A Technique for measuring radial error motions of ultra-high-speed miniature spindles used for micromachining

Abstract

Ultra-high-speed (UHS) miniature spindles are widely used for mechanical micromachining processes, such as micromilling and microdrilling, as well as for precision machining processes. The accuracy of features created in those processes depends on the trajectory of the tool tip as the spindle rotates. The tool-tip trajectory can be obtained by measuring the speed-dependent radial motions (which are sometimes referred to as the dynamic runout) at the tool tip. The main contributors to the tool-tip speed-dependent radial motions are the error motions of the spindle, the form accuracy of the cutting tool, the alignment of the tool with respect to the axis of rotation, and the vibrations resulting from the rotating eccentricity. This paper describes a methodology that uses two laser Doppler vibrometer (LDV) systems to measure the radial motions at two axial locations of a precision cylindrical artifact attached to the spindle, while the spindle is rotated at its operational speeds. Measured radial motions are then processed to obtain radial and tilt error motions of the UHS spindle in the rotating sensitive direction. An alignment procedure is developed to ensure the mutual perpendicularity of the two (X and Y) laser beams. The methodology is demonstrated on an UHS air-turbine driven spindle with aerostatic-bearings. Subsequently, an analysis is performed to determine the measurement uncertainty associated with the presented methodology. It is concluded that the presented methodology can be used to effectively measure radial and tilt error motions of UHS spindles. Furthermore, it is shown that the average radial motion, synchronous radial error motion value and the standard deviation of the asynchronous radial error motion vary significantly with the spindle speed due to dynamic effects.