Design of a Micro Milling Setup with an Active Magnetic Bearing Spindle

Abstract

This thesis describes the design of a micro milling setup with an active magnetic bearing spindle. Micro milling is the mechanical removal of material with sub millimeter tools. An active magnetic bearing typically consists of a set of magnetic actuators, a control loop and position sensors. Active Magnetic Bearings enable a very high rotational speed of 150.000 rpm and a high positioning accuracy. The use of bearing signals for process monitoring is a separate research topic in the same project. Special attention has been given to the design of magnetic actuators with low rotating losses. A design with an axial bias flux generated by permanent magnets has been applied. The rotordynamic modelling of the spindle is described as well as the controller design. The spindle has been realized and a rotational speed of 150.000 rpm has been achieved. The spindle has been integrated into a micro milling setup, where the work piece is positioned using an xy stage driven by Lorentz actuators. A novel tool clamping device had to be developed which matches the reduced dimensions of the spindle and which is able to withstand the very high rotational speed. A monolithic tool holder has been designed in which the clamping force increases with increasing rotational speed. The main improvements that can be made require the increase of the rotational speed and an increase of the flexural spindle resonance frequencies. A novel design has been presented to support a short, disk shaped rotor. This rotor shape has required the design of a controller which compensates the strong gyroscopic coupling in a rotating disk.