Optimal Process Tolerance Balancing Based On Process Capabilities And Quality Loss

The increasingly intensified global competition has forced manufacturing companies to look for all possible means to reduce production cost and improve product quality. The selection of appropriate process dimentions and process tolerances has been a common practice for cost reduction in the metal cutting industry. Many approaches have been presented.But these methods indicate a clear lack of integration approach in addressing the cost as related to manufacturing operations, process capability and quality as related to tolerancing issues. So, an integrated approach is presented to jointly solve the process capabilities and quality loss problems to avoid inconsistent decisions.Firstly, the methods of the dimention chain are presented. A detailed statement of the process dimension and graph theory is analyzed.The method of the process dimention is used for products with angular features, and the method of the graph theory is used for the products without angular feathures.The trigonometric formulae are derived to explain the tolerance accumulation when processing the angular feature suface.Secondly, the paper devided the relationship of the process capability and quality loss. The process capability is related to the manufacture company and the quality loss is related to the user, so the relationship of them is very important for establishing the programming model. The factors influencing the process capability and quality loss are analyzed and the relationship of them is devided.Then, based on the relationship of the process capability and quality loss, a nonlinear programming model is established for the process tolerance. The new programming model is used to jointly optimize the process tolerances of the required operations to achieve a feasible eclectic blue print. In the established model, the objective function is to minimize the total manufacturing cost. The constraint equations consist of the process tolerance chains, quality loss, process capability indices and the relationship of them, with economical tolerance bounds of machine tools.Finaly, two practical example are used to verify the proposed approach's validity,one is the workpiece without angular feactures, and the other is with angular feactures. The comparative results show that the proposed approach is more advantageous in relaxing tolerance requirements, reducing machining cost and avoiding the inconsistent of the cost and quality, compared with the existing methods.