Helical broach

A helical broach that forms a twisted groove at the inner circumference of a machined hole of a workpiece by a plurality of cutting edges that protrude from an outer-circumferential part of a broach body formed in a shaft shape to the radial outside of the broach body and are arranged in a spiral shape which is twisted around the axis of the broach body from the tip-end side of the broach body to the rear-end side thereof,

• wherein the plurality of cutting teeth include a plurality of circumference cutting teeth that are arranged on the tip-end side of the broach body, and include a plurality of tooth thickness cutting teeth that are arranged on the rear-end side of the broach body,
• the plurality of tooth thickness cutting teeth respectively include cutting edges that are formed at intersecting ridge parts between rake faces facing the tip-end side of the broach body and lateral surfaces facing one wall surface of both groove wall surfaces of the twisted groove of the workpiece, and include guiding edges that are formed at intersecting ridge parts between lateral surfaces on the opposite side to the lateral surfaces provided with the cutting edges and the rake faces,
• among the tooth thickness cutting teeth, the tooth thickness cutting tooth positioned closest to the rear-end side of the broach body is provided with the guiding edge instead of the cutting edge at the intersecting ridge part between the rake face and the lateral surface facing one wall surface of both groove wall surfaces of the twisted groove of the workpiece,
• the tooth thickness cutting tooth positioned closest to the rear-end side of the broach body is formed so as to be retreated in the tooth thickness direction more than the tooth thickness cutting tooth positioned right ahead thereof in a row to which the tooth thickness cutting tooth belongs,
• on the guiding edge of the tooth thickness cutting tooth positioned closest to the rear-end side of the broach body, a negative relief angle θc is formed in the range of 0.5°≦θc≦2°, and
• a retreat amount of the tooth thickness cutting tooth positioned closest to the rear-end side of the broach body with respect to the tooth thickness cutting tooth positioned right ahead thereof in a row to which the tooth thickness cutting tooth belongs, is set to a range of 0 to 20 μm.