Abstract: A ball-bar 26 for calibrating machine tools and coordinate measuring machines comprise a unit 30 containing a displacement transducer 34 for measuring axial displacement of a precision ball 28. The ball 28 is pivotably located on the head or spindle 12 of the machine, which is driven in a circle about a ball 22 mounted on the bed or table 10 of the machine at the other end of the ball-bar in order to perform a calibration. The ball-bar has a modular construction, its various components such as the unit 30, extension bar 40, magnetic sockets 24,29, supporting device 14 and magnetic clamp 16 being connected together via M6 screw-threads, e.g. at 18,42,44. This enables the unit 30 to be connected to the various other components in a variety of configurations, e.g. for use as an electronic dial gauge.

Abstract: 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 center defined by the center 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 center 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.