A mechanism for clamping a flexible metal printing plate to the circumferential surface of a rotatable plotter drum is provided. The clamping mechanism includes a mounting member affixed to the drum's surface. The mounting member has a passage aligned radially with respect to the drum. A slidable member extends through the passage and past the drum's axis of rotation. The slidable member has, at one end, a head and, at the other end, a weight. The clamp head is disposed further from the axis of rotation than the mounting member and adjacent the drum surface so as to permit clamping between the head and the drum surface. The slidable member is disposed so that its center of mass is opposite the drum's axis of rotation from the head. A mechanism, such as a compressed spring mounted between the weight and the mounting member, is provided for urging the slidable member so as to force the head towards the drum surface. An actuator having two modes is also provided. In the first mode, it forces the slidable member so as to move the head away from the drum surface and, in its second mode, it permits the head to be urged towards the drum surface by the urging means. The clamping mechanism is also provided with a mechanism for turning the slidable member, so that every time the actuator goes into the second mode the clamp head is turned 90° from the previous time the actuator went into the second mode.