| 摘要 |
PROBLEM TO BE SOLVED: To obtain a desired shape highly precisely in a short time at the time of forming a cylindrical member whose middle part is constricted. SOLUTION: By pressing core bars 27, 28 in both end parts of a base stock 6, the displacing part A and fixing part C of the base stock are restrained. Axial pressure is applied to the base stock 6 with the core bar 27, 28 and simultaneously relative rotation is applied to the displacing part A and fixing part C. Because the displacing part A and fixing part C are restrained with the core bar 27, 28, torsion is generated only in the deforming part B and the diameter is reduced toward the axial center. Moreover, by making the displacing part A and fixing part C approach as progressing diameter reduction by drawing, the deforming part B are inserted between the tip faces of the core bars 27, 28 and the cylindrical member 4 whose middle part is constricted is formed. At this time, by mutually adjusting the rotational speed and approaching speed of the displacing part A and fixing part C of the base stock 6, strain generated in the deforming part B is finely adjusted and the occurrence of wrinkles or the like is prevented.
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