The testing of axes of rotation has drawn much attention with ever-increasing demands on methods using a maste cylinder and dial gases in early 1930s, much improvement has been made on measuring principles and equipment. However, testing techniques existing today still rely upon mechanical masters for the datum surface of spindle motion and, for this reason, some measurement errors are inevitably encountered. One of the errors is the offset between the master and spindle which distorts the fundamental component of measured spindle motion. The other is the out-of-roundness of the master itself requiring special care to be removed. In addition, the master behaves as an additional mass which modifies the vibrational modes of the spindle system to be measured. This paper proposes a new method to measure the radial motion of a spindle without master. Optical moire technique is adopted for the purpose with two concentric-circle gratings. The radial motion is directly determined by analyzing the fringe patterns produced by optical interferences between the gratings. This method can inherently eliminate the undesirable measurement errors caused by the master. In addition, electromagnetic interferences do not affect measuring accuracy since no electrical displacement sensor is needed, as opposed to conventional techniques.
