Cutting insert for rotary cutting tool, and rotary cutting tool

A cutting insert for a rotary cutting tool, the cutting insert comprising:

• a first surface;
• a second surface opposite to the first surface;
• and
• an outer circumferential surface which is contiguous to each of the first surface and the second surface, wherein
• a ridgeline between the first surface and the outer circumferential surface includes a first cutting edge and a first straight line portion,
• a ridgeline between the second surface and the outer circumferential surface includes a second cutting edge and a second straight line portion,
• the first surface includes a first seating surface having a shape of a flat surface,
• the second surface includes a second seating surface having a shape of a flat surface,
• the first cutting edge includes a first main cutting edge portion and a first sub cutting edge portion,
• the second cutting edge includes a second main cutting edge portion and a second sub cutting edge portion,
• when viewed in a direction from the first surface toward the second surface, at least a portion of the second sub cutting edge portion is located outside the first main cutting edge portion, and at least a portion of the first sub cutting edge portion is located outside the second main cutting edge portion,
• when viewed in the direction from the first surface toward the second surface, each of the first sub cutting edge portion and the second sub cutting edge portion has a shape of a curved line,
• the outer circumferential surface includes
• a third seating surface having a shape of a flat surface, being located between the first main cutting edge portion and the second sub cutting edge portion, and being contiguous to the first main cutting edge portion,
• a fourth seating surface having a shape of a flat surface, being located between the second main cutting edge portion and the first sub cutting edge portion, and being contiguous to the second main cutting edge portion, and
• a fifth seating surface having a shape of a flat surface, being located between the first straight line portion and the second straight line portion, and being contiguous to the first straight line portion and the second straight line portion,
• in a cross section which is parallel to the direction from the first surface toward the second surface and which intersects the first main cutting edge portion, the third seating surface, and the second sub cutting edge portion, an angle formed between the third seating surface and the first seating surface is an obtuse angle, and an angle formed between the third seating surface and the second seating surface is an acute angle,
• when viewed in the direction from the first surface toward the second surface, in a circumferential direction, the first main cutting edge portion, the first sub cutting edge portion, and the first straight line portion are alternately located,
• when viewed in a direction from the second surface toward the first surface, in the circumferential direction, the second main cutting edge portion, the second sub cutting edge portion, and the second straight line portion are alternately located, and
• when viewed in the direction from the first surface toward the second surface, the ridgeline between the first surface and the outer circumferential surface has a shape with a rotational symmetry of 180° about a central axis.