Unified tolerance model for part based on key features and graph

The desired requirements for a tolerance model for part are: completeness, compatibility, computability and validity. We aim to meet these requirements in a way different from past researchers^[2-8]. Past researchers dealt with dimension tolerance, form tolerance and position tolerance separately. We build a unified tolerance model that deals with these three tolerances simultaneously. Our approach is based on key features (KFs) and directed graph. KFs include point KF, line KF and plane KF. We build a meta-tolerance model with five kinds of information: (1) a set made up of KFs; (2) a set made up of lines which connect KFs; (3) clear designation of type of tolerance; dimension, form or position; (4) attributes of tolerance which include size and type of tolerance value; (5) variational direction of KF, where variational direction is represented by unit-vector. We build our tolerance model with the above-mentioned meta-tolerance model and the principles of sum operation in graph theory. So a unified tolerance model comes into being. This model is propitious to representation of tolerancing experience from engineers and that from analysis of tolerance chain. As shown by an example of a common type of machine part, our approach does satisfy the desired requirements for a tolerance model for part: completeness, compatibility, computability and validity.