| Achieving the dimensional integrity for a complex structural assembly is a demanding task due to the manufacturing variations of parts and the tolerance relationship among them. While assigning tight tolerances to all parts would solve the problem, an economical solution is (1) taking advantage of small motions that joints allow, such that critical dimensions can be adjusted during assembly processes and (2) making parts and subassemblies properly constrained at every assembly step, which absorbs part variations to yield reduced assembly stress.; This thesis presents a systematic method that automatically synthesizes assemblies at an early stage of design, i.e., decomposes a given product shape, selects joint types, and generates assembly sequence, such that the combined outcome would provide the adjustability for the critical dimensions and properly constrained fit at every assembly step. Taking advantage of the fact that adjustability and proper constraint must be provided at every assembly step, the method inverts these desirable conditions into decomposition rules, which are recursively applied to a product shape until decomposed shapes are considered to be manufacturable. At each recursion, the product shape is decomposed into a pair of child shapes, joints are assigned to the interfaces between child shapes to ensure in-process dimensional adjustability and proper part constraints. Then, the reverse of this decomposition sequence yields a feasible assembly sequence, at every step of which, the adjustability and properly constrained fit are guaranteed. In addition, the enumeration of fixture schemes is incorporated into the assembly synthesis method, which, in combination, defines all feasible Datum Flow Chains for a given product shape and critical dimensions.; An augmented AND/OR graph is devised to enumerate and represent all feasible assembly and fixture scheme syntheses with corresponding assembly sequences, and a correct and complete algorithm to generate the AND/OR graph is proposed. Case studies are presented to demonstrate the application of the method to 2D skeletons of sheet metal products as well as 3D beam products, where Screw theory is utilized to analyze constrains. Pareto optimum solutions are identified by searching the AND/OR graph and the trade-offs among assembly design, sequence and fixture scheme are discussed. |
