Patent ID: 8425163

Claim:
A thread forming tap comprising: an external thread including: a complete thread portion having protruding portions and radial relief portions that are alternately formed; and a chamfered thread portion successive to the complete thread portion and decreasing in diameter toward a leading end of the tap, the chamfered thread portion having protruding portions and radial relief portions that are alternately formed, wherein the complete thread portion includes a front thread portion being successive to the chamfered thread portion, and a remaining rear thread portion, the front thread portion having a length in an axial direction of the thread forming tap five times or less a pitch P of the external thread; the front thread portion being formed of: one of a major diameter and a pitch diameter of the external thread having (1) a fixed radial size, equal to that of a proximal end of the chamfered thread portion with a gradient of change along the axial direction being zero, or (2) a back taper with a radial size gradually decreasing from the proximal end of the chamfered thread portion toward the rear thread portion with a predetermined gradient of change, and the other of the major diameter and the pitch diameter being formed with a back taper with a radial size gradually decreasing from the proximal end of the chamfered thread portion toward the rear thread portion with a predetermined gradient of change; the rear thread portion having each of the major diameter and the pitch diameter being formed with a back taper with a radial size gradually decreasing from a proximal end of the front thread portion toward a shank respectively; and the gradient of change at the rear thread portion being greater than the gradient of change at the front thread portion for the one of the major diameter and the pitch diameter of the external thread, and the gradient of change at the rear thread portion being similar as the gradient of change in the front thread portion for the other of the major diameter and the pitch diameter of the external thread.