| US 7,530,248 B2 | ||
| Plug for use in piercing mill | ||
| Kazuhiro Shimoda, Osaka (Japan); and Tomio Yamakawa, Osaka (Japan) | ||
| Assigned to Sumitomo Metal Industries, Ltd., Osaka (Japan) | ||
| Filed on Jun. 06, 2008, as Appl. No. 12/155,657. | ||
| Application 12/155657 is a continuation of application No. PCT/JP2006/324204, filed on Dec. 05, 2006. | ||
| Claims priority of application No. 2005-353665 (JP), filed on Dec. 07, 2005. | ||
| Prior Publication US 2008/0314109 A1, Dec. 25, 2008 | ||
| Int. Cl. B21B 19/04 (2006.01) | ||
| U.S. Cl. 72—97 | 3 Claims |
| 1. A plug for use in a piercing mill, comprising a tip end portion, a taper portion, and a middle portion sequentially in
the direction from the front end to the back end of said plug, wherein the surface of said tip end portion has a convex spherical
surface in the axial direction of said plug, its radius of curvature is R1, the length of said tip end portion is L1 that is smaller than said R1,
the surface of said taper portion is formed continuously with the surface of said tip end portion, the outer diameter of said
taper portion on the front end side is D1, the outer diameter of said taper portion on the back end side is D2 that is larger than D1, the length of said taper portion is L2,
the surface of said middle portion is formed continuously with the surface of said taper portion, the outer diameter of said
middle portion gradually increases in the direction from the front end to the back end of said plug, and
said plug satisfies Expression (1), Expression (2) if 0<L1/R1<0.5, and Expression (3) if 0.5≤L1/R1<1:
0.5D1<L1+L2≤2.5D1 (1)
1.0<D2/D1≤1.4 (2)
1.0<D2/D1<1.8−0.8L1/R1 (3).
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