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Measurement process capability

Agreement between customer and supplier

Basically, the reverification test of the measurement process capability involves a comparison of the achievable (characteristic-dependent) measurement uncertainty with the tolerance, which is also characteristic-dependent. A similar procedure is described in the factory standards mentioned above. In VDI/VDE 2617 Sheet 8, procedures for determining measurement uncertainty and evaluating measurement process capability are explained specifically for coordinate measuring machines and the three procedures "Measurement uncertainty budget", "Simulation" and "Measurement of calibrated workpieces" are described from the point of view of test process capability (also measurement process capability). This basically involves comparing the measurement uncertainty with the characteristic tolerance. To ensure measurement process capability, the measurement uncertainty must be significantly smaller than the respective dimensional tolerance. For economic reasons, a ratio of 1:10 is often required as a prerequisite for the suitability of the measurement process. However, for sizes with a very tight tolerance, it is sometimes necessary to accept compromises due to the lack of feasibility of this ratio and for some requirements even sharper criteria must be used.

Consider measurement uncertainty

The aim of these requirements is to avoid releasing parts that are out of tolerance or rejecting parts that are in tolerance. In order to avoid making wrong decisions at the tolerance limits, the existing measurement uncertainty must be taken into account. The procedure for this is described in numerous articles[14] and the ISO standard 14253.

In the upper part of Figure 69, the relationships for determining the remaining tolerance for the supplier (including internal suppliers) are imaged, based on the specified tolerance TS and the measurement uncertainty of the measuring equipment. In the general case, the specified tolerance becomes the basis of the supply contract and could therefore also be referred to as the contractual tolerance TV. In terms of product quality, the relationships described below also apply to internal production units. If it is to be avoided with certainty that out-of-tolerance parts are released, the remaining tolerance must be reduced by the measurement uncertainty.

This should be done according to a prior estimation of this measurement uncertainty for each characteristic by changing the drawing tolerances in the test plans. If the quality of the metrology is low, the manufacturing tolerances TL must therefore be severely restricted, thereby placing higher demands on the stability and accuracy of the manufacturing process. The additional manufacturing costs can significantly exceed the additional costs of purchasing a modern coordinate measuring machine.

The lower part of Figure 69 images the corresponding relationship for the customer. He cannot reject a product that is outside the tolerance by only the value of the measurement uncertainty of his own incoming goods inspection. This means that the tolerance for the acceptance TA must be increased by the measurement uncertainty based on the specified tolerance. This aspect has considerable consequences. With this procedure, the customer has the choice of accepting parts at the tolerance limit and at the same time rejecting them for further use or releasing parts out of tolerance because the acceptance tolerance does not correspond to the specified tolerance. This either violates the requirement for economic efficiency or responsible quality assurance. The reason for this is that the specified tolerance is simply used as the contract tolerance without taking the measurement uncertainty into account.

Consider measurement uncertainty
<p>Fig. 69: Influence of the measurement uncertainty on the remaining tolerance: T<sub>S</sub> specified tolerance; T<sub>V</sub> contract tolerance; U<sub>L</sub> uncertainty of the supplier's measuring machine; T<sub>L</sub> tolerance for delivery release; U<sub>A</sub> uncertainty of the measuring machine in the goods receiving department; T<sub>A</sub> tolerance for the release of goods receiving of delivered parts; A Parts with an actual value within the specified tolerance must also be rejected by the supplier due to the measurement uncertainty. B Parts with an actual value outside the specified tolerance must also be accepted by the customer due to the measurement uncertainty, although they may not be used. T<sub>A</sub> and T<sub>S</sub> are contradictory. The numerical examples are for illustrative purposes.</p>

Variants for taking measurement uncertainty into account

In order to avoid working with "two different sizes" for the drawing tolerance depending on the type of decision-making process, the treatment of measurement uncertainty between the customer and supplier should be regulated according to one of the following procedures:

  • Buyer and supplier agree on a one-time inspection. It is assumed that only parts with tolerances will be delivered or that the measurement reports are part of the delivery. There is no additional incoming goods inspection at the customer's premises.
  • The customer and supplier agree on a contractual tolerance[14] that differs from the specified tolerance and also takes into account the customer's measurement uncertainty.

Dispensing with an incoming goods inspection places the responsibility for the part quality and its effects on the end product entirely in the hands of the supplier. Clarification of the associated liability issues is then of great importance.

Agreement of a contract tolerance ...

Figure 70 images the definition of a contract tolerance. To make it easier to understand, only one example of a specific characteristic using one type of measuring machine or one measurement uncertainty is explained. The contract tolerance is determined for the corresponding characteristic by reducing the specified tolerance by the customer's measurement uncertainty (Fig. 70 left: UA). The supplier must narrow this contract tolerance further to the tolerance for delivery release due to its measurement uncertainty (Fig. 70 left: UL). This results in the following tolerance chain:

TV = TS – UA

TL = TV – UL

Agreement of a contract tolerance ...
<p>Fig. 70: Contract tolerance: T<sub>A</sub> tolerance for the release of goods acceptance of delivered parts; T<sub>S</sub> specified tolerance; U<sub>A</sub> uncertainty of the measuring machine in the customer's goods acceptance department; T<sub>V</sub> contract tolerance; U<sub>L</sub> uncertainty of the supplier's production measuring machine; T<sub>L</sub> tolerance for delivery release; A Parts with an actual value within the contract tolerance must also be rejected by the supplier due to the measuring uncertainty U<sub>L</sub>. B Parts with an actual value outside the contractual tolerance must be rejected by the customer due to the measurement uncertainty. C Parts with an actual value outside the specified tolerance are certainly outside the tolerance and must also be rejected. T<sub>A</sub> and T<sub>S</sub> coincide. The numerical examples are for illustrative purposes.</p>

... creates clear conditions

This procedure allows the measurement uncertainties of the supplier and customer to be taken into account during the inspection in relation to the contractual tolerance. In the most unfavourable case, this results in the value of the tolerance for delivery release for the supplier and the specified tolerance as the tolerance for acceptance for the customer (Fig. 70 right: UA or UL). This avoids the customer having to accept parts that are outside the specification and therefore unusable, as the tolerance for acceptance and the specified tolerance are identical. This ensures clear contractual conditions.