Process Monitoring for a Machine Tool Spindle with Magnetic Bearings

This paper talks about the application of active magnetic bearings (AMBs) in a high frequency machine tool spindle, emphasizing the possibility of direct process monitoring with the AMBs. Process monitoring, i.e. the measurement of cutting forces and the recognition of process anomalies (tool breakage, tool wear), is a very important task in machining technology today, as it is necessary for automation in machining lines. For conventional motor spindles, several approaches have been made, including force measurement by a piezoelectric equipment, power measurement, spindle current measurement and acoustic emission. AMB motor spindles provide the possibility to do the process monitoring directly with the AMBs. The control output of each bearing is proportional to the magnetic force applied to the rotor shaft. By measuring these control outputs, the cutting forces of the milling process can be determined without any extra equipment. Short-time variations of the cutting forces, as e.g. the entrance of each cutting edge into the material, can be determined by measuring the spindle shaft dislocation with the position sensors of the AMBs. Good results could be achieved by this method. The cutting forces could be determined, and tool breakage and tool wear could be well recognized.