Claims
[Claim 1] A cutting insert having a polygonal shape for right angle machining,
comprising:
opposed upper and lower surfaces;
a peripheral side surface interconnecting the upper surface and the lower surface at outer peripheries of the upper surface and the lower surface; and
at least one main cutting edge and at least one secondary cutting edge alternately formed along the outer peripheries of the upper surface and the lower surface,
wherein an adjacent pair of the main cutting edge and the secondary cutting edge form a substantially right angle therebetween, wherein the peripheral side surface includes: a first side surface adjoining the main cutting edge of the upper surface; a second side surface adjoining the secondary cutting edge of the lower surface; a third side surface adjoining the secondary cutting edge of the upper surface; and a fourth side surface adjoining the main cutting edge of the lower surface,
wherein the first side surface forms a first obtuse angle relative to the upper surface, the second side surface forms a second obtuse angle less than the first obtuse angle relative to the upper surface, the third side surface forms a first acute angle relative to the upper surface, and the fourth side surface forms a second acute angle less than the first acute angle relative to the upper surface, and
wherein the upper surface and the lower surface are rotationally sym¬metrical around an axis perpendicular to a central axis of the cutting insert.
[Claim 2] The cutting insert of Claim 1, wherein the main cutting edge comprises
a convex curve in a radial direction of the cutting insert when viewing the cutting insert from above and below.
[Claim 3] The cutting insert of Claim 2, wherein when viewing the cutting insert
from above and below, the first and fourth side surfaces comprise a convex curved surface along the main cutting edge and the second and third side surfaces comprise a plane surface.
[Claim 4] The cutting insert of Claim 3, wherein the main cutting edge comprises
a curve concave toward a center in thickness of the cutting insert when viewing the cutting insert from side.

[Claim 5] The cutting insert of Claim 4, wherein widths of the first and fourth
side surfaces become narrower away from the main cutting edge and widths of the second and third side surfaces become narrower toward the secondary cutting edge.
[Claim 6] The cutting insert of any one of Claims 1 to 5, wherein the secondary
cutting edge comprises a straight line when viewing the cutting insert from side and from above and below.
[Claim 7] The cutting insert of any one of Claims 1 to 5, wherein the secondary
cutting edge comprises a convex curve in a thickness direction of the cutting insert when viewing the cutting insert from side and comprises a convex curve in a radial direction of the cutting insert when viewing the cutting insert from above and below.
[Claim 8] The cutting insert of any one of Claims 1 to 5, wherein the cutting
insert comprises three main cutting edges and three secondary cutting
edges at the upper surface and three main cutting edges and three
secondary cutting edges at the lower surface, and
wherein an imaginary circle inscribed with the upper or lower surface
of the cutting insert is inscribed with the main cutting edges at three
points.
[Claim 9] A milling cutter, comprising:
the cutting insert of any one of Claims 1 to 5; and
a cutter body having a plurality of insert pockets each receiving the
cutting insert,
wherein the insert pocket has a supporting surface abutting and
supporting the upper surface or the lower surface of the cutting insert,
the supporting surface forming a positive rake angle relative to a central
rotational axis of the cutter body, and
wherein the cutting insert is mounted on the insert pocket at a positive
axial rake angle.
[Claim 10] The milling cutter of Claim 9, wherein the axial rake angle of the
cutting insert is greater than the rake angle of the supporting surface of the insert pocket.