An approach to the economic manufacture of an aerostatic lead screw for micro-scale machine tools

Abstract

A new manufacturing method is presented for an aerostatic lead screw actuator (ALSA) designed for use in a hard-turning micro-scale machine tool. The actuator is designed for a travel of 20 mm and to achieve a static stiffness of 50 N/mm and dynamic stiffness of 40–60 N/mm for a range of frequencies from 50 Hz to 500 Hz, respectively. The design of the aerostatic bearing is based on a orifice-restricted groove-fed collar bearing and uses replication of the lead screw geometry by casting with a low-shrink epoxy to produce the nut. The nut is designed to achieve a static stiffness of 50 N/μm and dynamic stiffness of 40–60 N/μm with friction below 0.1 N m. Equations for the aerostatic bearing are adopted from a mathematical model of an aerostatic collar bearing and used to determine the screw size, orifice diameter and air gap. The design evaluation led to the selection of an air gap of 9 μm and an orifice diameter of 1 mm. By using a two-revolution nut, the stiffness requirements of the ALSA are achieved. Methods to produce the orifices, grooves, nut housing and desired surface roughness on the nut are presented. Several alternative nut designs used to construct the ALSA are evaluated according to stability and friction characteristics. Stability is found to be improved by using a single revolution nut due to better control of the orifice size, but at the expense of some system stiffness. Potential methods to control the air gap size are also presented.