Internal groove insert and tool holder therefor

An internal groove insert, comprising:

• a rectangular main body having a top surface, a lower surface, first, second, third and fourth lateral surfaces formed in a counter-clockwise direction when seen from a top view;
• a clamping hole passing from said top surface to said lower surface, the clamping hole having a first central axis;
• and
• first and second projections each projecting from adjacent first and second lateral surfaces, respectively;
• wherein said first projection has a first cutting edge on its end portion at a side of the top surface of the main body, and said second projection has a second cutting edge on its end portion at a side of the lower surface of the main body;
• wherein
• said first and fourth lateral surfaces are inclined by an obtuse angle with respect to the top surface of the main body,
• said second and third lateral surfaces being inclined by an acute angle with respect to the top surface of the main body;
• and wherein
• the first central axis of said clamping hole is formed to be parallel to an insert corner edge where the first and fourth lateral surfaces meet.