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).