Tolerance Control

The issue of part tolerancing and, in particular, the statistical assignment and assessment of tolerances are excellent examples of the need for design and manufacturing to understand what each other is doing and why. The best intentions of the design process can go unmet if the manufacturing process is not operated in a manner totally consistent with design intent. To more clearly appreciate the relationship that must exist between the design and manufacturing operations, some of the basic assumptions of the tolerancing activity and their relationship to the manufacturing process are examined. The following sections clearly point to the importance of statistical process control relative to the issue of process capability.

The key concepts in statistical tolerancing are:

• The use of a statistical distribution to represent the design characteristic and therefore the process output for the product/part in question relative to the design specifications

• The notion of random assembly, that is, random part selection from these part process distributions when more than one part is being considered in an assembly

• The additive law of variances as a means to determine the relationship between the variability in individual parts and that for the assembly

To assume that the parts can be represented by a statistical distribution of measurements (and for the assumption to hold in reality), the part processes must be in a state of statistical control. The following example illustrates the importance of statistical process control in achieving design intent in a tolerancing problem.

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