Hybrid Spindle - An approach for a milling machine tool spindle with Extended working range for HSC and HPC

Commercial motor-driven spindles for milling machine tools are generally developed for specific machining operations, such as HPC (High Performance Cutting) or HSC (High Speed Cutting). HPC and HSC require significantly different working ranges regarding spindle speed and torque. E.g. in HPC operations, high torque at low spindle speeds is needed. The two working ranges are covered only limited by a conventional spindle so far. However, a spindle system that can switch the working range allows cost-effective manufacturing of a wider range of materials within the same machine tool. The spindle bearings and the power electronics supply are challenges, when switching between both working ranges. HPC operation requires high bearing stiffness in order to transmit high forces occurring during the cutting process. However, the associated high bearing preload leads to high frictional losses at high speeds for HSC operations. Furthermore, the wide operating range generates contrary requirements for the motor winding. To increase the spindle speeds with a winding being assembled for high torques, the current has to be increased. For this purpose, expensive and non-standardized motor inverters are required. This paper introduces a concept to overcome these challenges. Firstly, an approach to adjust the bearing preload is presented, using a compact, electrically controlled bearing preload element, which is suitable for industrial applications. Next, an approach for a reconfigurable winding design of the spindle motor is shown. This allows electrical switching between the working ranges using conventional motor inverters. Finally, an analysis of the conditions of use of the hybrid spindles is presented.