| 摘要 |
PROBLEM TO BE SOLVED: To prevent the drop of strength in a connecting part between a collar part and a hollow-shaft part what is called a neck part while reducing the weight by manufacturing the hollow-shaft parts from a pipe material. SOLUTION: A short pipe material which is thick and larger in the diameter than the outside diameter in the hollow shaft part of a desired hollow bolt with a collar part is used as a base stock A. In the 1st stage, by forcing the base stock A into a die from one direction, a 1st base stock B having a thick head part B2 on one end and a hollow-shaft part B1 which is smaller in the diameter and thinner in the thickness than the base material A on the other end is formed. In the 2nd stage, by forcing a forming pin into the head part of the 1st base stock B and also hammering the head part B2, the mouth part of the head part B2 is formed into a prescribed shape by expanding and simultaneously this head part C2 and the connecting part C4 between the head part C2 and the hollow-shaft part C1 are formed so as to be thicker than that of the hollow-shaft part C1.
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