Motorized spindies are in common use in high-speed milling . The dynamic behavior cf the spindie depends on the actual number of revolutions and the temperature. The performance cf the spindie during milling, particularly, the behavior concerning chatter, is crucially affected by the speed dependent dynamic behavior cf the mechanical system. This paper presents the reasons for the speed-sensitive stiffness and the shifting stability lobes. A new method cf measuring and calculating the dynamic behavior during milling by means of sub-space-state-space-identification methods is introduced. Finally, the computed stability lobe diagrams are compared with experimentally determined stability lobe diagrams.