Indirect Force measurement of Micromilling with an Active Magnetic Bearing spindle R.S. Blom 1,2* , A.M. Hoogstrate 1 , P.M.J. Van den Hof 2 , H.H. Langen 1 1 Precision and Microsystems Engineering 2 Delft Center for Systems and Control Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands * corresponding author: r.s.blom@tudelft.nl Abstract While scaling down the toolsize in micromilling, the challenge of maintaining a stable and accurate machining process increases. Hence robust online process monitoring and control are important for obtaining a reliable micromilling process. Online indirect cutting force measurement is proposed that uses the active nature of an Active Magnetic Bear- ing spindle. Using a model-based estimation scheme, force measurements are obtained using data from the displacement sensors and coil current sensors in the magnetic bear- ing system. It is demonstrated that the bandwidth that can be achieved with this ap- proach suffices for the micromilling application. 1 INTRODUCTION The main motivation for scaling conventional milling to the microdomain is that it enables the fabrication of individual micro-components with 3D features. Applications of such components can be found in areas like Medical, Aerospace, Automotive, and Electronics & Communica- tions. Reducing the tool diameter to the range below 1 mm results in a number of issues that make micro milling fundamentally different from conventional machining. This is due to a change of dominant effects when scaling, which include vibrations, the influence of tool geometry, offsets and rigidity, temperature ef- fects and workpiece homogeneity. As a result process stability, surface generation and proc- ess accuracy are affected. To obtain a stable, reliable and high quality micromilling process, much research is still needed. This addressed from different angles: - Efforts are done to increase more under- standing about the chip formation at the mi- cro scale, which is to be used for process optimization and tool design; - Research is done to develop new concepts for meso-scale machine design to realize faster and more precise machinery; - Investigated are methodologies enabling process monitoring and control during 3D micromilling. The presented research is in- cluded in this theme. Implementation of monitoring and control tech- niques tailored to the challenges of this process is of paramount importance. To reach the de- sired accuracies, more control mechanisms are needed alongside highly accurate motion con- trol of the tooltip. This may include vibration control, cutting force control, tool deflection control and (thermal) error compensation. Fur- thermore, workpiece quality will benefit from tool condition monitoring techniques. In the mi- cro scale, wear of the tool edge has a signifi- cant impact on the process quality and stability. Tool wear monitoring may allow for better con- trol of the surface roughness, burr formation, and other process phenomena. Tool breakage avoidance and detection is needed to prevent damage to workpiece and machine. In all of the above mentioned techniques two elements are essential: (1) obtaining reliable information about the process and (2) using this information online in a closed-loop setting to adapt the milling process. In this research our objective is to perform both steps in a model- Figure 1: Experimental setup.