| 摘要 |
PROBLEM TO BE SOLVED: To improve the fatigue strength of a female thread part with small torque by executing the press-machining by the screwing of a cold-forged tap having a male thread part for cold forging, and having an effective diameter larger than that of the female thread part, to a surface layer of the female thread part. SOLUTION: A high strength female thread part 2 has an enlarged diameter larger than an effective diameter D2 of a female thread part 4 formed by the cutting work, to be provided with an effective diameter D1, and is further expanded to be larger than a screw outer diameter D4 of the female thread part 4 to be provided with a screw outer diameter D3. A high strength female thread part 2 can be formed by executing the pressure machining to the female thread part 4 formed by a cutting tap 10, by means of a cold forging tap 5 having a male thread part 6 for cold forging having an effectively diameter D1 larger than the effective diameter D2 of the cutting tap 10. A surface layer of a valley part 2A and a thread operating surface 2B of the high strength female thread part 2 is refined and reinforced, and simultaneously the residual compression stress is applied thereto to reduce the tensile stress, whereby the fatigue strength of the high strength female thread part 2 can be improved.
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