Tap the thread forming with inside diameter finishing edges

Groove inside diameter finishing edges (40) is provided (42), three of the relief portion between the axial center (O) chamfer at around the projecting portion of the plurality of rows in (24) (20a) ~ (20f) ( is provided so as to be positioned at the center portion of the 22), the protrusion in the groove (tap rotation direction (a) and the chamfer an opening edge opposite the 42) (24) (20a) (20c), is provided with (20e) a predetermined from the tap rotation direction (a) an angle (theta) shifted inside diameter finishing edges in position (40). Therefore, by forming projecting portions of a plurality of rows of the chamfered portion (24) and (20a) ~ the inner peripheral surface of the prepared hole by using all of (20f) (52) is plastically deformed internal thread (54) can be the number increases of protrusions that are involved in the thread forming process of the internal thread (54) (20a) ~ (20f). Obtained by this protrusion (20a) ~ (20f), one per load (deformation margin of plastic deformation) is decreased, excellent durability without the axial dimension is increased in the chamfered portion (24) It is as to become.