The paper describes the dynamical real-time simulation of the Z axis of a five axis machining center for high speed milling. The axis consists of a mechanical structure: machine head and electro-mandrel, a CNC Control System provided with feed drives and a Pneumatic System to compensate the weight of the entire vertical machine head. These three sub-systems have been studied and modeled by means of: - FEM modeling of the mechanical structure; - an equation set to represent the main functions of the CNC; - an equation set to represent the functioning of the Pneumatic System. These different modeling sub-systems have been integrated to obtain the entire actual dynamical behavior of the Z axis. A particular analysis was developed to represent the friction phenomena by a specific analytical model. Experimental activity was developed to test and validate the different modeling sub-systems, and other experimental tests were performed on the machining center to compare simulation outputs with experimental responses.