Cutting insert

1. A cutting insert having a forward end and a rear end opposite the forward end, the cutting insert comprising:

an end cutting edge at the forward end of the cutting insert;

side cutting edges extending from opposite ends of the end cutting edge and toward the rear end of the cutting insert;

a rake face including

a positive rake face extending from the end cutting edge and toward the rear end of the cutting insert, the positive rake face terminating at a rear end of the positive rake face, the positive rake face inclined downward from the end cutting edge so that a positive rake angle is obtained,

a breaker wall rising obliquely from the rear end of the positive rake face or rising obliquely from a position rearward of the rear end of the positive rake face to an upper end; and

inclined surfaces extending from the end cutting edge along the side cutting edges, the inclined surfaces formed so as to incline downward from the side cutting edges toward a central portion of the rake face between the side cutting edges;

wherein the rake face defines a recessed groove located downward of the inclined surfaces in the central portion between the inclined surfaces, the recessed groove extending rearward from the end cutting edge toward the rear end of the cutting insert and beyond an upper end of the breaker wall into a portion of the cutting insert rearward of the breaker wall,

wherein the recessed groove is formed such that, as viewed from a rake face side in a region between the end cutting edge and the upper end of the breaker wall, from a position of the end cutting edge toward the rear end of the cutting insert, a groove width of the recessed groove gradually increases, then gradually decreases such that the recessed groove has a narrowest portion at a position forward of a position of the upper end of the breaker wall and rearward of a positive rake angle rear end position which is the rear end of the positive rake face, and then gradually increases from the position of the narrowest portion until the position of the upper end of the breaker wall, and

wherein the recessed groove is formed such that, when the groove width at the position of the end cutting edge is denoted by W 1 , the groove width at the position of the narrowest portion is denoted by W 2 , and the groove width at the position of the upper end of the breaker wall is denoted by W 3 , then the groove widths W 1 , W 2 , and W 3 satisfy a dimensional relation W 1 <W 2 <W 3 .

2. The cutting insert according to claim 1 , wherein each of the inclined surfaces includes:

a first inclined surface inclined downward at a small angle α 1 from a corresponding one of the side cutting edges toward the central portion of the rake face between the side cutting edges; and

a second inclined surface continuous with the first inclined surface and inclined downward at an angle α 2 larger than the small angle α 1 .

3. The cutting insert according to claim 1 , wherein, where Wa is a length of the end cutting edge when the end cutting edge is viewed from the rake face side, W 1 and Wa satisfy a dimensional relation W 1 ≤Wa/2.

4. The cutting insert according to claim 1 , wherein, as viewed from the rake face side, corners formed by the end cutting edge and the side cutting edges are rounded.

5. The cutting insert according to claim 1 , wherein the cutting insert is configured for grooving or cutting-off.