Thread forming tap

A thread forming tap having a full thread portion and a leading portion disposed contiguous with the full thread portion and reducing in diameter toward its tip, the full thread portion and the leading portion being provided with an external thread on which lobes and recesses are alternately formed, wherein

• when, with respect to a shape around a tool center line O of the lobe in the leading portion, δ represents an angle around the tool center line O toward thread forming side from a top of the lobe of 0° having a maximum radial dimension, in a range on at least the thread forming side from the top of the lobe and beyond a working region where the leading portion cuts into a workpiece for thread forming,
• in a finishing plastic deformation section where the angle δ is less than or equal to an inflection point angle θ that is predefined within the working region, the shape of the lobe is defined so as to vary along a quadratic curve relative to the angle δ so that relief amount R, which is amount of recession from a radial dimension at the top of the angle δ=0°, increases toward the inflection point angle θ, whereas
• in a rough plastic deformation section where the angle δ is greater than the inflection point angle θ and is defined to exceed the working region, the shape of the lobe is defined so as to vary along an Archimedean curve with a certain clearance angle α1 so that the relief amount R increases linearly relative to the angle δ, and
• a margin section with a radial dimension equal to the radial dimension of the top is substantially zero or is within a range where the angle δ is less than or equal to 1°.