Abstract: A cut off tool comprises an elongated support blade of uniform thickness adapted for mounting in a tool holder and having a head projecting above said blade at one end thereof. An elongated carbide insert having an undercut slot is mounted over the top of said head and is adapted for securing thereon. The insert is of uniform cross section having side walls which taper downwardly and inwardly, end walls which taper downwardly and inwardly and a top surface which is transversely concave along its length to define with an end face a concave leading cutting edge. An elongated lock body is loosely mounted upon and along the support blade and includes at one end a locking head defining a chip breaker and carbide insert lock. The body is notched at its forward end defining with said lock head a hold down part which overlies and retainingly engages said insert.