Some New Developments in Tolerance Design in CAD

Abstract

Abstract This paper presents some recent developments in tolerance design in CAD. An Oriented Functional Relationship Graph (OFRG) is developed to identify all the relevant functional components which have an impact on the (evaluated) performance of a product. It is proposed to use expert system technique to determine all the functional surfaces and necessary relational tolerance types on functional surfaces. Using an OFRG, it is easy to determine dimensional chains, error transmission chains, functional dimension trees and the functional dimensioning mode of a component. We propose to use a transformation matrix technique to determine the sensitivity of each tolerance. A new practical cost model, serving as the criterion for tolerance allocation, is presented. In contrast to empiric data (they are nearly uncollectible) based cost model, the cost model presented in this paper is a process data (they are collectable) based one. The discussion on optimal tolerance shows that optimal functional tolerances can not be obtained without considering the method in machining the workpieces. Hence, the concept of concurrent engineering must be introduced into tolerance design. Finally a mathematic model for tolerance design, called simultaneous tolerancing, in the context of concurrent engineering is proposed.