Cutting insert, cutting tool, and method of manufacturing a machined product

1. A cutting insert, comprising:

an upper surface with a polygonal shape which has a side part, and a first corner part and a second corner part each being adjacent to the side part;

a lower surface with a polygonal shape which corresponds to the upper surface;

a side surface disposed between the lower surface and the upper surface;

a first corner cutting edge that is an intersection of the upper surface and the side surface and is located on the first corner part;

a second corner cutting edge that is an intersection of the upper surface and the side surface and is located on the second corner part;

a flat cutting edge that is an intersection of the upper surface and the side surface and is disposed on the side part;

a major cutting edge that is an intersection of the upper surface and the side surface and is disposed between the second corner cutting edge and the flat cutting edge on the side part; and

a connecting edge that is an intersection of the upper surface and the side surface and is disposed between the flat cutting edge and the major cutting edge,

wherein the flat cutting edge and the major cutting edge have a straight line shape, and the connecting edge has an outwardly protruding curvilinear shape in a top view,

wherein the major cutting edge has an upwardly protruding shape, and comprises an upwardly protruding curvilinear portion, a first straight line portion extending from the curvilinear portion toward the connecting edge, and a second straight line portion extending from the curvilinear portion toward the second corner cutting edge in a side view, and

wherein the flat cutting edge, the connecting edge, and the first straight line portion are located on a straight line in the side view.

2. The cutting insert according to claim 1 , wherein the curvilinear portion is located closer to the second corner cutting edge than the first corner cutting edge.

3. The cutting insert according to claim 1 , wherein the second straight line portion is longer than the first straight line portion in the side view.

4. A cutting tool, comprising:

a holder having a plurality of insert pockets on a front end part of the holder; and

a cutting insert according to claim 1 , which is attached to each of the insert pockets in a state in which the major cutting edge projects beyond the holder.

5. The cutting tool according to claim 4 , wherein, in a state in which the cutting insert is attached to the insert pocket, an inclination angle of the first straight line portion of the major cutting edge is larger than an inclination angle of the second straight line portion of the major cutting edge with respect to a plane direction along the flat cutting edge in the side view.

6. A method of manufacturing a machined product, comprising:

rotating the cutting tool according to claim 4 ;

bringing the major cutting edge of the cutting tool being rotated into contact with a workpiece; and

separating the cutting tool from the workpiece.

7. The cutting insert according to claim 2 , wherein the second straight line portion is longer than the first straight line portion in the side view.