| 摘要 |
A plug has a tip end portion, a taper portion, and a middle portion. The surface of the tip end portion is a spherical surface whose radius of curvature is R1 and length is L1 that is shorter than R1. The outer diameter of the taper portion on the front end side is D1, and the outer diameter on the back end side is D2 that is larger than D1, and the length is L2. The plug satisfies Expression (1), Expression (2) if 0<L1/R1<0.5 and Expression (3) if 0.5‰ L1/R1<1. The curve of the spherical surface of the tip end portion is gentler than that of the semi-spherical surface, and the diameter of a hole formed in a billet by the tip end portion is larger than that in the case of the semi-spherical surface. Therefore, the taper portion does not contact with the billet. 0.5 �¢ D �¢ 1 < L �¢ 1 + L �¢ 2 ‰ 2.5 �¢ D �¢ 1 1.0 < D �¢ 2 / D �¢ 1 ‰ 1.4 1.0 < D �¢ 2 / D �¢ 1 < 1.8 - 0.8 �¢ L �¢ 1 / R �¢ 1 |
