Centrifugal force induced dynamics of a motorized high-speed spindle

Trend of the high-speed and high efficiency machining has pushed the continuous demand of higher spindle speed and power for the machining center application. Because the extremely high speed produces significant centrifugal force, it creates a need to predict the spindle dynamical characteristics at dynamic states. This work presents analysis results of the spindle dynamic of a motorized high speed spindle with angular ball contact bearings. For a machining center, two major subsystems determining the overall spindle stiffness are the shaft/bearing subsystem and the draw bar mechanism subsystem. Shaft/bearing stiffness as well as the natural frequency decreased at high speeds due to the bearing softening and gyroscopic effect. The bearing softening is the major reason of the reduced spindle stiffness, while the gyroscopic effect plays the secondary effect. Angular contact ball bearing softening at high speed is due to the reduced contact load and increased contact angle at the ball/inner-raceway contact interface caused by the centrifugal force. For the draw bar mechanism, analysis results show that the dynamic draw force at high speeds is significantly increased from that designed at the static state. Because the toolholder/spindle interface stiffness is proportional to the draw force, centrifugal force theoretically contributes a plus to the spindle stiffness at dynamic state. The dynamic draw force, however, is dependent on the friction loss inside the draw bar mechanism. Because of the low friction coefficient, the ball-type mechanism is superior to the wedge type mechanism.