1 Copyright © 2004 by ASME INTEGRATED SYNTHESIS OF ASSEMBLY AND FIXTURE SCHEME FOR PROPERLY CONSTRAINED ASSEMBLY Byungwoo Lee and Kazuhiro Saitou * Department of Mechanical Engineering University of Michigan Ann Arbor, Michigan 48109-2125 Email: {byungwoo, kazu}@umich.edu * Corresponding author 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 parts and fixtures are properly constrained at every assembly step. A case study on a simple frame structure is presented to demonstrate the method. INTRODUCTION Structural enclosures of modern mechanical products, such as ship hulls, airplanes and automotive bodies, typically are made of hundreds or thousands of parts due to their geometric complexity and sizes. As the number of parts increases, however, achieving the dimensional integrity of the final assembly becomes more difficult due to the inherent variations in manufacturing and assembly processes. Figure 1. Two box designs (a) without and (b) with adjustable height during assembly [1]. A solution is to adjust critical dimensions in assembly processes when parts or subassemblies are located and fully constrained in fixtures. This in-process dimensional adjustment is typically facilitated by slip planes, mating surfaces at joints that allow a small amount of relative motions. For example, Figure 1 shows two designs of a rectangular box. In contrast to design in (a) with no in-process adjustability of the critical dimensions (length between sections 1 and 3), design in (b) provides slip planes such that relative location of parts can be adjusted along the critical dimension. Figure 2. Two box designs (a) without and (b) with properly constrained parts [1]. The dimensional integrity of an assembly is also affected by the post-assembly distortion due to the internal stress induced by joining parts with dimensional mismatches. A solution is to ensure the proper constraining of subassemblies at each assembly step. For example, part 1 in Figure 2 (a) is not properly constrained and therefore the post-assembly distortion might occur, if the length of sections 2 and 4 are slightly different due to manufacturing variation. With two slip planes perpendicular to each other, the design in (b) can absorb manufacturing variations within parts 1 and 2-3-4, provided that variations in angles are negligible. In addition to the assembly design including joint types at part interfaces, the assembly sequence also influences in- process dimensional adjustability and proper part constraints. In the assembly sequence in Figure 3 (a), the critical dimension 1 2 4 3 (a) (b) 1 2 4 3 1 2 3 4 (b) 1 2 3 H Proceedings of DETC’04 ASME 2004 Design Engineering Technical Conferences and Computers and Information in Engineering Conference September 28-October 2, 2004, Salt Lake City, Utah, USA DET C20 04-5 7 771