| The problems of high efficiency and low accuracy have existed in WEDM-HS (High Speed Wire Electrical Discharge Machining) which is unique to China for a long period of time. These problems result in a low WEDM-HS processing index, and make it difficult to compete with foreign WEDM-LS (Low Speed Wire Electrical Discharge Machining).The multi-cutting technique has been proved to be an effective way of improving WEDM-HS processing accuracy and surface quality. As far as multi-cutting technique applying to WEDM-HS is concerned, in addtion to the machine itself precision, the moving system should guarantee the stability of wire electrode space position. To deal with the above mentioned problems, in this paper, the dynamics and space position stability of multi-cutting wire electrode in WEDM-HS are researched deeply. A fluid-solid coupling nonlinear vibration model is derived. Meanwhile, many factors influence on the dynamic characteristics of the wire electrode are analyzed by simulation and experiment. In addition, the wire electrode space position stability technique based on MR (Magneto Rheological) double wire winding drums and adjustable hydrojet are discussed for effectively solving the multi-cutting existing problems.The major researches of this paper are as follows:1. Through the analyses of multi-cutting processing characteristics in WEDM, the influence of multi-cutting wire electrode offset caused by unilateral discharge on the friction coefficient of kerf flow channel is researched. Based on these, a very narrow cutting kerf flow channel is transformed into a concentric annular flow channel by defining an average friction coefficient. At the same time, based on Einstein’s theory about two phase fluid viscosity, a new research is found that the working fluid average friction coefficient Cf decreases gradually with the debris volume concentration Cv increases. However, when the Cf, exceeds a threshold level, the Cf increases gradually. In the condition of normal machining parameters, the threshold level Cv=0.2when the debris density pe=8.7×103Kg·m3, the threshold level Cv=0.225when the debris density pe=11.34×103Kg·m-3. Based on these, a new concept is put forword that a certain concentration of debris is helpful to wire electrode stability. The velocity expressions of the shear flow UT and differential pressure flow U。 are derived in a laminar condition. These studies can be served as the theoretical bases to analyse thoroughly the "electrode wire, cutting kerf, working liquid and debris" complex coupling system and to predict rightly the dynamic response of wire electrode.2. Aimed at the bottleneck of the instability of wire electrode space position, in this paper, the multi-cutting wire electrode between two guides is simplified into a pinned-pinned (simply supported) elastic string. Based on these, the multi-cutting wire electrode spatial mechanics model is established by analyzing the force loading status of the small element of wire electrode located in the dielectric fluid. Based on the force balance equation, a multi-cutting wire electrode fluid-solid coupling nonlinear vibration model is established by adding additional axial force TG caused by wire extension deformation. Finally, the vibration model of pinned-pinned cylinder in axial flow which is established by Hamilton principle in M.P.Paidoussis’s paper in2005is employed to prove the rationality of vibration model which is established by Newton’s second law in this paper.3. The fourth order Runge-Kutta method is employed to simulate multi-cutting wire electrode dynamic behavior after nondimensionalization and discretization. Based on these, the factors such as the debris volume concentration Cv, the wire electrode axial tension To, the discharge force FE(t), the wire moving speed Vo and the multi-parameters, which influence on wire electrode displacement are further studied. Meanwhile, the factors like the wire electrode span L, wire electrode axial tension T0, high frequency pulse discharge and guides eccentricity, which influence on wire electrode vibration frequency are also further researched. By establishing a high speed photography experimental system and designing a matched image processing algorithm in HALCON soft, the electrode dynamic behavior are studied experimentally. The results show that the first, the second and the third nature frequency are820Hz,1489Hz and2425Hz respectively when the molybdenum wire diameter is0.20mm, span is100mm and Cr12MoV workpiece thickness is20mm. Meanwhile, the kerosene has obvious inhibition effect on wire electrode vibration. The wire offset caused by discharge is3.2μm. In addtion, the reverse inertia impact has also a great influence on the amplitude of wire electrode, the maximum amplitude is about±50μm.4. The wire electrode space position is especially sensitive to tension fluctuation, so the influences of guides and wire winding drum on the wire electrode tension are analyzed from the point of the wire moving system friction, and the influence of creep relaxation on the wire electrode tension is analyzed from the aspect of the wire strain energy. In additon, the influences of wire moving speed and the cutting time on wire strain energy are discussed as well. Moreover, Through the analyses of current wire moving system of multi-cutting HS-WEDM, we find that the reciprocating moving wire is the main cause of the fact that wire electrode tension fluctuates and workpiece generates inverting stripes. Combining with the characteristics of MRF, a mathematical model of wire electrode tension control system is established based on double wire winding drums. Through studing the influence of wire moving speed and exciting current on the wire electrode tension fluctuation, a new technique of wire electrode space position stability is put forward based on MR double wire winding drums.5. The surface mean heat flux model of multi-cutting wire electrode is established. The heat transfer situation of kerf flow channel is analyzed deeply. The influences of working liquid pressure of kerf entry on kerf flow field and wire temperature field are studied by FLUENT soft. The simulation results show that when inlet pressure of working fluid increases from0.15MPa to0.25MPa,The area of above crystallization temperature decreases from1.02×lO-3mm2to0.89×10-3mm2. At the same time, The affected area of temperature gradient decreases with the increase of inlet pressure as well. The result also shows there is an area where working fluid speed is zero in kerf. This area decreases with the increase of inlet pressure of working fluid. Based on these, a new approach to restraining wire electrode vibration, to reducing the disturbance from the hydraulic jump, to increasing kerf working fluid flow, to accelerating the debris removal, to decreasing wire electrode the temperature, and to ultimately stabilizing wire electrode space position is presented by adjustable hydrojet. |
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