Technology Research On Micro-shaft Fabrication By Coplanar Twin-wire Tangential Feed Electrical Discharge Grinding

The growing demand for micro-parts or parts with micro-structures has promoted the development of micro-fabrication.As an important product and tool in the field of micro-fabrication,the accuracy of the micro-shaft directly affects the operability of the product,especially as a tool for micro-fabrication,such as the processing of micro-holes,micro-grooves,and micro three-dimensional structures on the surface.The accuracy of the structures or parts to be processed depends on the uniformity of the micro-shaft,including the uniformity of the diameter in the axial direction of a single micro-shaft and the repeatability of the diameter for repeatedly processed micro-shafts.At present,the manufacturing technology of micro-shafts is mainly based on micro-electrical discharge machining(Micro-EDM),because of its high processing accuracy,ease of machining difficult-to-cut metals,convenience of process control,almost no cutting force during the machining process,and relatively low-cost.In the field of Micro-EDM,wire electrical discharge grinding(WEDG)is one of the optimal methods of the micro-shafts fabrication with high accuracy.Mainly,the effect of wire electrode wear on the accuracy of micro-shaft diameter can be controlled through the machining process.In addition,the point discharge and small discharge energy between the wire electrode and the microshaft facilitate the control of the uniformity and repeatability of the diameter.In view of the lack of diameter control or the complexity of diameter control strategy in WEDG processing technology,the processing method of coplanar twin-wire tangential feed electrical discharge grinding(CTTF-WEDG)was proposed.The preparation of the micro-shafts was achieved by the narrow slit formed by the coplanar twin-wire electrodes in the same plane,and the way that the micro-shafts feed tangentially relative to the arc top of the guide along the symmetrical centerline of the slit.The narrow slit width changes nonlinearly with the tangential feed.In the CTTF-WEDG,the resolution of the material removal thickness in the radial direction of micro-shaft is improved by tangential feed,and the axial diameter of micro-shafts is constrained by the width of the slit and the axial feed,which achieve high uniformity of a single micro-shaft and repeatability of the diameter in repeated machining,respectively.Besides,the application of a two-channel RC pulse power supply provides the availability to improve the efficiency of the micro-shaft fabrication.The crucial elements of the micro-shaft diameter control in the CTTFWEDG were analyzed,including the width of the narrow slit,the tangential distance from the location of the narrow slit with minimum width,the discharge gap,and the radius of curvature of the wire electrode.The theoretical variation of the diameter and rate of micro-shaft with different conditions of the above factors were obtained,which provides a theoretical basis for the feasibility of the CTTFWEDG method.To realize the CTTF-WEDG method,high-performance microEDM equipment has been developed,including a high-precision three-axis linear motion platform based on a granite bed,a twin-wire electrode transport system,coplanar twin-wire device with narrow slit width adjustment based on piezoelectric ceramics,spindle with micro translational platform,auxiliary equipment,two-channel RC pulse power supply with mutual insulation,PMACand C#-based software of CNC system.Research on the basic technology for processing micro-shafts by CTTFWEDG has been carried out.The strategy for determining the initial machining position of the micro-shaft and the feed strategy for the micro-shafts was proposed,respectively.The control parameters for the operation of the wire electrodes were determined,based on the smallest fluctuation of the front edge of the wire electrodes in the processing region.The machining process of the micro-shaft was divided into three stages of roughing,semi-finishing,and finishing.With the goal of improving processing efficiency,the division method of the three processing stages was determined according to the axial diameter deviation of the micro-shaft in each stage.The processing parameters of each stage were preliminarily determined.The finishing process determines the diameter uniformity and repeatability of the micro-shafts,thereby the parameters of the finishing process were optimized based on orthogonal experiments.With the application of optimal parameters,the relationship was studied between the change of the micro-shaft diameter and the wire electrodes wear after the axial feed.In addition,a comparative study on the efficiency of processing micro-shafts was carried out,which proved that the CTTF-WEDG is more efficient than a single power supply or single wire electrode processing.To further improve the uniformity of a single micro-shaft diameter,the effects of errors in the CTTF-WEDG system were analyzed,including the motion accuracy and positioning accuracy of machine tool,the effects of fluctuation of wire electrodes front edges on micro-shaft diameter,and the effects of the noncoplanar of twin-wire.The corresponding control strategies were proposed,and experiments were conducted to verify the effectiveness of the strategies.The optimized feeding strategy of the micro-shaft avoids the taper due to wire electrodes wear,which improves the uniformity of the micro-shaft diameter in the axial direction.The uniformity was controlled within ±0.5?m in the range of800?m length.To fabricate micro-shafts with small diameters and large aspect ratios,the factors for the tapering of the machining process were analyzed,and micro-shafts with diameters less than 10 ?m and aspect ratios up to 47 were successfully fabricated by applying strategies.To achieve the high-repeatability control of the micro-shaft diameter in CTTF-WEDG,the factors affecting the repeatability of the diameter during the repeated preparation process were conducted to analyze,including the effects of the cylindricity error of the micro-shaft blank and the clamping error on the initial processing position determination,the effects of the asymmetry of the coplanar twin-wire,and the effects of the deviation of the processing position of the microshafts in the error sensitive direction(radial of micro-shafts).The focus was on the material removal thickness in the radial of the micro-shaft.Besides,the relationship between the wire electrode loss and the diameter of the micro-shaft after continuous tangential feed during the finishing process was also the focus of research.Based on the micro-shaft diameter control model,the brass and tungsten micro-shafts of (?)45?m were repeatedly prepared with high precision,and the repeatability of the micro-shafts was controlled within ±1?m.In summary,on the basis of the CTTF-WEDG,a series of accuracy control strategies for the uniformity and repeatability of the micro-shaft diameter were proposed,and the basic research on the relationship between the wire electrodes wear and the variation of the micro-shaft diameter was carried out.The fabrication of micro-shafts with high uniformity and repeatability was realized.CTTFWEDG technology enriches the method and accuracy control strategy of microshafts fabrication by micro-EDM and provides theoretical guidance and technical guarantee for expanding the application of micro-EDM technology in the field of micro-fabrication.