METHOD FOR PRECISION CALIBRATION OF ROTARY SCALE ERRORS AND PRECISION DETERMINATION OF GEAR TOOTH INDEX ERRORS

A method using the ‘closure principle’ is developed for precision calibration of rotary scale errors of gear measurement machines and precision calibration of gear tooth index errors (accumulated tooth-spacing errors) after ‘removal’ of rotary scale errors. The method uses the standard machine procedure for measuring gear tooth index errors applied to a spur gear artifact mounted between machine centers. Therefore, the rotary scale calibrations include consistent effects arising from eccentricities of machine gear-mounting centers and scale-mounting center relative to the instantaneous axis of table rotation, wobble of the instantaneous axis of table rotation, as well as scale graduation errors, etc. Gear-artifact index errors are referenced to the axis connecting the mounting centers located on the gear. Successful implementation of the method does not require super precision of the gear artifact. A method for obtaining approximate uncertainties (standard deviations) of both scale and index error calibrations is developed that utilises the same measurement data required for the scale and index error calibrations. The developed methods are illustrated by applications to scale and index error calibrations obtained from multiple sets of measurements. Typical standard deviations achieved are under 0.05 μm (1/10th of the wavelength of light). Good consistency is achieved between predicted and measured results. The general methods developed should be useful for other types of rotary axis calibrations.