Patent ID: 8186915

Claim:
A spiral tap including a thread part being provided with an external thread corresponding to an internal thread to be worked, a helical flute dividing the external thread, and a cutting edge formed along the helical flute, when the thread part is screwed into a prepared hole formed in a workpiece, the cutting edge cutting the internal thread on an inner circumferential surface of the prepared hole, and chips being discharged toward a shank via the helical flute, wherein the helical flute is formed with a constant lead LA in a constant lead part provided at a tip end, and is connected smoothly to the constant lead part with a lead gradually increased to be not less than 1.2×LA at a flute terminal part, in a gradually increased lead part extending from the constant lead part to a flute terminal at side of the shank, and the constant lead part has an axial length thereof ranging from 0.3×LA to 1.2×LA at the tip end.