A new perspective on tolerance analysis based on tolerance zones

Abstract: "Universal interchangeability of parts or sub-assemblies is one of the goals of most designs. The knowledge of all possible variations in an assembly due to the specified tolerances is essential to ensure universal interchangeability. Tolerance zone, as defined in [1], is a virtual region formed around a true feature. Tolerance zone propagation can be effectively used to observe variations due to individual part tolerances in an assembly. Although the concept of tolerance zones is not explicitly used in most existing tolerance analysis methods, the underlying principles can be readily used to generate a tolerance zone.However, it is seen that tolerance zones generated by Worst Case or Root Sum Square (RSS) models do not encompass all possible variations inan assembly made of parts that are within tolerance specification, because the effect of non-worst case stacking situations is not considered. The new perspective on tolerance analysis, discussed in this paper, addresses the effect of non-worst case stacking situations by studying angular accumulation in addition to linear accumulation of tolerances. In a general plus/minus tolerance part there is a high probability that the part conforms to the specified tolerances with a surface that is not at the true angular position with respect to other features.Such parts when assembled together lead to angular stack-up. In this paper, angular accumulation is addressed using the concept of tolerance zones and a mathematical basis is developed for the representation and analysis of angular accumulation based on classical kinematic theories. Possible applications of this analysis are also discussed.