| 摘要 |
<p>PROBLEM TO BE SOLVED: To form a uniform thread on a metallic pipe by making the radius of curvature of a tip forming part of plural main forming punches equal to the radius of curvature of the groove part of an objective screw, making the dimension along the peripheral direction of the metallic pipe of sub-forming punches smaller than those of the main forming punches and moving the sub-forming punches forward and backward independently from the main forming punches. SOLUTION: A device for forming a double-start thread on a metallic pipe is provided with the main forming punches 1 and sub-forming punches 2. The main forming punches 1 are movable forward and backward toward the center of the metallic pipe 3 set at a forming position and the radius of curvature of a tip forming part 1a is equal to the radius of curvature of a groove part of the screw to be formed. The shape of a tip forming part 2a of the sub-forming punches 2 which is arranged between the main forming punches 1 is round. To form the thresd, first, the sub-forming punches 2 are advanced and after executing the preliminary forming for slightly recessing a part 3a of the metallic pipe 3 inward in the radial direction, the sub-forming punches 2 are retreated and simultaneously the main forming punches 1 are advanced.</p> |
