| Fixtures are used to locate and hold workpieces during manufacturing operations, such as machining, assembly and inspection. Therefore, fixture design is an important part of manufacturing process planning. Traditionally, a craftsman approach has been used, which involves an experienced design engineer determining fixture number and locations through "trial and error". In the past decade, research in fixture design has been extensive and a number of design rules and methodologies have been proposed. Most of these advances, however, are only applicable to solid workpieces only.; Fixture design for sheet metal parts is unique for the following reasons: (1) Sheet metal parts are very flexible, and a good fixture should be able to restrain workpiece deformation; (2) The fixture design should consider multiple parts, instead of a single part as is the case for solid workpieces; (3) Springback can introduce post-assembly problems and should be considered in fixture design stage; (4) The parts are assembled through resistance spot welding, which introduces unstable perturbations to the process.; This dissertation addresses these unique features for sheet metal parts by using an optimal fixture design approach, with the following achievements:; (1) The "N-2-1" principle is proposed for sheet metal fixturing. Since sheet metal parts are flexible in the surface normal direction, "N {dollar}>{dollar} 3" block locators are generally required to restrain the rigid body motion, the excessive workpiece deformation, and possible buckling. "2-1" pin locators are generally enough to locate the workpiece in the surface directions.; (2) Robust fixture design is proposed for 3D workpieces. When applied to sheet metal parts, the optimal design for "2-1" locating pins can be achieved. Through robust design, the influence of workpiece surface errors and fixture set-up errors can be minimized by appropriately arranging the fixture configuration, leading to an optimal design from among an infinite number of feasible fixture schemes. To accommodate the different workpiece surface profile, both infinitesimal error analysis and small error analysis methods are presented. As a very successful application on sheet metal part, the robust design algorithm can be used to determine the optimal "2-1" locating pin design.; (3) The "N" block locator optimization methodology is developed for both single part holding and multiple part assembly. The optimal fixture design algorithm has been developed using structural optimization techniques, i.e., finite element method and nonlinear programming approach, to achieve the best "N" block locator configuration. Two modules for "N" locator optimization are developed. One is suitable for single part holding for manufacturing processes such as part measuring, drilling. The other one is suitable for multiple sheet metal part assembly, where the assembly variations can be minimized when welding variabilities and assembly springback are considered.; As a result of this research, an integrated Optimal Fixture Design software called OFixDesign is developed using two commercial packages, MSC/NASTRAN (for finite element analysis) and VMCON (for optimization), plus a CAD system called SDRC/IDEAS for pre- and post-processing. |
