Male screw body, female screw body, screw body design method, screw thread structure

A threaded body design method comprising:

• a boundary root diameter extracting process of, in the case of implementing a fastening strength test of performing a relative separation in an axial direction by screwing a female threaded body for verification into a male threaded body for verification using a plurality of male threaded bodies for verification having a constant nominal diameter and different thread angles and root diameters and a plurality of female threaded bodies for verification configured to screw with the male threaded bodies for verification, extracting a degree of variation caused by the thread angle variate from the root diameter (hereinafter, referred to as boundary root diameter) around a boundary of a shaft fracture form and a thread collapse form by causing breakage of both the shaft fracture form in which a fastening state is released by separating the male threaded body for verification from a shaft portion and the thread collapse form in which the fastening state is released by deforming or shearing of a thread of the male threaded body for verification;
• a shaft fracture predominant thread angle selecting process of selecting the specific thread angle (hereinafter, shaft fracture predominant thread angle) based on the degree of variation of the boundary root diameter and at which the boundary root diameter is maximized;
• and
• a thread angle determining process of applying a thread angle close to the shaft fracture predominant thread angle to the actual male threaded body and/or the female threaded body in the nominal diameter.