Decomposition-based Assembly Synthesis for In-process Dimensional Adjustability

This paper presents a method of assembly synthesis focused on the in-process adjustability, where assembly synthesis is defined as the decomposition of the end product design prior to the detailed component design phase. Focusing on the effect of joint configurations on dimensional integrity of complex assemblies, the method recursively decomposes a product configuration and assigns joint configurations according to simple rules, in order to achieve a designed dimensional adjustability and non-forced fit. The rules employed during the decomposition process are drawn from the previous works of assembly design. An augmented AND/OR graph is utilized to represent a process of assembly synthesis with the corresponding assembly sequences, and the algorithm for generating the AND/OR graph is discussed. The method is applied to two dimensional skeletons of product designs at very early stage of the design process. The relation of the assembly synthesis to Datum Flow Chain (Mantripragada and Whitney, 1998) is discussed. It is also shown that each final design from the assembly synthesis defines its own Datum Flow Chain. INTRODUCTION Body frames of most mechanical products such as ships, airplanes, and automotives are fairly complex, hence it is very expensive to manufacture them from a single piece of material if it is not impossible. Typically, human designers would decompose a complex body structure into parts such as panels and beams so that each part could be manufactured with reasonable cost while satisfying its structural and functional requirements. As the number of parts increases, however, achieving the dimensional integrity of the final assembly becomes more Corresponding author demanding work due to the inherent manufacturing variations in fabrication and assembly operations. For body structures or frames in which parts are typically forged or bent, it is not economical to manufacture every part with tight tolerance such that tolerance stack-up could be compatible with required dimensional integrity of the final product. Hence, in this type of assemblies, while relative dimensions among parts are specified, the locations of joints are not specified at the part design. Instead, during assembly operations, parts are located and fully constrained in fixtures and they are welded or stamped or drilled for fasteners. In order to adjust relative locations, the contact areas where joints will be placed should be designed in such a way that a small amount of relative motion is allowed, which is why those contact areas are called slip planes. Figure 1. Examples of decomposition and joint configuration for dimensional adjustment. The design in (b) provides adjustability along the critical dimension, while the design in (a) lacks proper slip planes. Designers still have to decide how to decompose the product and how to orient slip planes so that they could provide adjustability during the assembly operations. See Figure 1, for example, where two candidate designs of a rectangular box are shown. Suppose the distance between section 1 and 3 is critical 1