An apparatus for mounting a coordinate positioning apparatus accessory, such as a tool setter, to the workpiece mounting surface of a coordinate positioning apparatus, the workpiece mounting surface having a slot formed in it. The apparatus includes an insert for inserting into the slot of the workpiece mounting surface, and a mount attachable to a coordinate positioning apparatus accessory. The insert includes structure to fix the insert within the slot; the insert is substantially contained within the slot when inserted therein. The mount includes at least one protruding member for engaging the insert when contained within the slot, thereby fixing the mount, and therefore any coordinate positioning apparatus accessory attached to the mount, to the workpiece mounting surface of the coordinate positioning apparatus.