Abstract: A spindle assembly of a type adapted to receive a readily removable disc for rotation senses the thickness of the disc with respect to a predetermined thickness and in response to so sensing the thickness of the disc an indication is provided as to whether the thickness of the disc varies from the predetermined thickness. The disc is clamped by movement of a clamping body while a portion of the spindle assembly serves to insure that the clamping body moves into and out of engagement with the disc surface in a direction substantially normal to the disc under control of the spindle assembly. In addition, the force with which the clamping body acts is modulated selectively as desired to accommodate the need for greater forces when mounting larger discs.