Provided is a pipe-tapered-thread machining spiral tap that restrains occurrence of tearing and formation of cutting edge mark in the machined internal thread upon stop of rotation of the tap.
The pipe-tapered-thread machining spiral tap 10 has three, four or five spiral flutes 20 each having a helix angle β of not smaller than 30° and smaller than 50°. A flute width ratio AG/(AG+AL) is 0.3-0.5, where AG represents a central angle which is subtended by each of the spiral flutes 20 and which is defined at a center corresponding to a rotation axis C in a cross section that is perpendicular to the rotation axis C, and AL represents a central angle which is subtended by each of the lands 22 and which is defined at the center corresponding to the rotation axis C in the cross section. Thus, it is possible to obtain the pipe-tapered-thread machining spiral tap 10 in which occurrence of tearing is restrained owing to presence of the spiral flutes 20 each having the helix angle β that is not smaller than 30° and smaller than 50°, and in which formation of mark of the cutting edge in the machined internal thread upon stop of rotation of the tap is retrained owing to a short distance between the cutting edge 24 and the back edge 26.