In the manufacture of molds and dies, it is, sometimes, necessary to mill deep cavities with complex geometries. Therefore, tools with a large length/diameter ratio (L/D) are usually used in these processes, which stimulate tool vibration. The goals of this study are to develop a tool with an impact damping system and verify the effectiveness of this system applied to the cutting tool in high-speed milling operations of hardened steel. Aiming this goal, a ball nose end mill with L/D ratio of 7 was used, machining a curved, convex surface of hardened steel. Results of workpiece roughness, variation of cutting forces and tool wear when using the cutting tool with and without the damping system were compared. In addition, three diameters of steel spheres (used as elements to produce impact) were tested in the damping system. The results showed differences in the workpiece roughness and variation of the cutting forces when using the tool with and without the impact damper and the influence of the size of the spheres on the damping effect, mainly in the region of tool-workpiece contact with more tendency to generate vibration. The main conclusions were that the use the impact damping system used is really effective to decrease tool vibration, and the bigger the spheres used in the damper, the higher is their effects to damp the system Ver menos