Cutting insert

1 . A cutting insert comprising:

an upper surface;

a lower surface on an opposite side to the upper surface;

a peripheral side surface that connects the upper surface and the lower surface to each other; and

an attachment hole that extends through the upper surface and the lower surface, wherein

the upper surface is rotationally symmetrical about a central axis extending through a center of the upper surface and a center of the lower surface, and the lower surface has substantially the same shape as the upper surface,

an edge where the upper surface and the peripheral side surface intersect is divided into a plurality of adjacent sections, and each of the sections includes one minor cutting edge and one major cutting edge that is longer than the minor cutting edge,

in each section, the minor cutting edge and the major cutting edge are arranged in a same order counterclockwise from one of the sections to an adjacent one of the sections in a circumferential direction along a circumference centered on the central axis,

each of the major cutting edges is inclined upward in a right-hand direction extending away from the lower surface gradually while extending away from the minor cutting edge, and

as the upper surface extends from each of the major cutting edges toward the attachment hole, a height position of the upper surface in an up-down direction, in which the upper surface and the lower surface oppose each other, does not change, or the height position decreases in a direction from the upper surface toward the lower surface.

2 . The cutting insert according to claim 1 ,

wherein, in a plan view as seen from the upper surface, an angle formed by the minor cutting edge and the major cutting edge is 140° or more and 155° or less.

3 . The cutting insert according to claim 2 ,

wherein, for each of the major cutting edges, a virtual straight line extending in the up-down direction in which the upper surface and the lower surface oppose each other, through a lowest point, which is a point on the major cutting edge that is closest to the lower surface, and a highest point, which is a point on the major cutting edge that is farthest from the lower surface, is inclined at an angle of 1° or more and 10° or less relative to a virtual plane perpendicular to the central axis.

4 . The cutting insert according to claim 3 ,

wherein the upper surface is six-fold or five-fold symmetrical about the central axis.

5 . A cutting insert comprising:

an upper surface;

a lower surface on an opposite side to the upper surface;

a peripheral side surface that connects the upper surface and the lower surface to each other; and

an attachment hole that extends through the upper surface and the lower surface, wherein

the upper surface is rotationally symmetrical about a central axis that extends through a center of the upper surface and a center of the lower surface, and the lower surface has substantially the same shape as the upper surface,

an edge where the upper surface and the peripheral side surface intersect is divided into a plurality of adjacent sections, and each of the sections includes one minor cutting edge and one major cutting edge that is longer than the minor cutting edge,

in each section, the minor cutting edge and the major cutting edge are arranged in a same order counterclockwise from one of the sections to an adjacent one of the sections in a circumferential direction along a circumference centered on the central axis,

each of the major cutting edges is inclined upward in a right-hand direction extending to be away from the lower surface gradually while extending away from the minor cutting edge, and

each of the major cutting edges is inclined to be away from the lower surface gradually while extending away from the minor cutting edge,

the upper surface includes

a flat surface parallel to a virtual plane perpendicular to a central axis extending through the center of the upper surface and the center of the lower surface,

a rake surface that is provided on a periphery of the flat surface and is inclined at a first angle relative to the virtual plane, and

a land having a linear contour in cross-section that is provided between the rake surface and each of the major cutting edges and is inclined at a second angle relative to the virtual plane, the second angle being smaller than the first angle, and

for each of the major cutting edges, when

in a plan view as seen along the central axis, a shortest linear distance from the major cutting edge to an outer edge of the attachment hole is taken as a reference value, and when

in a cross-section taken perpendicular to the major cutting edge at a position along the major cutting edge, a length measured, along a direction perpendicular to the central axis, from a virtual intersection point where a virtual straight line that is an extension of the linear contour of the land intersects the flat surface to the major cutting edge, is taken as a variable which changes depending on the position at which the cross-section is taken,

all of the variables are larger than half of the reference value.

6 . The cutting insert according to claim 5 ,

wherein, in a plan view as seen from the upper surface, an angle formed by the minor cutting edge and the major cutting edge is 140° or more and 155° or less.

7 . The cutting insert according to claim 6 ,

wherein, for each of the major cutting edges, a virtual straight line extending in the up-down direction in which the upper surface and the lower surface oppose each other, through a lowest point, which is a point on the major cutting edge that is closest to the lower surface, and a highest point, which is a point on the major cutting edge that is farthest from the lower surface, is inclined at an angle of 1° or more and 10° or less relative to a virtual plane perpendicular to the central axis.

8 . The cutting insert according to claim 7 ,

wherein the upper surface is six-fold or five-fold symmetrical about the central axis.