Sub-assembly partitioning choice for complex assemblies based on an action-count-closure criterion

Taking manufacturing and assembly issues into consideration during the design phase has been shown to be capable of reducing production costs of a given design. Although Design-ForAssembly (DFA) methods currently exist that aid the designer by suggesting systematic redesign, it is felt that while these methods are useful for simple designs, e.g. those having a limited number of parts, they may be inadequate for more complex assemblies. This thesis attempts to address how DFA may be used successfully for such complex assemblies through subassembly repartitioning or minor redesign. Complex assemblies are generally characterized by having a large parts count with the assembly organized as a collection of subassemblies. In addition to having little or no design freedom, a complex assembly may contain assembly moves in which a large number of kinematic degrees of freedom (actions) must be closed, i.e. fixed, during the assembly move. This action count closure can then be taken as a quantifiable measure of difficulty of a particular assembly move. It has been determined that the design freedoms that are available to a designer for redesigning a product for easier assembly consist primarily of detail redesign, case (nonfunctional) redesign, and subassembly repartitioning. By using Assembly Sequence Analysis (ASA), an approach for suggesting how detail redesign may be applied successfully to an assembly has been studied. To address the issue of subassembly repartitioning, a tool has been developed that suggests how an assembly may be repartitioned favorably according to specified criteria. This tool searches a space of possible subassembly repartitions using a genetic algorithm. Although the developed tool is capable of handling multiple criteria, the criterion used in this thesis was based on measuring difficulty of assembly by using the action counts of the assembly. This tool is also capable of determining favorable assembly sequences in addition to subassembly repartitionings with respect to the specified criteria. It has been found that while the action count criterion is useful in locating difficult assembly moves, it may not be the driving criterion in determining good assembly sequences and subassembly partitioning schemes suggesting that other criteria, e.g. minimizing the number of reorientations or refixturings during assembly, should also be considered. Case studies of real industry assemblies subjected to these approaches are illustrated. Thesis Supervisor: Daniel E. Whitney Senior Research Scientist Center for Technology, Policy, and Industrial Development Lecturer, Department of Mechanical Engineering