Abstract: A tool or workpiece holder retention mechanism which engagably retains the holder retention portion of the holder by creating a clamping force which draws the holder retention portion into a cooperatively formed area between two retaining jaws. Two abutting surfaces, one on the holder and one on the abutting surface on the retention mechanism housing, have formed thereon cooperatively engagable surfaces at least one of which is generally spirally formed, which meshedly engage to self-center the tool holder. Preferably the cooperatively engagable surfaces are in the form of mating spiral gear teeth specifically the jaws and the holder retention portion are constructed and actuated so as to be radially freely floatable to avoid interference with the self-centering function of the mating spiral surfaces.