Thread forming tap

A thread forming tap comprising a complete thread portion and a leading portion which is provided to be contiguous with the complete thread portion and which has a diameter reduced in a direction toward a distal end of the thread forming tap, such that the complete thread portion and the leading portion are provided with an external thread in which protruding portions and relieved portions are alternately formed, wherein the thread forming tap is to be screwed into a prepared hole provided in a workpiece, so as to cause an inner circumferential surface of the prepared hole to be plastically deformed for thereby forming an internal thread corresponding to the external thread provided in the complete thread portion,

• wherein a screw thread of the external thread has a leading flank and a trailing flank, which are to be positioned on a front side of the trailing flank and on a rear side of the leading flank, respectively, when the thread forming tap is screwed into the prepared hole,
• wherein the screw thread is shaped such that, in the leading portion, an indentation depth into the workpiece made by the leading flank is smaller than an indentation depth into the workpiece made by the trailing flank,
• wherein the screw thread of the external thread has a symmetrical isosceles triangular shape in a cross section thereof,
• wherein a pitch of the screw thread in at least a part of the leading portion is larger than a pitch of the screw thread in the complete thread portion by a range in which the indentation depth into the workpiece made by the leading flank is not zeroed, and
• wherein the pitch of the screw thread in the part of the leading portion is larger than the pitch of the screw thread in the complete thread portion.