Tool Path Planning In High Feed Speed Surface NC Machining With Kinematical Constraints

The Numerical Control(NC)machining of curved surface with high speed and high efficiency is the unremitting pursuit of mechanical manufacture.Due to the complexity of sculptured surface profile and strenuous exercise of machine tool axis,vibration and shock will occur with the feed rate increasing,which can affect the machining quality directly,or even damage the internal structures of machine tools.Aiming at the problem that the kinematic characteristics of machine tool axis exceed the limit of the machine tool in surface NC machining with high feed rate,a machining path planning method with kinematical constraints is presented in this thesis.The main idea is to study on the tool path planning in high feed speed surface machining based on the way of surface machining and the relationship between machining path and the kinematic characteristics of machine tool axis,considering the kinematical constraints of each axis,by which it can be realized ultimately to make the movement of each axis more stably in the same processing efficiency,to avoid the vibration and shock and to improve the quality of surface.The main work is as follows.(1)Analyses of machine tools' kinematic characteristics.Based on the kinematic transformation of five axis NC machine tool,the mathematical relationship between the cutter location data and the motional coordinate of each axis is derived.The calculation method of the kinematic characteristics parameters and the kinematical constraint conditions for tool path planning is presented by analyzing the kinematic characteristics of the five axis NC machining process deeply,and kinematical constraint values of each axis are estimated.(2)Acquisition of cutter location points with kinematical constraints.Aiming at surface similar to revolution body,based on the cutter locations and the cutter contact points generated by the method under helix driving mode,the B spline curve is used to fit the initial contact path generated.Considering the kinematic characteristics of each axis and machining error,the final cutter contact path is obtained.The method of obtaining final normal vector by initial normal vector interpolation at the initial contact point is proposed according to the analysis of the correspondence between cutter contact points and nodes on the B spline curve,and then the final cutter location points are determined newly.(3)Cutter orientation planning with kinematical constraints.Firstly considering the velocity constraint,a method of cutter orientation planning meeting velocity constraint is put forward by analyzing the relationship between the cutter orientation and the speed of machine tool axis,and the feed speed of the machine is adjusted.Secondly,analyzing the acceleration characteristic of motion axis,the mathematical optimization model is established to obtain the final cutter orientation on the basis of the planning considering velocity characteristic and the feed speed is adjusted.Then,the feed rate of individual cutter location points where the jerk exceeds the limit is reduced to ensure that the jerk also satisfy the kinematical constraint conditions.Finally,a calculation example is given to verify the effectiveness of the method for cutter orientation planning.(4)Experiments of shoe last machining by five axis machine tool and acceleration measurement.Taking the five axis NC machining of shoe last surface as an example,after generating NC program by post processing,the finish machining experiment is carried out on the five axis NC machine tool and the experiment of measuring acceleration is conducted at the same time.After that,Experimental results are analyzed to verify the correctness of the theoretical calculation for kinematic characteristics and the validity of the tool path planning method.