Research On Variable Pitch End Mill In Milling Titanium Alloy

Titanium alloys are extensively used in aerospace industry due to their excellent performance in aerospace environment. However, it is difficult to machine for their poor thermal conductivity, low elastic modulus and high chemical activation. Vibration and high cutting temperature during machining leads to fast wear of tools and gives poor surface quality of the processed parts at high cutting speed. Simultaneously, most materials need to be removed from roughcast due to the characteristics of component design, so machining cost becomes very high when these parts are processed at low cutting speed. These are some of the constraints in the development of aerospace industry. In order to overcome these constraints, vibration characteristics in machining of titanium alloys Ti-6A1-4V are considered in this study. Vibration minimizing mechanism and structure parameters optimization of variable pitch mill which may significantly support in structural optimization of mills are presented.A three dimensional force model for dynamic milling which includes mill eccentricity and deflection is proposed and simulated. Least square method is used for obtaining milling force coefficients by fitting the cutting force data of experiments. Furthermore, the effect of pitch angles, helix angles, the axis depth and the radial depth on milling force is also analyzed; then, the effect of eccentricity on centrifugal force is analyzed. Results show larger helix angle, larger difference value of adjacent pitch angles, small radial depth of cut and small feed are preferred to produce smaller cutting force Fy.Dynamic models of displacement and force for variable pitch and variable cutting depth are considered. The effect of different parameters on the limit cutting depth is also analyzed and stability lobe diagram is obtained from MATLAB software. Results indicate that large difference value of adjacent pitch angles, large stiffness, large damping, small radial depth and small feed are significant to produce higher depth of cut. Milling force in time, frequency and time-frequency domain is also analyzed. Experiment is carried out at dry, downing milling at variable milling speeds from 80m/min to 360m/min. Quality of titanium parts is compared at stable and unstable cutting and milling speed is optimized by the experimental analysis. The experiment results show that when the chatter occurrence, milling forces were found to increase dramatically by 61.9%-66.8% compared with that of at stable cutting, machining surface quality became poor and machined surface roughness increase by 34.2% -40.5% compared with that of at stable cutting.Vibration minimizing mechanism of variable pitch mills is analyzed in frequency domain.25 different types of mills are designed with orthogonal design method. The effects of the core diameter, helix angle, rake angle, clearance angle and pitch angle on the stress and strain are analyzed in FEM simulation software ABAQUS. Pitch angles of variable pitch mills are optimized based on frequency domain in MATLAB. Optimized pitch angles of 83°-97°-83°-97°and 87°-93°-87°-93°are obtained for mills containing 4 cutting edges. Furthermore, the core diameter between 11.4mm and 13.4mm and the rake angle between 7°and 11°for 20mm'diameter mill are found.According to the above analysis 10 types of mills with variable pitch angle, variable helix angle and variable groove depth structure are also designed for high-speed milling experiment. The effects of structural parameters of mills and milling speed on milling force and vibration displacement of mills are compared after time domain, frequency domain and time-frequency domain analysis. At last, the effective laws of structural parameters on cutting performance and the optimal milling speed are also presented. Results show that variable pitch have best vibration minimizing effects, and the vibration displacements is smaller when the difference value of adjacent pitch angles is larger; the resonance was produced because the excitation frequency of cutting force close to the machine natural frequency when milling speed are 120m/min and 150m/min, thereby cutting vibration was increased.