Abstract: A top jaw system for securing a top jaw in position above a chuck on a milling machine, lathe or the like is provided. A protrusion extends upward from the chuck and is preferably attached to the chuck by rigid attachment to a jaw nut which is positioned in a common master jaw on the chuck. The protrusion has a flat face which extends upward and over the jaw nut to form an acute angle with the jaw nut. A top jaw having a slot sized to receive the protrusion is placed over the protrusion. The top jaw has a pin aperture extending through the top jaw through the slot at a right angle to the protrusion when the top jaw is placed on the protrusion. When the top jaw is placed on the protrusion, an elongated pin having a flat surface extending along its elongated axis is placed in the pin aperture. The flat face of the protrusion is pushed into contact with the flat surface of the pin. This is preferably done a set screw extending through the top jaw and contacting the protrusion opposite the flat face.