A Comprehensive System for Computer-Aided Tolerance Analysis of 2-D and 3-D Mechanical Assemblies

Abstract

Tolerance analysis of assemblies promotes concurrent engineering by bringing engineering requirements and manufacturing capabilities together in a common model. By further integrating the engineering modeling and analysis with a CAD system, a practical tool for product and process development is created. It provides a quantitative design tool for predicting the effects of manufacturing variation on performance and cost in a computer-based design environment.

A comprehensive method based on vector assembly models is presented for modeling and analyzing variations in 2-D and 3-D mechanical assemblies. The method is consistent with engineering design practice. The models are constructed of common engineering elements: vector chains, kinematic joints, assembly datums, dimensional tolerances, geometric feature tolerances and assembly tolerance limits. The method is well suited for integration with commercial CAD systems.

Three sources of variation may readily be included in the models: dimensional, geometric and kinematic. Dimensional variations account for small changes in size due to manufacturing processes. Geometric variations describe changes in shape, location and orientation of features. Kinematic variations describe the propagation of variation through an assembly by small adjustments between mating parts.

Design intent is expressed by assembly tolerance specifications, which may be added to the model and used in computing predicted quality levels. A set of assembly design constraints patterned after ANSI Y14.5 geometric feature controls is described

Systematic modeling procedures and rules for creating vector assembly models are outlined. Examples of a 2-D and a 3-D assembly are presented with corresponding vector assembly models.

A complete computer-aided tolerancing system is described, which is tightly integrated with a commercial CAD system. Vector assembly models are created graphically and analyzed for variation statistically in an interactive design environment.