Patent ID: 7815512

Claim:
A thread forming tap for forming an internal thread, comprising: a working portion including a margin portion forming the internal thread to be formed, and a relief portion having a smaller diameter than the margin portion, having an external thread corresponding to the internal thread, the margin portion and the relief portion being disposed in plurality so as to be continuous in an axial direction in parallel to an axis and to be positioned alternately around the axis, wherein by screwing the working portion into a prepared hole, the margin portions bites and plastically deforms a surface layer of the prepared hole to thereby form the internal thread, and at least at a chamfer part, each of the margin portions has a following margin disposed at a following flank of a ridge of the external thread, and a leading margin disposed at a leading flank positioned oppositely across a root of the external thread, and the following margin is shifted more in a screw-rotation direction than a symmetry position of inversion of the leading margin onto the following flank with the root as a symmetry axis.