Abstract: A pneumatic cylinder and piston assembly is provided with a piston rod on the end of which is mounted a punch. The cylinder is provided with an extension tube through which the piston rod and punch extend. The assembly is mounted on a support frame have a bored body and a pair of spaced lateral brackets. The assembly is held in the body with the extension tube passing freely through the brackets. A die is mounted on the frame at a position in counter opposition to said punch whereby a rivet may be driven into materials to be joined and clinched by said punch on driving of said piston. The apparatus includes a stopper mounted about the piston rod to limit the movement of said piston rod in said cylinder and restrict the maximum protruded position of the punch, a spacer mounted about the extension tube and located between the body and the lower bracket to define the optimum driving of the punch, and a nut threaded over the extension tube for securing the extension tube to the lower bracket.