IMAIZUMI Haruki Faculty of Engineering, Shizuoka university, Research assistant, 工学部, 教務員 (80303535)
HAYAKAWA Kunio Faculty of Engineering, Shizuoka university, Assistant professor, 工学部, 助手 (80283399)
NAKAMURA Tamotsu Faculty of Engineering, Shizuoka university, Professor, 工学部, 教授 (70023322)
A new flexible process for micro sheet metal forming was investigated, by which millimeter sized very thin shells can be formed from flat blanks without the need for any expensive dies. This process involves the use of the incremental forming technique. The elementary operation of the process is indentation and travel of a small stylus-like tool along one closed contour line of an objective shape. By repeating this operation for all contour lines, the sheet is formed into the final shape gradually. This process would be useful technique for flexible micro-fabrication of micro components such as a micro-machine shell-body. In the present study, several kinds of thin metal foils ranging in thickness from 10 to 100 micrometers were used and the scaling effect on the fracture limit of the incremental process was examined experimentally.
With the miniaturization of the components to be formed, it was difficult to complete the process without any defect. When a very thin metal sheet was forme
d into a conic shell with steep wall, a catastrophic rupture of the wall occurred. The thickness of the shell wall is shown by the sine law of thickness. The sine law indicates that the wall of the shell becomes thinner when the half apex angle of the cone becomes smaller. The influence of sheet thickness on the fracture limit of annealed stainless steel was investigated. The experimental results show that the fracture limit strain decreases with decreasing the initial thickness. However, the fracture limit strain is greater than that for the conventional press forming. But the experiment was conducted to keep the path-pitch, tip radius and size of sheet constant and thus downscaling refers only to initial thickness of the blank. In order to estimate the substantial scaling effect, in the next experiment, the feed and the tip radius were varied and the effects of them were investigated. The second experimental results show that by decreasing the feed and the tip radius proportional to the initial thickness, the fracture limit of thinner sheet was improved. However, the limit strain of the thin sheet was smaller than that of thick one.
Since the surface roughening of sheet metals with the plastic deformation develops the non-uniformity in thickness, the fracture limit strain of very thin sheet metal appreciably depends on the phenomenon. The foils were subjected to various amount of incremental deformation and the non-uniformity in thickness was examined. The experimental results show that the roughness of the rear surface, which did not contact the stylus, increased as increasing the total plastic strain, while that of the front surface kept constant. The all sheets tested had almost the same maximum height roughness. Therefore, the non-uniformity in thickness of the thinner sheet was greater than that of the thicker sheet, and the thinner sheets had weaker portions to induce a catastrophic rupture. Less