Importance

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Abstract

A spiral tap is disclosed having a thread portion having an external thread, spiral flutes which is fluted in the same direction as a cutting/rotating direction, as seen in a direction away from a shank side, to divide the external thread, and cutting edges formed along the spiral flutes; the thread portion including a full thread portion having a fixed outer diameter, and a chamfer portion decreasing in outer diameter toward a tap end; the chamfer portion being screwed into a prepared hole to cut an internal thread on an inner circumferential wall of the prepared hole and to discharge chips along the spiral flutes toward the shank; the chamfer portion having an external thread with screw threads which is formed such that outer circumferential portions are removed, at a linear line for cutting as seen from a direction perpendicular to a cross-section involving an axis O, from the screw threads having the same dimension as that of the full thread portion; and the linear line intersecting the axis O at an inclined angle θ falling in a range of −15°≦θ≦30′ with a side decreasing in diameter toward the tap end being defined positive.

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First Claim