Patent ID: 8714885

Claim:
A throwaway rotary cutting tool that is used for performing rough cutting of a blade root of a turbine blade that is to be mounted on an outer circumference of a rotor, the blade root being machined such that the blade root gradually becomes narrower towards a tip end and a width of the blade root increases and decreases, the throwaway rotary cutting tool comprises: a tool body whose diameter becomes narrower in a stepped manner towards a tip end of the tool body in an axial direction and that includes a plurality of step portions on a side face of the tool such that all portions of each subsequent external surface of the plurality of step portions when moving toward the tip end are equidistant from or closer to a central axis of the tool body than a portion from all preceding external surfaces closest to the central axis of the tool body; a first tip seat and a second tip seats that are formed as recessed areas on an outer circumference of and in a circumferential direction of each of the plurality of step portions of the tool body and that are disposed in a staggered pattern in relation to the axial direction in a side view of the tool body; a first tip that is rectangular, that is fastened in the first tip seat, and that includes a side that is parallel to the axial direction; and a second tip that is rectangular, that is fastened in the second tip seat, and that includes a side that is parallel to the axial direction, wherein: in the side view of the tool body, a portion of a rotation path of the side of the first tip and a portion of a rotation path of the side of the second tip overlap each other on a same line in a direction parallel to the axial direction in each of the plurality of step portions, the first tip and the second tip configure a right-hand cut, a groove that is formed between the first tip seat and the second tip seat is a left-hand helix, and axial rake angles of the first tip and the second tip are positive and gradually increase with each of the plurality of step portions in order in the axial direction from the tip end to a back end of the tool body, the axial rake angles being rake angles, in the axial direction of the tool body, of the first tip and the second tip.