Tool Path Optimization Method For Efficient Machining Of High Temperature Alloy Impeller

Impellers in turbo engines are usually made of high-temperature alloys,such as Nickel-based material,and have narrow tunnels.These factors have brought several problems for the machining process of these impellers.First,the yield strength of these kinds of materials is huge and the cutting force coefficients are usually large.Second,cutters with large length-diameter ratio are usually used because of the narrow tunnels and long blades,and these cutters are usually flexible.In current machining processes,these two factors make the improvement of efficiency difficult because the cutter is tend to chatter in those conditions.To protect the cutter and improve the surface quality,low radial depth and axial depth have to be used.To avoid chatter without losing efficiency,methods for finding a global spindle speed are usually used.However,this method can hardly be used in 5-axis milling of high-temperature alloys.First,in the milling of these materials,the linear velocity of cutting is usually low,which means the spindle speed is relatively low.In the scope of low speed in lobe diagram,possible axial depth and absolute axial depth are similar and the scope for feasible axial depth is small.Therefore,it is hard to find the stable working point beyond the absolute axial depth by just changing the spindle speed.Second,the change of engagement between tool and workpiece along the tool path will lead to the change of lobe diagram,which make it harder to find a global stable spindle speed.For ball-end milling,this paper improved the stability by changing the tool orientation along the toolpath.According to the experiments in current literature,the changing of tool orientation can increase the absolute axial depth by 2 to 3 times,so it can avoid chatter without reducing the cutting efficiency.However,methods for stability judgement in current literature do not consider the change of FRF coordinate and engagement coordinate caused by the change of tool orientation.For tunnel rough milling,this thesis uses the plunge milling to make the main cutting force be along the tool axis which has higher stiffness.Methods in this thesis solved several drawbacks of methods in current literature for the plunge milling of high-temperature alloys.First,the radial depth is not well controlled in current plunge milling methods.According to current researches and experiments,the radial depth in plunge milling has a great influence on cutting force and stability.In the plunge milling of high temperature alloy,large radial depth can destroy the cutter and the holder.Second,the retracting process in plunge milling needs to be improved.In every circle of plunge milling,because of the radial cutting force,there exist frictions between the cutter and the side wall during the retracting process.Though the retracting process is not designed to remove the material,low feedrate has to be used.This thesis solves the above problems for the machining of impellers made from high-temperature alloy through toolpath optimization.The main contributions are as follows:(1)A method is proposed to eliminate the chatter in ball end milling by the change of tool orientation.This paper uses the kinematic chain and tool path to establish the characteristic equation in frequency domain for each different tool orientation on different cutter locations.For each cuter location,several feasible tool orientations are found based on stability and collision check and one is selected according to the smoothness of tool orientation.Experiments show that this method can improve the stability of ball-end milling process.(2)This paper proposed a plunge milling toolpath generation method to control the radial depth and decrease the number of plunge milling cutter locations.Radial depth has great influences on plunge milling cutting force and stability and the machining time is most influenced by the number of cutter locations in plunge milling.In current plunge milling tool path generation methods,it is hard to calculate and control the radial depth and the number of cutter locations at the same time.A new plunge milling tool path generation method based on medial axis transform is proposed to improve both of them.Experiments show that both the cutting efficiency and the cutter life are improved.(3)A method is developed for the control of radial depth at the bottom and optimization of retraction process for 5-axis plunge milling.In every circle of plunge milling,the increase of radial depth at the end of plunging phase and the friction during the rising process leads to the quick wear of cutters.The radial depth should be decreased and the rising process should be improved.In five-axis plunge milling,the complexity of the intersection part between the cutter and the stock make the optimization difficult.This paper use simulations to get the intersection parts and discretize them into pieces.Then the geometry information is calculated.Based on that,new cutter locations and moving directions are calculated to optimize the toolpath.Experiments show that this method can improve the tool path generated by commercial software Cimatron on cutting efficiency and cutter life.(4)Methods are applied on an impeller made from nickel-base alloy.In current machining processes,it is hard to improve the cutting efficiency because of the material and flexible cutters.This paper uses the above tool path optimization method to improve the processes.For tunnel rough milling,plunge milling toolpath with radial depth control is generated and optimized.In 5-axis plunge milling,the tool path generated by commercial software MAX is optimized by decreasing the radial depth and adding moving phases.In milling of leading edge,the tool orientation is optimized to improve the stability.Cutting experiments show that these methods improve both the cutting efficiency and the cutter life for the machining of high temperature alloy impeller.