Basic studies on accuracy management systems based on estimating welding deformations*
SHOJI TAKECHI1, KAZUHIRO AOYAMA2, and TOSHIHARU NOMOTO2
1Sumitomo Heavy Industries Ltd., 19 Natsushima-cho, Yokosuka 237-8555, Japan
2Department of Environmental and Ocean Engineering, Graduate School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract： Accuracy management concerns various aspects of all assembly industries, e.g., final product quality, manufacturing efficiency, and manufacturing costs. In the near future, accuracy management will be even more important in manufacturing systems because of mechanization and automation. This paper considers the concept of computer-aided accuracy management systems, which are based on the concept of integrated manufacturing systems. We propose two management systems, accuracy planning and accuracy estimation, and concentrate on accuracy planning. A model of welding deformations and an easy estimation method for such deformations by computer analysis using the finite element method (FEM) are described. A prototype system based on the accuracy management concept was implemented in the authors' integrated manufacturing systems, and some examples of estimating welding deformations are given here.
Key words： accuracy management, FEM, product model, integrated manufacturing systems
Address correspondence to： S. Takechi
Received for publication on Sept. 29, 1999; accepted on Jan. 25, 1999
*Translation of an article that appeared in the Journal of The Society of Naval Architects of Japan, vol. 181 (1997)： The original article won the SNAJ prize, which is awarded annually to the best papers selected from the SNAJ Journal, JMST, or other quality journals in the field of naval architecture and ocean engineering.
Introduction
In the late 1950s, statistical quality control (SQC) was introduced in Japanese shipbuilding. Since then, SQC has been widely applied at all hull construction stages, contributing to improved ship quality and productivity.
In particular, the geometric accuracy of the hull shape has progressed considerably. Maintaining good geometric accuracy in the hull shape during construction is very important, because accuracy has an enormous effect on：
― ensuring the high quality of ships;
― manufacturing efficiency and cost;
― mechanization and automation.
First, the accuracy of the hull shape is important not only to the beauty of the ship's appearance, but also to the final quality of the ship in terms of propulsion performance, structural strength, etc. Second, extra adjustments would be necessary if the hull shape was inaccurate. Maintaining the accuracy of the fabrication blocks means that manufacturing efficiency improves, and unnecessary expenditure is reduced. In particular, productivity at the dock construction stages of hull fabrication increases considerably when the hull shape is in good accurate conditions.1 Third, mechanization and automation are significant factors in maintaining global competitiveness. In order to ensure the geometric accuracy of the fabrication blocks within an appropriate range it is necessary to use new machines, such as welding robots, in various stages of production.
A great effort to improve the management of accuracy control activities has been made in every Japanese shipyard. In the past, SQC has contributed to supporting accuracy management activities. However, statistical methods tend to show the general tendency or situation, and not to analyze how the various factors correspond with each other theoretically.2 This is not always suitable for a production system such as ship-
building. Thus a new approach to accuracy management, which deals with each block and each stage individually, is required.
In recent years, computer hardware has become more powerful and economic in performance, and highly developed software technology is being used in various
