Robust Design by Tolerance Allocation Considering Quality and Manufacturing Cost

Abstract

Abstract Involving customer values in the design process is necessary to improve the total quality of a product. The purpose of this work is to establish a theoretical base for tolerance allocation which allows both quality and manufacturing cost to be considered. The paper addresses functional tolerance chains, i.e. tolerance chains that involve a dimension important for the function of the product or component. The total loss to customer is determined as the sum of two tolerance dependent properties; the functionality loss and the component price. The functionality loss represents the customer’s economical loss due to poor functionality. The optimal tolerance limits are found by minimizing the total loss to customer. These are the limits that represent the best trade-off between cost and quality, from the customer’s point of view. This work specially emphasizes a method for treating asymmetrical functionality loss, i.e. when the design is more sensitive to a deviation of a critical parameter in one direction than in the other. By moving the manufacturing target in a direction away from the most sensitive part, the total loss to customer can be reduced. This paper describes how the optimal manufacturing target and corresponding symmetrical tolerance band are found. This method thus increases the robustness of the design. The method may be used for single tolerances or any resulting tolerance of a tolerance chain.