A metal cutting tool having an exchangeable cutting insert mounted on a replaceable cartridge seat releasably fitted in a tool holder of the cutting, the cartridge seat comprising a body portion with a receiving pocket formed therein, an axially directed external surface of the body portion and a substantially planar rear surface thereof; the receiving pocket being formed with a substantially planar insert receiving surface; the tool holder being formed with an insert receiving cavity having an axially directed internal surface adapted to receive the body portion and a substantially planar end surface adapted to abut said rear surface; one of said axially directed surfaces being substantially cylindrical and the other being formed with three suitably located, axially directed protrusions so that upon the screw coupling together of the cartridge seat in the receiving pocket the body portion pivots about its region of abutment so as to be clamped against the protrusions.