Cutting insert having outwardly inclined side surface and inwardly inclined lower surface, and rotary cutting tool

A cutting insert (1) comprising a first end surface (2), a second end surface (3) opposing the first end surface (2) and a peripheral side surface (4) connecting the first end surface (2) and the second end surface (3), the cutting insert having an axis (O) penetrating the first end surface (2) and the second end surface (3), wherein:

• the first end surface (2) has a substantially quadrangular shape with four sides, including two long sides (7a) and two short sides (7b), and major cutting edges (8) being formed at the intersection of the first end surface (2) and the long sides (7a);
• the peripheral side surface (4) has two outwardly inclined first side surface parts (4a) associated with the two long sides (7a), and two second side surface pats (4b) associated with the two short side (7b);
• the second end surface (3) has two inwardly inclined parts (3a), each connected to one of the two first side surface parts (4a) and being inclined towards one another, each of the two inwardly inclined parts (3a) extending between the two short sides (7b) along a length of the second end surface (3);
• a vertical plane (P1) containing the axis (O) and intersecting the first and second end surfaces (2, 3) passes between the two long sides (7a);
• the outwardly inclined first side surface parts (4a) gradually become more distant from the vertical plane (P1), as they head from the first end surface (2) to the second end surface (3);
• and
• the inwardly inclined parts (3a) gradually become more distant from the first end surface (2), as they head from respective outwardly inclined first side surface parts (4a) toward the vertical plane (P1).