Evaluating Process Capability for Geometrically Toleranced Parts: A Practical Approach

Abstract

The practice of geometric tolerancing has gained in industry popularity since the 1990s. This approach has advantages over conventional tolerancing in defining both geometry and associated tolerance and, thus, generating a more realistic “acceptable design space.” There have been a number of highly mathematical treatments of the subject over the years which have not found their way into popular usage. We look at a specific example of geometric tolerancing and derive a simple approach based on the geometry of the situation and standard C p and C pk calculations. The study describes an approach based on understanding the limiting conditions of acceptable operation. In the example of a pin and clearance hole, we derive the limiting condition as a zero radial gap for a hole and a perfectly centered pin at maximum metal condition. We then performed a standard C pk calculation with the limiting condition acting as the effective tolerance limit. Because we are dealing with radial gaps, we have an effective one-sided tolerance with a minimum acceptable value of 0 for the radial gaps. We tested the approach using Normally distributed simulated data and found that it provides an accurate evaluation of process capability and projected scrap levels. With minor cautions, we conclude that this methodological approach could be extrapolated to other geometrically toleranced situations.