Abstract: An assembly for fastening and locating an added head of a numerical machine tool comprises two cylinder mounts located in the periphery of a main shaft of the numerical machine tool. A piston is movably disposed between the two cylinder mounts such that the piston is driven by the hydraulic-pneumatic loops to move up and down so as to control the movement of an engagement block for retaining a retaining block of the added head. The main shaft of the numerical machine tool is provided with a crank-tooth coupler to facilitate the adjusting of the angular position of the added head in accordance with the operational requirement for processing a workpiece.