Calibration device for machine

Telescopic ball bar 24 has a male member provided by a sphere 22 at one end and a female member provided by a pair of forks 30 at the other. The ball bar is attached to the spindle 10 of a machine tool by a universal pivotal coupling between the sphere 22 and a magnetic socket 14 retained on the spindle 10. The ball bar 24 is attached to the table by a further universal pivotal coupling between a sphere 28 retained on the table 12 in a clamping device 40, and the forks 30. In use, the spindle is operated to move in a circular path about a centre defined by the centre of the sphere 28. Any deviation of the spindle movement from such a circular path will be detected by a transducer 36, provided in the bar 24, which bears against the surface of the sphere 28. The position of the centre of the sphere 28 is determined by bringing the sphere 28 into engagement with the socket 14, and measuring the position of the spindle 10 at that point. Easy engagement between the sphere 28 and the socket 14 is facilitated by the clamping device 40, which enables the sphere 28 to move relative to the table to engage the socket 14. Once the sphere 28 has engaged the socket 14 (and the position of its centre has been measured by measuring the position of the spindle 10), the sphere 28 may be clamped rigidly relative to the table 12 for a calibration operation.