Patent ID: 8708621

Claim:
A spiral fluted tap which includes a threaded portion having an external thread corresponding to an internal thread to be cut and a cutting edge formed along a spiral flute formed to divide the external thread, and configured to be screwed into a prepared hole that is provided on a workpiece to cut an internal thread by the cutting edge on an inner circumferential surface of the prepared hole with discharging chips toward a shank via the spiral flute, wherein the spiral flute has a stepped portion at a rear end of a chamfer portion or in a portion on a shank side from the rear end of the chamfer portion in the threaded portion, a flute bottom diameter on a distal end side of the spiral fluted tap from the stepped portion is smaller than a flute bottom diameter on a shank side from the stepped portion, and a diameter difference Δd between the flute bottom diameters in opposite portions with respect to the stepped portion is determined to satisfy a following formula (1), Δ d≧H /( n×m ) (1) where H is a height of a thread of the internal thread to be tapped, n is a number of a chamfer thread of the threaded portion and m is a number of the spiral flute.