Active Control of Tool Deflection and Chatter During Precision Milling Operations Using Magnetic Bearing Spindles

North Carolina State University, Raleigh NC

Investigators

Abstract

This grant will be used to determine the feasibility of quasi-static and dynamic error compensation for precision machining applications using small diameter end mills. The prime mover for these active control schemes will be a commercially-available, magnetically-suspended, 70,000 rpm spindle. Such a spindle is capable of creating the high surface speeds needed for miniature milling tools, can adjust the center of rotation to compensate for tool deflection, and has the high bandwidth necessary to provide active chatter compensation. The control strategies will have two modes: low-frequency path correction for tool deflection and high-frequency compensation for chatter. The controller used to position the magnetic bearing spindle will require major modifications to implement the algorithms described above. The controller supplied with this spindle senses the rotor displacements and manipulates electromagnet currents to keep the spindle rotating around the inertial center. A high-speed processing board will be added to implement the proposed control strategy of estimating the magnitude and direction of dynamic tool deflections and modifying the spindle rotor setpoint to compensate for these static and dynamic deflections. If successful, the techniques demonstrated in this project will improve the capability to machine precision surfaces such as injection molds dies for plastic optical and mechanical parts. But the results will have much broader applications including a scientific basis for generalized tool deflection compensation, actively reducing the effects of chatter on surface finish, and high-bandwidth control of magnetic bearings. In addition, real-time frequency analysis techniques will be developed to analyze the displacement signals and extract high-frequency components for chatter detection and correction. It is anticipated that these results will have immediate application to both high-precision and full-scale, high-speed milling applications.