Developmental Research of Hydrostatic Air Spindle for Ultra-Precision Machine Tools

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1) Development of the maximized damping design at resonance frequency of bearing-rotor system : Based on the frequency characterisitics of squeeze film damping, we established a design method of maximizing air film damping at resonance frequency of the bearing-rotor system by fabricating the lattice grooves on the radial and thrust bearing surfaces. From this design concept we can increase the damping ratio at resonance up to more than 0.1.2) Vibration analysis of hydrostatic air spindle including elastic deformation of rotor : Based on the finite element method, we developed the structural modal analysis program of the bearing-rotor system including the elastic deformation of rotor and the equivalent stiffness of the connecting bolts. As a result it is found that not only the three stractural natural frequencies of rotor up to 11 kHz but also air bearing resonances can be predicted more accurately within 5% errors.3) Development o air spindle design method of minimizing the rms value … More of rotor response caused by the machining force with various kinds of frequency spectrum : We first determined the radial and thrust bearing configurations so as for the static displacement at machining point to be minimized. Next we developed a design method of compromizing the static stiffness and resonance damping ratio in order to minimize the rms value of dynamic response at machining point. The optimal air bearing designs for narrow and wide band spectrum external forces are exemplified. Further the lowest bending vibration frequency of rotor can be increased up to 7kHz.4) Realization of ultra-precision air spindle with 10nm/N dynamic compliance in terms of 3 sigma : We made an air spindle prototype whose rms value of the dynamic response is minimized to the imposed machining force with uniform amplitude for the frequencies from 0.5 to 5 kHz. From measurement of the bearing characterisitics, it is found that, because of the machining inaccuracy, the effective bearing clearance is 4 to 4.5 mum, so that the static stiffness is 50 to 70% of the theoretical value for 5 mum nominal clearance. The dynamic air film stiffnesses are larger than the calculated ones by 20 to 30% and the damping ratios are 60 to 80% of the calculated ones. However, it is confirmed that the 3 sigma value of the dynamic compliance of the spindle supported freely can be within 10nm/N in both the radial and thrust directions. It is also found that the rms value of nonrepeatable vibration is less than 10 nm. Less