A thread-cutting tap includes: a thread portion having an external thread, and including a chamfered portion and a complete thread portion that is axially adjacent to the chamfered portion; a flute extending from an axially distal end of the tap toward an axially proximal end of the tap; a rake face defined by one of widthwise opposite end portions of the flute; and a cutting edge defined by one of widthwise opposite edges of the flute, so as to be adjacent to the rake face and extend along the flute. The cutting edge is provided with a protrusion portion at least in a radially outer end portion of the chamfered portion, such that the protrusion portion protrudes from the rake face forwardly in the rotation direction and extends along the cutting edge.

A pipe-tapered-thread machining spiral tap includes a tapered thread portion that is circumferentially divided by flutes into a plurality of lands. Each of at least one of the lands has a cutting edge that extends along a corresponding one of the flutes. The flutes are made of a plurality of spiral flutes that are three, four or five spiral flutes each having a helix angle that is 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 flutes and which is defined at a center corresponding to the rotation axis in a cross section that is perpendicular to the rotation axis, and AL represents a central angle which is subtended by each of the lands and which is defined at the center in the cross section.

A pipe-tapered-thread machining spiral tap includes a tapered thread portion that is circumferentially divided by flutes into a plurality of lands. Each of at least one of the lands has a cutting edge that extends along a corresponding one of the flutes. The flutes are made of a plurality of spiral flutes that are three, four or five spiral flutes each having a helix angle that is 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 flutes and which is defined at a center corresponding to the rotation axis in a cross section that is perpendicular to the rotation axis, and AL represents a central angle which is subtended by each of the lands and which is defined at the center in the cross section.

In one aspect, thread forming taps are described herein. In some embodiments a thread forming tap comprises a shank disposed at a first end adapted to engage a tool holder and an elongated working portion disposed at a second end. The working portion comprises at least one helical thread and a plurality of longitudinally-extending linear grooves. The longitudinally-extending linear grooves intersect the helical thread forming a plurality of lobes, wherein adjacent lobes differ from one another in relief type and/or relief rate.