured that the process tolerance is less than the drawing tolerance by an amount equal to that of the measuring uncertainty (ISO 14253-1). In the end, the lower the quality of the measuring technology, the higher the requirements regarding the stability and accuracy of the manufacturing processes. Any additional manufacturing costs thus incurred may be much higher than the extra costs for purchasing a modern, multisensor coordinate measuring machine.

The measuring uncertainty of the corresponding measuring processes plays an especially important role with respect to the interface between the supplier and the customer. The supplier has tolerance limits which he must maintain and guarantee in order to reduce the measuring uncertainty of his coordinate measuring machines (process tolerance). On the other hand, the customer must extend the tolerance of the measuring machines installed in his incoming inspection department to include the amount of the measuring uncertainty (Fig. 55).

Fig. 55: Comparison of the effect of different measuring uncertainties according to ISO 14253-1.

Since he cannot make the supplier accountable for his own measuring uncertainty, the customer can only file claims if these extended limits are violated. This is especially likely to lead to a contradiction if the customer’s own measuring uncertainty is excessively high. The customer must instruct his own quality assurance department to refrain from using any parts for which a certain tolerance deviation has been measured. This applies especially in cases where the measuring machine used in the incoming inspection department has a measuring uncertainty equal to or lower than the one used by the supplier. To prevent this from happening, the customer should prescribe a closer tolerance to the supplier [7]. The term “contractual tolerance” can be used to describe this situation. The following tolerance chain results:

Contractual tolerance = specified tolerance
                                       – measuring uncertainty of customer
Process tolerance      = contractual tolerance
                                       – measuring uncertainty of supplier

Under these conditions, it can be ensured that the terms of the contract are clearly defined and verifiable. One example of this case is shown in Figure 56. Here, the tolerance for the supplier has been reduced to the contractual tolerance. This means that the customer can accept all parts approved by the supplier as within tolerance without having to make any concessions with respect to quality assurance. Since this procedure could result in higher costs, it creates a demand for sufficiently accurate coordinate measuring technology both in the supplier’s and in the customer’s inspection department.

Fig. 56: Restriction of the supplier’s tolerance to ensure conformity with the customer’s specified tolerance.