A complete tolerance charting system in assembly

Abstract

The process of assigning tolerances in product design, more often than not, will affect the functionality and cost of the product. An unnecessarily tight tolerance leads to higher cost while an excessively loose tolerance may lead to malfunctioning of the product. Traditionally, this important phase of tolerance allocation is done intuitively to satisfy design constraints based on the skills and experience of the designers. In most cases, the result is not optimum and modification of the allocated tolerances is required. After the tolerances are allocated, the next step is to ensure that the assembly can be manufactured with the resources available. This can usually be verified by performing tolerance charting for the assembly. However, if the result shows that the blueprint (B/P) tolerances specified are too tight, the entire process of tolerance allocation and tolerance charting has to be repeated. This paper presents an efficient method of integrating the process of allocating optimum B/P tolerances in product design with the process of tolerance charting in process planning. This method ensures that the assembly is manufactured with optimum working dimensions and tolerances and yet satisfies all the assembly requirements. Two models are constructed. One based on process sequence and another based on the relationship between each component. Using the models coupled with unique algorithms, sets of linear equations are formulated based on the design constraints and assembly requirements. With another set of equations derived in terms of the process capabilities, the optimum B/P tolerances, working dimensions and tolerances can be determined by solving the equations.