How many dimensional data points are enough to accurately describe a part feature? The key to answering this question is understanding the stability of the manufacturing process. In general, components should be measured only as often as required to ensure the stability of manufacturing processes. This requires identifying and monitoring part features that are critical to the part’s end-use function and developing a strategy to control the dimensions of those features.

The choice of manufacturing technique is the key factor in choosing a process control method. If, for example, the manufacturing process reliably produces a critical bore with good form, its size or position may vary. In this case, control of the size and position will be important but not necessarily roundness or cylindricity control. By contrast, if the machining process produces features with significant form variation (i.e., the variability of the form is a significant proportion of the form tolerance), then understanding where and how the form errors occur becomes important.

Some features may need to mate with other parts for the end-use product to work correctly. In many cases, the form or profile of these features is critical to the functional fit; consequently, the processes used to make these features must be precisely controlled.

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