Study On The Feedrate Planning Of Jump Motion In Multi-axis EDM For Shrouded Blisks

Shrouded blisks are crucial components of aeroengines,rocket engines and gas turbines.The advanced integral shrouded structure of the blisks could benefit the performance and reliability of turbomachinery,however,it brings great challenges to the manufacture of the blisks.With the unique advantages in machining high precision components which are made of difficult-to-cut materials,multi-axis EDM is regarded as the most commonly used manufacturing method for shrouded blisks.As part of NC system,motion control is responsible for controlling the tool and workpiece of the machine tool to move in strict accordance with the given tool path,which is very important for a machine tool to realize high precision and high efficiency machining.Thus,it has been a hot issues in the field of numerical control technology for a long time that how to give a compromise solution with less computational complexity and less storage space consumption on the premise of satisfying the given accuracy.In addition,the servo feed/back function and jump motion of the multi-axis EDM machine require frequent and accurate forward and reverse interpolation of the tool path.However,most of the existing interpolation methods are designed for cutting processing,which cannot meet the functional requirements of multi-axis EDM.Machining accuracy and efficiency is the criterion for evaluating the performance of machine tools and processing technologies.The aim of the dissertation is to address several key problems in improving the machining accuracy and efficiency of muti-axis EDM.The main achievements of this dissertation are summarized as follows:First,the basic requirements of high-speed jump motion in the process of EDM are analyzed in detail,an S-shaped acceleration and deceleration(ACC/DEC)method as well as a multi-axis EDM NC system based on the EtherCAT protocol applicable to high-speed jump motion are proposed.The S-shape ACC/DEC method is divided into three functional modules for analysis.On this basis,the S-shape acceleration and deceleration method for Z-axis high-speed jump motion with continuous jerk is proposed.The results show that accelerate to the maximum velocity with the same maximum jerk,the acceleration planning method of sine curve can reduce the maximum acceleration by about 25%.Second,in view of the fact that frequent acceleration and deceleration will greatly reduce the machining efficiency in the process of jump motion which must go through a number of short line segments,a real-time look-ahead transition algorithm and a motion planning scheme for jump motions with short line segments are proposed.a parametric spline curve is adopted to blend adjacent short line segments.The discharge gap is used as the approximation error tolerance in the design of a transition curve.With the help of a bidirectional scanning and a jerk-limited S-shaped feedrate scheduling,the feedrate of a jump motion can be obtained.Simulation and experimental results demonstrated that the PSCTA can bring over a smoother velocity profile as well as a higher machining efficiency.Machining tests for a workpiece with complex curves shows that the PSCTA can reduce the machining time by12.16%as compared with the traditional method.Third,to solve the problem of low machining efficiency caused by the jump feedrate fluctuation in multi-axis EDM for shrouded blisks,a multi-axis corner rounding solution(MCRS)is proposed to smooth multi-axis jump paths.Two three-dimensional subspaces,a linear subspace and a rotary subspace,are built to describe six-axis jump paths for a six-axis EDM machine tool.A three-axis jump path smoothing solution can be applied for the subpath in each subspace.By using a bidirectional scanning method as well as an S-shaped profile,the smoothness of the multi-axis feedrate schedule is obtained.A transformation model is built based on the CAD models of both a shrouded blisk and an electrode as well as the kinematics of a six-axis EDM machine tool.Simulation results show that the MCRS achieves a smoother scheduled feedrate profile as well as a higher efficiency.Experimental results show that the machining time for shrouded blisks can be decreased by 19.68%with the use of the MCRS.Finally,in order to reduce the real-time calculation of the feedrate planning module and improve the overall response speed and reliability of the CNC system,the dual-NURBS parallel curve in machine tool coordinate system(MCS)and generalized arc length unit increment(GUALI)interpolation methodare proposed.On the basis of the"atomic level"approximation point obtaining from the parametric curve with GUALI,a method of generating the velocity limit curve(VLC)of the double NURBS parallel curve based on the unit arc length increment scanning(UALIS)method is proposed.Then,the implementation of feedrate planning method based on the digitizer/player architecture is proposed.This method adopts dual-NURBS parallel parametric curves trajectories with C~2 continuous,which will reduce speed loss in servo feed movement comparing with the short line interpolation.Also,in the process of the jump motion,smoothing jump path can offer larger"turn speed”,so as to improve the machining efficiency.