| 摘要 |
PROBLEM TO BE SOLVED: To make an improvement in the productivity of a grooved plug and an improvement in the strength thereof, etc. SOLUTION: One end of a columnar blank is subjected to diameter reduction by cold-forging, by which its external diameter is made smaller than the diameter in other segments. While the base end side of the small-diameter section is restrained by using segmental dies 25 and 25, its front end side is expanded in its diameter by cold-forging, by which the head of the front end and a circumferential groove succeeding to the same are formed on one end side of a solid shaft section.
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