Tolerancing mechanical assemblies with CAD and DOE

Component tolerances for assembled products are often set with the help of the error transmission formula. However, this approach requires knowledge o f the partial derivatives of the functional relationship between the compon ent dimensions and the assembly quality characteristic. In many practical s ituations, those quantities are not easily obtained. In this article we wil l demonstrate how a combined use of computer-aided design (CAD) and design of experiments (DOE) can be used to obtain partial derivatives of the funct ional relationship without knowing an explicit mathematical expression for it. With knowledge of the partial derivatives, we can use the error transmi ssion formula to establish functional tolerances, The intent of the present article is to demonstrate, with some examples, an idea and a set of techni ques that can be used to set functional tolerances for mechanical component s and assemblies.