Towards an Effective Virtual Sheet Metal Assembly Development Process Securing Geometrical Quality and Equipment Utilization

Licentiate thesis, 2009

Automobiles are an integral part of modern society and its way of life. In addition, automotive manufacturers are important institutions in society, summing the work of a vast array of sub suppliers, employing many. The margins are however moderate and competition is fierce. The automotive industry furthermore faces shifts of paradigms regarding propulsion as well as styling. Fundamental technical innovations, primarily to meet environmental requirements, that are moreover not a trivial issue to foresee, will affect all parts of the operation. It is thus important that product realization can respond quickly and effectively to market impulses using no more resources than necessary. Today most aspects of development are conducted in a virtual environment. However, design decisions are still based on experience rather than mathematical analysis. In this work we target the car body, or the Body-in-White, which is perhaps the most defining part of any car, with a significant influence on safety, aesthetics, handling, fuel economy and top speed. In the automotive industry, sheet metal assembly design has moved in the past 20 years from a physical to a virtual engineering environment. Virtual models of products and processes are now developed long before any physical prototypes are built. However whereas product and process now are represented in a virtual environment, the design parameters are still separately determined based on experience rather than holistic mathematical analysis. Moreover, quality analysis is mainly separated from cost/capacity analysis, leading to either suboptimal quality or cost/capacity. Therefore, the objective of this research is to investigate the prerequisites for holistic sheet metal assembly design, where influencing design parameters are treated together. This project aims at answering the following overall research question: What characterizes an effective, virtual sheet metal assembly development process, securing the key characteristics of geometrical quality and equipment utilization? The research question is primarily targeted by investigating how influencing design parameters should be optimized with respect to key characteristics. Moreover, supporting information flows and analytical tools enabling the process are identified. This work presents a chronological framework for virtual sheet metal assembly design. The chronological couplings between the design parameters are identified and discussed. The framework conveys the order in which design parameters that influence the key characteristics need to be introduced to the project, to ensure that prerequisite data from previous parameters are available in each subsequent parameter analysis. This work furthermore identifies the trade-offs between geometrical quality and equipment utilization. It is concluded that fixture geometries that are effective in terms of geometrical quality often constitute bulky obstacles that have a significant negative influence on station cycle time and equipment utilization. It is moreover found that welding concepts that favor geometrical quality usually entail longer robotic travel and require lower robot operating speeds. This work moreover presents a dynamic packing method and algorithm. At early project stages, product and process design are of an approximate nature and are subject to frequent design changes and part restriction volume conflicts. This inhibits the use of automatic path planning. Therefore, a method that suggests a minimal part geometry design change that results in a collision-free assembly path is developed and implemented.