Four-sided insert for milling cutters with negative radial angle of the cutting edges for three-dimensional milling from the solid

A four-sided insert for milling cutters with negative radial angle of the cutting edges for three-dimensional milling from a solid, comprising:

• a lower face;
• an upper face, or rake face, that is delimited by four sides that constitute the cutting edges of the insert, said four sides being mutually opposite in pairs and being connected to corresponding sides of the lower face by lateral faces, two mutually opposite cutting edges constituting main cutting edges of the insert, the other two mutually opposite cutting edges constituting secondary cutting edges of the insert;
• regions where said main cutting edges encounter said secondary cutting edges forming four tips, respectively two outer tips, which are arranged diagonally on the body of the insert, and two inner tips, which are alternated with said outer tips, the insert being associable with the body of the milling cutter and being able to rotate through an angle of substantially 180° about an axis that passes through the center of the rake face and is substantially perpendicular to the upper surface of the insert, in order to place in an active position, as the outer main cutting edge, one of said main cutting edges and, as active secondary cutting edge, one of said secondary cutting edges with one of said two outer tips so as to form the outer cutting circle of the milling cutter and with one of said inner tips so as to form the inner cutting circle of the milling cutter, and wherein the lateral faces that connect said main cutting edges to the lower face of the insert have, proximate to said inner tips, positive lower relief angles that are greater than the lower relief angles proximate to said outer tips.