Functional tolerancing of complex mechanisms: Identification and specification of key parts

The functional tolerancing process for complex mechanisms needs the study of the behavior of mechanical joints. The use of the Computer-Aided Design system helps in carrying out this task. This paper presents an effective new approach for decomposing a global geometric functional requirement of the mechanism into geometric specifications defined on key components (parts and sub-assemblies). A recursive tolerancing method serves to identify and functionally specify the key components related to a tolerance-chain. A geometric variation model, based on the invariant degrees of freedom (DOFs) of the datum reference frames and tolerance zones, enables validating a datum reference system built on the positioning features of a component and then deducing the influential mechanical joints. Formalized simple rules based on the topology of parts have also been developed for validating a datum reference system. A geometric specification defined on a sub-assembly has been introduced as a new geometric functional requirement: this provides the designer with an effective recursive tool for ensuring the functional tolerancing of the entire assembly. The proposed approach is straightforward to implement and has been devised from the concept of standardized specifications, with the particularity that it is capable of treating cases in which the positioning features are not perpendicular.