A cutting insert of an embodiment includes an upper surface including a first upper cutting edge and a second upper cutting edge, a lower surface including a first lower cutting edge and a second lower cutting edge, and a side surface. The side surface includes a first surface extending from the second upper cutting edge, a second surface extending from the first lower cutting edge, a third surface extending from the first upper cutting edge, and a fourth surface extending from the second lower cutting edge. The first surface and the fourth surface are parallel to a central axis. The second surface is inclined so as to depart from the central axis as going from the lower surface toward the upper surface. The third surface is inclined so as to depart from the central axis as going from the upper surface toward the lower surface.

An object is to provide a cutting insert for a milling cutter with which efficiency in machining can be increased. The cutting insert includes two polygonal main surfaces facing each other and side surfaces each connected to a corresponding side of each of the main surfaces. The side surfaces include a major side surface through which an attachment hole extends, a first minor side surface perpendicular to the major side surface, and a second minor side surface disposed between the first minor side surface and the major side surface. A ridge between each of the main surfaces and the major side surface forms a major cutting edge, and a ridge between each of the main surfaces and the second minor side surface forms a minor cutting edge. An angle between the major cutting edge and the minor cutting edge is 145 or larger.

A cutting insert includes a first and a second pair of opposing side surfaces. As seen towards a first side surface, a first axial relief face and a first axial abutment face form part of a first surface grouping on a first side of a longitudinal plane. As seen towards a second side surface, a second axial relief face and a second axial abutment face form part of a second surface grouping on the first side of the longitudinal plane. The first axial abutment face forms a substantially flat surface and extends perpendicularly to a median plane of the insert. The second axial abutment face forms a substantially flat surface and extends perpendicularly to the median plane. A second side surface of the first pair of opposing side surfaces has a first and a second radial abutment face.

A cutting insert according to an embodiment of the present invention is a double-sided cutting insert comprising an upper surface, a lower surface, a side surface, a cutting edge, and a mounting hole penetrating the upper surface and the lower surface, in which the upper surface is provided with a single upper surface boss portion that surrounds the mounting hole and is flat, the cutting edge includes a main cutting edge, a corner cutting edge extending from the main cutting edge, a sub-cutting edge extending from the corner cutting edge, and a side portion extending from the sub-cutting edge, a first main cutting edge flank surface extending from the main cutting edge forms an acute angle with an extension line of the upper surface boss portion, a sub-cutting edge chip breaker surface extending from the sub-cutting edge is at a gradually increasing angle with the extension line of the upper surface boss portion, as the sub-cutting edge chip breaker surface is distanced away from the corner cutting edge, and when the cutting insert is viewed into the upper surface, a side portion surface extending from the side portion is concavely recessed.

A substantially parallelogramatic insert having a downward inclined main cutting edge and an auxiliary cutting edge in part of a long-side ridgeline and a short-side ridgeline adjacent to a pair of corner cutting edges of an upper surface; the main cutting edge being constituted by first to third straight cutting edges; the first to third straight cutting edges being connected in a bent-line shape with inward obtuse crossing angles when viewed from above, such that the second straight cutting edge is positioned outermost; and cutting edge angles .sub.1, .sub.2, .sub.3 between the rake faces and flanks of the first to third straight cutting edges meeting the relation of .sub.1>.sub.2>.sub.3.

A cutting insert has an upper surface having a long side, a short side, a lower surface, a lateral surface and a through hole. The lateral surface has a first lateral surface. The first lateral surface has a first plane that is overlapped with a central axis of the through hole in a front view and is parallel to the central axis, a second plane that is located closer to the short side than the first plane and is parallel to the central axis, a third plane that is located more away from the short side than the first plane and is parallel to the central axis, and a first inclined surface that is located between the second plane and the upper surface and comes closer to the central axis as coming closer to the long side. The first inclined surface is located away from the first plane.

A rotary cutting tool, which is excellent in defect resistance and has a stable lifespan, includes a rake surface composed of one surface, a cutting edge angle that is larger than 0 and smaller than 90, a true rake angle that is not smaller than 42 and not larger than 13, and a major cutting edge inclination angle that is not smaller than 5 and smaller than +5. A wiper insert is preferably formed in a rounded convex shape.