| 摘要 |
PROBLEM TO BE SOLVED: To control thickness nonuniformity of a billet by preventing inclination of a plug during expanding and misalignment between the plug and a mandrel without linking the plug and the mandrel. SOLUTION: A mandrel 30 is provided at its end face with a convex portion 31 whose sectional area is gradually reduced toward its end. A plug 20 to be brought into contact with the end face of the mandrel 30 is provided at its rear end face with a concave portion 24 to be closely engaged with the convex portion 31. When a stem 10 descends and the end portion of the plug 20 is initially inserted in the top portion of a guide hole 51 of a billet 50, inclination of the plug 20 is eliminated by the time when the expanding is started. During the expanding, the plug 20 is fixed in the radius direction by the mandrel 30. It is also possible that the mandrel 30 is provided at its end face with the concave portion and the plug 20 is provided at its rear end face with the convex portion.
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