Performing measurement or calibration on positioning machines

An apparatus for performing measurements on a machine, the machine having relatively movable parts one of which is capable of carrying a tool or probe for performing an operation on a workpiece, relative movements of said parts of the machine being driven under the control of a machine controller, the apparatus comprising:

• first and second structures detachably connectable to respective ones of the relatively movable parts of the machine, at least one of the structures having three supports and the other having at least one support,
• each of the supports on the first and second structures having part-spherical support surfaces thereon, which are centered on a point thereby defining calibration points on each structure, the relative positions of said calibration points being accurately known in relation to the other calibration points, if any, on the respective structure,
• at least three pre-calibrated extendible measuring bars having part-spherical surfaces at their ends which are shaped to fit the support surfaces on the structures, said measuring bars being connected between the two structures so that the shaped surfaces at their ends are located in predetermined relationships with said calibration points, each of the supports on the two structures having at least two of the pre-calibrated extendible measuring bars connected to it,
• said measuring bars being undriven but operating during relative movements of the movable parts of the machine as the machine performs said operation on a workpiece, to provide information as to the relative positions of said calibration points of all of said supports at any instant during said relative movement,
• said position information being passed to the machine controller as feedback data for the control of the relative movement of the machine parts.