Cutting insert and indexable ball end mill

1. A cutting insert to be detachably attached to an insert mounting seat formed at a tip end portion of an end mill main body of an indexable ball end mill rotatable about an axis, the cutting insert comprising:

a rake face facing a direction of rotation of the end mill main body;

a seating face facing an opposite side to the rake face and configured to be seated on a bottom surface of the insert mounting seat; and

a flank face extending around the rake face and the seating face,

wherein two cutting edges each having an arcuate cutting edge portion extending in an arc shape in plan view when viewed from a direction opposed to the rake face and a linear cutting edge portion extending so as to be in contact with the arcuate cutting edge portion are formed on an intersecting ridgeline portion between the rake face and the flank face so that the arcuate cutting edge portion and the linear cutting edge portion are alternately positioned in a circumferential direction of the rake face,

a groove portion having a wall surface configured to be capable of coming into contact with a projection portion protruding from the bottom surface of the insert mounting seat is formed on the seating face,

the groove portion has a narrow width portion in which a groove width decreases from one end side toward the other end side in a direction along which the groove portion extends from the arcuate cutting edge portion to the linear cutting edge portion, and

at the other end side of the narrow width portion, the narrow width portion is opened in the flank face.

2. The cutting insert according to claim 1 ,

wherein one of the two cutting edges is a main cutting edge and the other is a minor cutting edge,

the arcuate cutting edge portion of the minor cutting edge is formed to have a same radius as a radius of the arcuate cutting edge portion of the main cutting edge and have a shorter length than a length of the arcuate cutting edge portion of the main cutting edge in the circumferential direction,

the groove portion extends from an arcuate cutting edge portion side of the minor cutting edge toward a linear cutting edge portion side of the main cutting edge, the linear cutting edge portion side of the main cutting edge is set as the one end side, and the arcuate cutting edge portion side of the minor cutting edge is set as the other end side, and

wherein the groove portion is opened in the flank face connected to the arcuate cutting edge portion of the minor cutting edge.

3. The cutting insert according to claim 2 ,

the arcuate cutting edge portion of the minor cutting edge is formed in a convex curved shape in such a way that the arcuate cutting edge portion extends to the seating face side after being extended away from the seating face side as more apart from the linear cutting edge portion of the minor cutting edge, and

the opening portion to the flank face of the groove portion is positioned closer to the linear cutting edge portion side of the minor cutting edge than the rotationally most projecting point where the arcuate cutting edge portion of the minor cutting edge projects most with respect to the seating face.

4. The cutting insert according to claim 3 ,

wherein in the bottom view when viewed from the direction opposing the seating face, a first intersecting angle formed by a straight line connecting the rotationally most projecting point and a center of the arcuate cutting edge portion of the minor cutting edge and a straight line connecting the end portion toward the seating face side of the intersecting ridgeline portion between a wall surface of the groove portion directed toward the linear cutting edge portion side of the minor cutting edge and the flank face and the center of the arcuate cutting edge portion of the minor cutting edge is in a range of 5° to 60°.

5. The cutting insert according to claim 3 ,

wherein in the bottom view when viewed from the direction opposing the seating face, a second intersecting angle formed by a straight line connecting the rotationally most projecting point and the center of the arcuate cutting edge portion of the minor cutting edge and an extended line of a linear cutting edge portion of the minor cutting edge toward the arcuate cutting edge portion side of the minor cutting edge is in a range of 10° to 50°.

6. The cutting insert according to claim 1 ,

wherein the groove portion is free of an opening in of the flank face on the one end side in the direction in which the groove portion extends.

7. The cutting insert according to claim 1 ,

wherein the groove portion has a constant groove width in at least one of the one end side and the other end side with respect to the narrow width portion.

8. The cutting insert according to claim 1 ,

wherein a groove width at the other end of the narrow width portion in the groove portion is set to be in a range of 0.05×r to 0.18×r with respect to a radius r of the arcuate cutting edge portion of the cutting edge positioned at the other end side of the groove portion.

9. An indexable ball end mill in which the cutting insert according to claim 1 is detachably attached to an insert mounting seat which is formed at a tip end portion of an end mill main body rotatable about an axis by bringing a wall surface of the groove portion into contact with a projection portion protruding from a bottom surface of the insert mounting seat.

10. The indexable ball end mill according to claim 9 ,

wherein the projection portion is formed so as to be wider from the other end side toward the one end side in a direction in which the groove portion extends, in a portion which the narrow width portion of the groove portion abuts.