Computer aided tolerance charting for products with angular features

Abstract

Manual tolerance charting has been used in manufacturing enterprises since the 1950's, and computer aided tolerance charting has been developed since the early 1980's. However, most of the research work focused on one dimensional (1D) tolerance charting that can only deal with square shouldered features. This thesis focuses on the development of a theory and methodology on computer aided angular tolerance charting for products with angular features. Firstly, a surface model is established by defining a surface with two points and the angle between its normal and the positive x-axis. Two universal trigonometric formulae are derived to calculate the surface changes caused by the machining cuts. Then, a two-dimensional (2D) tolerance chart is proposed to illustrate the surface changes in both axes. Furthermore, an algebraic method is used to trace the surface changes during the operations. The reverse dimensional chains are obtained, and the solid stock removals are determined automatically. The forward dimensional chains are obtained from inverting the reverse dimension chains. The reverse dimensional chains are used to calculate the working dimensions if the blueprint dimensions and the stock removals are given. On the other hand, the forward dimensional chains are used to check whether the part can be made to meet the blueprint requirements. They are also used to allocate proper process tolerances to the machining cuts. Based on a discrete interval cost tolerance model of that each machining cut may result in several different manufacturing cost due to different tolerance grades, the tolerance allocation problem is formulated as a multi-choice knapsack model, where a genetic algorithm is used to find its optimal solution. From these models and approaches, a prototype computer aided angular tolerance charting system is developed to help process engineers to determine the working dimensions for a new process plan and help quality control engineers to determine whether the process plan can be used to produce the product according to the requirements as stated in the blueprint.