Integrated Synthesis of Assembly and Fixture Scheme for Properly Constrained Assembly

Abstract

This paper presents an integrated approach to design an assembly, fixture schemes, and an assembly sequence, such that the dimensional integrity of the assembly is insensitive to the dimensional variations of individual parts. The adjustability of critical dimensions and the proper constraining of parts during assembly process are the keys in achieving the dimensional integrity of the final assembly. A top-down design method is developed which recursively decomposes a lump of initial product geometry and fixture elements matching critical dimensions into parts and fixtures. At each recursion, joints are assigned to the interfaces between two subassemblies to ensure that parts and fixtures are properly constrained at every assembly step. A case study on a simple frame structure is presented to demonstrate the method. Note to Practitioners-Achieving dimensional integrity of complex assemblies is a very demanding task due to the dimensional variations of parts and their propagation. Keys to achieving the goal are to adjust critical dimensions using proper fixtures and joint configurations and to minimize unpredictable deformations caused by overconstrained parts, such that assembled products can be robust to dimensional variations of parts. Equally important are arranging fixtures and sequencing assembly steps as these are what really define which critical dimension is adjusted and which subassemblies are put together on what fixture at each assembly step. In this paper, we propose a top-down decomposition-based approach that generates assembly design, assembly sequence, and fixture plan ensuring adjustability for critical dimensions and properly constrained subassemblies at every assembly step. The method does not only prevent costly trial and error following bottom-up approaches, but also enumerates all feasible solutions for a given product geometry and critical dimensions. As the number of solutions can be fairly large, for real-word application, it is crucial to incorporate practical constraints and to identify other important criteria to search optimal designs.