Variable pitch end mills are widely used in the high-speed milling process due to having better vibration reduction properties. However, because of the unequal pitch angles and the asymmetrical structure of end mills, there are principle error, poor dynamic balance accuracy, and serious tool vibration problems in the milling process. In order to improve the dynamic balance accuracy of variable pitch end mills, the structure of the end mill is designed and optimized based on the minimum eccentricity criterion in this paper. Firstly, by analyzing the dynamic balance of variable pitch end mills, the relationship between it and eccentricity is defined, and the end section model and the centroid equation are developed through the structure design of end mills. Then, the optimization method of variable pitch end mills is analyzed, the eccentricity decreases from e₀= 150um to e'₀ = 3um based on this method. the structure of the end mill is optimized to meet the design criteria, which improves the balance accuracy level of end mills. Finally, through the modal analysis for the optimized structure of the variable pitch end mill, the maximum relative displacement in the mode is reduced by about 17%, it is verified that the optimized end mills have better dynamic performances, which is of great significance for alleviating the cutting vibration and improving the cutting stability.

The development of the machining technology is restricted by the problems of serious vibration and low system stability. The variable-pitch mill has certain vibration reduction property due to the unequal pitch angles. In this work, firstly, according to the features of the structure of the variable-pitch end mill, the cutting mechanism is analyzed, and a theoretical model of the dynamic cutting force is developed. Secondly, the cutting force coefficients of the model are determined based on the test of hardened steel for verifying the significance of the theoretical model. Then, the time-frequency characteristics and vibration reduction mechanism of variable-pitch mills are analyzed. The frequency characteristics of different types of pitch angle mills are also explored by the spectrum analysis. Finally, based on the energy and variance methods of the amplitude, the multi-objective optimization of the pitch angle is carried out, where the spectrum lines are found evenly distributed, and the optimal result is 80°-97°-100°-83°. For satisfying the machining requirements, the pitch angle of the variable-pitch end mill is reasonably selected to reduce the amplitude of the forced vibration, which can play an important role in reducing the vibration of the tool and improving its cutting stability.