A theoretical and experimental study of forced spindle vibration under unbalanced magnetic forces in ultra-precision machining

Abstract

In ultra-precision machining (UPM), a brushless permanent magnet DC motor employed for an air bearing spindle exhibits unbalanced magnetic forces (UMFs). The UMFs exert on the spindle rotor to induce forced spindle vibration, named UMFs induced forced spindle vibration (UMF-FSV). It would significantly deteriorate surface quality in UPM. However, the UMF-FSV is not well understood with its effect on surface generation in UPM. In this study, an analysis model of magnetic forces for a spindle motor is developed along with a surface generation model to study dynamics of UMF-FSV and its effect in UPM. The theoretical and experimental results signify that UMF-FSV is harmonic at a fundamental frequency 2pω (motor’s pole number 2p and spindle speed ω), which produces 2p straight radial patterns without phase shift at a machined surface in UPM.