Effects of gas composition on asynchronous error motion in externally pressurized spindles

Stable, externally pressurized aerostatic spindles exhibit picometer-level vibration as a result of the interaction of the high-pressure working fluid and its path through a spindle's compensation features (e.g., grooves, orifices, pockets, etc.). This vibration presents a challenge as end users seek the lowest possible asynchronous error motion in manufacturing, metrology and data storage applications. This technical brief describes experimental testing to quantify this low-level vibration and its significant variation with both supply gas composition and pressure. Of the gases tested, helium and neon generate the lowest vibration while other gases including air lead to an order of magnitude higher vibration. Vibration levels with carbon dioxide and nitrous oxide are an additional order of magnitude higher. At present, the root cause of the vibration amplitude dependence on supply gas composition is unknown although kinematic viscosity and mean free path length correlate well with the results presented here.