How to Establish Manufacturing Specifications – Donald J. Wheeler
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The idea behind manufacturing specifications is to define those measurement values which correspond to conforming items. That is, when the measurement falls within the manufacturing specifications we want to be able to say that the product is within the customer specifications. In the absence of measurement error we could achieve this objective by simply using the customer specifications as the manufacturing specifications. But when we have to make allowance for measurement error there will need to be some gap between the manufacturing specifications and the customer specifications. How to determine the size of the gap, D, and the kind of statements that we can make about the product is the topic of this paper.
If we choose a large value for D we can be very confident that the product is conforming, but we could end up with very tight manufacturing specifications. When this happens there will be an increased risk of rejecting conforming product. By choosing a small value for D we will not have as much confidence that the product is conforming, but we will have looser manufacturing specifications with a correspondingly smaller risk of rejecting conforming product.
Thus the main problem of obtaining manufacturing specifications is the problem of striking a balance between the size of the adjustment, D, and the likelihood of having a conforming item. While the exact values will also depend on other aspects of the situation, it is these two properties that dominate the problem. In this paper I give a set of rules for constructing manufacturing specifications that have different likelihoods of conforming product, and then I show how these rules were developed.
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