| 摘要 |
A circular hole is defined in a top plate stock of thin sheet steel, and a peripheral edge of the circular hole is raised and radially outwardly expanded into a substantially frustoconical first flange blank. Then, a peripheral edge of a proximal portion of the first flange blank is raised and radially outwardly expanded into a substantially frustoconical second flange blank contiguous to the first flange blank, the second flange blank having a lower portion beneath the peripheral edge of the proximal portion. Finally, the third flange blank is drawn into a tubular flange which has a predetermined inside diameter and a predetermined height.
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