Throwaway rotary cutting tool

A throwaway rotary cutting tool that is used for performing semi-finishing 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 is bilaterally symmetrical with respect to a longitudinal direction of the blade root, 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 comprising:

• a tool body that rotates about an axis, whose diameter becomes narrower in a stepped manner towards a tip end of the tool body in an axial direction, and that includes on a side face of the tool body a plurality of step portions;
• first tip seats that are even in number, and that are provided as recessed areas on an outer circumference of and in a circumferential direction of a step portion at a tip end side of the tool body in the axial direction;
• and
• second tip seats that are even in number, that are provided as recessed areas in a staggered pattern in the axial direction, and that are formed on an outer circumference of and in a circumferential direction of a step portion that is disposed farther toward a back end than the step portion at the tip end side of the tool body,
• wherein:
• a first tip having a total of four sides where the four sides are two rectilinear sides that are parallel to each other and two circular arc sides disposed at opposite ends of the rectilinear sides to directly connect the rectilinear sides to each other, the first tip being fitted into at least one of the first tip seats, and fastened,
• a second tip that is rectangular and a third tip that is rectangular are fitted into the second tip seats alternately in the circumferential direction of the tool body, and fastened,
• the first tip is disposed such that one of the circular arc sides is positioned toward an outer side of the tool body in a radial direction and the rectilinear sides are oriented obliquely in relation to the axial direction in an intersecting manner,
• the second tip is disposed such that a corner portion of the second tip is positioned toward the outer side of the tool body in the radial direction,
• the third tip is disposed such that the third tip is closer to a center of the tool body in the radial direction than the second tip and is offset to the tip end, and a corner portion of the third tip is positioned toward the outer side of the tool body in the radial direction,
• an arc-shaped cutting path is to be formed by the one of the circular arc sides of the first tip and by a side that is continuous with one end of the one of the circular arc sides, and
• a V-shaped cutting path is to be formed by the corner portion of the second tip that is positioned toward the outer side in the radial direction and by one of two sides that flank the corner portion of the third tip that is positioned toward the outer side in the radial direction.