Rotary cutting tool cutting insert and rotary cutting tool

A rotary cutting tool comprising:

• a rotary cutting tool cutting insert;
• and
• a main body portion that holds the rotary cutting tool cutting insert, wherein the rotary cutting tool cutting insert includes
• a top surface,
• a bottom surface located opposite to the top surface, and
• an outer peripheral surface contiguous to each of the top surface and the bottom surface, wherein
• a ridgeline between the top surface and the outer peripheral surface forms a cutting edge,
• the cutting edge includes
• a first line segment,
• a second line segment opposed to the first line segment,
• a third line segment inclined relative to each of the first line segment and the second line segment,
• a fourth line segment opposed to the third line segment,
• a first curved portion connecting the first line segment and the third line segment,
• a second curved portion connecting the second line segment and the fourth line segment,
• a third curved portion connecting the second line segment and the third line segment, and
• a fourth curved portion connecting the first line segment and the fourth line segment,
• a straight line along the first line segment and a straight line along the third line segment form an acute angle,
• a straight line along the second line segment and a straight line along the fourth line segment form an acute angle,
• the straight line along the second line segment and the straight line along the third line segment form an obtuse angle,
• the straight line along the first line segment and the straight line along the fourth line segment form an obtuse angle,
• each of the third curved portion and the fourth curved portion is larger in radius of curvature than the first curved portion and larger in radius of curvature than the second curved portion,
• the top surface has a flat portion spaced apart from the cutting edge,
• in the direction perpendicular to the bottom surface, the flat portion is located between the cutting edge and the bottom surface,
• an entirety of the top surface is not located above the cutting edge in the direction perpendicular to the bottom surface when a direction from the bottom surface toward the top surface corresponds to an upward direction.

A rotary cutting tool cutting insert comprising:

• a top surface;
• a bottom surface located opposite to the top surface;
• and
• an outer peripheral surface contiguous to each of the top surface and the bottom surface, wherein
• a through hole is opened at each of the top surface and the bottom surface,
• a ridgeline between the top surface and the outer peripheral surface forms a cutting edge,
• the cutting edge includes
• a first line segment,
• a second line segment opposed to the first line segment,
• a third line segment inclined relative to each of the first line segment and the second line segment,
• a fourth line segment opposed to the third line segment,
• a first curved portion connecting the first line segment and the third line segment,
• a second curved portion connecting the second line segment and the fourth line segment,
• a third curved portion connecting the second line segment and the third line segment, and
• a fourth curved portion connecting the first line segment and the fourth line segment,
• a straight line along the first line segment and a straight line along the third line segment form an acute angle,
• a straight line along the second line segment and a straight line along the fourth line segment form an acute angle,
• the straight line along the second line segment and the straight line along the third line segment form an obtuse angle,
• the straight line along the first line segment and the straight line along the fourth line segment form an obtuse angle,
• each of the third curved portion and the fourth curved portion is larger in radius of curvature than the first curved portion and larger in radius of curvature than the second curved portion, and
• in a direction perpendicular to the bottom surface, a distance between the top surface and the bottom surface is equal to a distance between the cutting edge, and the bottom surface,
• wherein the top surface includes
• a first land surface contiguous to the first line segment,
• a second land surface contiguous to the second line segment,
• a third land surface contiguous to the third line segment, and
• a fourth land surface contiguous to the fourth line segment, and
• in a cross section perpendicular to the bottom surface and located at a center of the through hole, each of a width of the third land surface in a direction perpendicular to the third line segment and a width of the fourth land surface in a direction perpendicular to the fourth line segment is larger than a width of the first land surface in a direction perpendicular to the first line segment and larger than a width of the second land surface in a direction perpendicular to the second line segment.