A Compact Aerostatic Spindle Integrated with an Axial Positioning Actuator for Micro and Ultra-Precision Machine Tools

An aerostatic spindle integrated with an axial positioning actuator and an air motor has been proposed and fabricated for use in a compact high-precision milling machine. The main shaft of the spindle itself can be axially positioned using a built-in voice coil motor and rotated using an air motor so that the compact spindle system can realize both functions of a one-DOF positioning table and the spindle. The prototype main shaft, which is 20 mm in diameter, 147.5 mm in length and 0.26 kg in weight, could rotate at over 15,000 rpm. The main shaft can also be positioned using a laser interferometer system with an axial positioning range of..5 mm, and axial resolutions of 1.24 nm at rest and 24.7 nm at a speed of 15,000 rpm. Furthermore, the disturbance observer using the axial displacement and the current in the voice coil motor could react to an axial step force of 2 mN.