Computer-aided Analysis of Engineering Tolerances

Abstract

In the design of mass-produced components, it is essential that manufacturing tolerances should be analysed to make sure that assemblies fit together satisfactorily and that parts are not produced to unnecessarily tight specifications. The analysis may be divided into three stages: (a) calculation of the sensitivity of a feature's position to the magnitude of the tolerances upon which it depends, (b) ensuring that the permitted tolerances which together influence a critical measurement are allocated in the most economical way, and (c) analysing the statistical distribution of tolerances on critical measurements. This thesis describes a method of performing stage (a). Stages (b) and (c) have been dealt with elsewhere. It is demonstrated that the analysis of tolerances in all but the most straightforward cases is not a trivial operation and a model is developed to assist with the calculation. This is a location element derived from the classical six-point system for locating a body in three dimensions. Elements may be combined to describe multi-datum machining operations, assemblies and drawing dimension systems by a tree-like structure. The model is analysed mathematically, a compendium of commonly-occurring cases is appended and algorithms for obtaining results of interest to the engineer designer are described. A computer program for the sensitivity analysis is also described and the integration of the method into a full tolerance-analysis system is discussed.