"Abstract Several methods exist for the analysis of the dynamic stability of milling processes. The methods based on the time domain analysis have been developed for systems with natural modes parallel to the coordinate system of the machine, that is, axes defined by the spindle (Z), the feed direction (X), and perpendicular to both (Y). For finishing operations, at which the main flexibility comes from the spindle / tool side, the approach is acceptable, but for roughing operations, or for any process in which the structural modes of the machine are involved, the method can not be applied for the analysis of the stability against chatter vibration. The paper shows a modification of the method of Semi-discretization based on a modal content approach. The new method allows to include in the simulation modes in any spatial direction, and further is not limited to a maximum number of three modes, as the previous developments. A comparison of the results with the methods of Semi-discretization and Multi-frequency Analysis is performed, showing excellent agreement."