| Wire electrical discharge machining(WEDM)is a high efficiency and quality electromachining method which replaces molding electrode with wire electrode based on traditional electrical discharge machining(EDM).Because of small wire diameter(0.02-0.3mm),no cutting stress,with the capacity of machining complex surface and different top and bottom shaped workpiece,WEDM is widely used in many industries which has highly requirements of geometric accuracy,such as precision mould,instrument and aerospace,etc.However,in the practical machining,it can be found that:the kerf width can reach up to 420?m when wire diameter is 250?m,and the corner error ranges from 10?m to 200?m,and the tapper error can reach 20%.So obvious form-position error will seriously restrict the generalization and application of WEDM in high-end manufacturing industries.This paper aims to study and improve the geometric accuracy of workpiece in WEDM,and the main research contents are as follows:(1)Aiming at repeatedly discharge caused by inappropriate feed speed,the thermal model is developed to predict material removal rate(MRR)in the multi-pulse discharge process.Firstly,comprehensive considering several factors,two thermal models are built to forecast MRR in the single pulse and multi-pulse discharge process.And then,on the basis of MRR predicted by thermal model,the most suitable feedrate can be set in CNC system to reduce repeatedly discharge.Experiment data shows that:the most suitable feed rate can simultaneously acquire narrow kerf width and fast cutting speed under the certain machining condition.(2)Aiming at the complexity of machining parameters interactions,the method of parameters optimization is adopted for improving workpiece geometric precision.Firstly,27 groups experiment of corner error are implementd at different cutting angle.Additionally,three fitting models are developed to obtain the relationship between machining parameters and corner error by general regression,neural network forecasting and support vector regression.And then,the optimal parameters combinations are obtained according to fitting models.Experiment data indicates that the method of parameters optimization can decrease corner error by 50%when the feed speed is 5mm/min.Similarly,the method of parameters optimization can be also used for improving kerf width and tapper error.(3)Aiming at the form-position error resulted by irregular wire trajectory,two theoretical models of wire deflection and vibration are built,and several corresponding suppression methods are proposed to restrain wire motion.Firstly,when the thickness of workpiece is larger than 10mm,a theoretical model of wire deflection between guide wheels is developed through forces analysis and temperature distribution of wire electrode.Additionally,when the thickness of workpiece is smaller than 2mm,a three-dimensional coupling model of wire vibration is established based on three-dimensional temperature field,electromagnetic field and solid mechanics.And then,experiment data and the comparative result with previous models show that these theoretical models of wire trajectory have high precision,and they can be utilized for the track of wire compensation in CNC.(4)Aiming at wire deflection and vibration,a constant wire tension control system is designed to restrain workpiece form-position error.Firstly,the control requirement,principle and configuration of constant wire tension control system is illustrated.Secondly,the constant wire tension control system is set up by the ways of system identification and intelligent PID.And then,the wire tension control and form-position error experiments are carried out.Experiment results indicate that this control system has several good performances,such as:high control precision,quick response speed and strong anti-interference capability.In addition,the constant wire tension control system can significantly reduce corner error(by 15?35%),kerf width(by 9?15%)and taper error(by 6.4%). |
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