| 摘要 |
PROBLEM TO BE SOLVED: To make the statistical minimum stripping torque sufficiently larger than the statistical maximum tapping torque by providing a primary thread structure, and a secondary thread structure formed among the threads defined by the primary thread structure, with the axial intervals. SOLUTION: A shank 20 is continuous, and is a screw itself, and a secondary thread structure 120 defining the threads 122 axially arranged at intervals among several threads 112 defined by a primary thread structure 100, is formed along an approximately cylindrical part 22 of the shank 20. The secondary thread structure 120 starts from an area 124 near an end of the approximately cylindrical part 22. The radial dimension of the secondary thread structure is gradually changed to be outwardly expanded in such manner that the radial dimension of the secondary thread structure 120 is continuously increased over approximately three thread pitches from the area 124 toward an area 126 on the approximately cylindrical part 22, having the maximum radius dimension of the secondary thread structure 120.
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