| In micro-EDM process,the material is removed by instantaneous high temperature of discharges between electrodes.Due to the weak macroscopic force between electrode and workpiece,the micronization of electrode can be greatly improved.Meanwhile,the material physical properties such as hardness,strength and brittleness of materials is not limited in micro-EDM,which still shows good processing performance for many difficult-to-cut materials such as alloy materials,composite materials,semiconductor materials and so on.Owing to these technical advantages,micro-EDM technology plays an important role in military and civil manufacturing fields.However,some problems exsit in micro-EDM,such as low machining efficiency,large electrode loss and poor controllability of machining quality,and these problems have certain relationship with current rotary electrode machining mode.Therefore,after consulting a large number of literatures,a new method of micro-EDM with ultrasonic translational electrode was proposed in this study,and the driving device for ultrasonic translational electrode is developed.Further,the micro-EDM characteristics with ultrasonic translational electrode were deeply studied through simulations and experiments.The research work in this study can produce important theoretical and practical significance for improving the performance and expanding the application fields of micro-EDM.Firstly,based on the inverse piezoelectric effect of piezoelectric ceramics,a driving device for ultrasonic translational electrode was designed using dual-sandwich transducer.Aiming at the design requirements of frequency consistency and anti-interference of working modes,the mathematical model between design index and key dimension variables was established by central composite design and response surface method,and the structural dimension of driving device was optimized by genetic algorithm.The dynamic characteristics of the driving device were analyzed by finite element method.The results showed that the natural frequencies of symmetrical and antisymmetrical modes of designed driving device are very close,and they can effectively avoid the influence of interference modes.Moreover,the working frequency band of driving device is more than 500 Hz,and it can output satisfactory circular trajectory at electrode clamping position.The harmonic response frequency and amplitude characteristic of driving device were tested by impedance analyzer and laser displacement sensor,and the experimental platform of micro-EDM with ultrasonic translational electrode was built.A simulation model of gap flow field with debris particles was built for micro-hole machining with ultrasonic translational electrode.The flow field of working fluid and movement of debris particles in discharge gap were studied.The simulation results showed that the flow field in bottom and side gap are quite different compared with that of rotating electrode.Under the condition of ultrasonic translational electrode,the flow velocity of working fluid is much larger than that of rotating electrode,and the quantity and efficiency of debris particles that expelling from the working gap have been greatly improved.By the finite-successive pulse discharge experiments,the influences of ultrasonic translational electrodes on discharge stability were analyzed.It was found that compared with the rotating electrode,the number of spark discharge and the uniformity of discharge crater distribution using ultrasonic translational electrode respectively increased by 9.4% and 18.4% under the same optimized parameters.The results of microhole with large aspect ratio machining experiments showed that the backoff frequency of electrode is reduced and the machining time is shortened under the condition of ultrasonic translational electrode.The ultrasonic translational electrode also has an obvious effect on reducing chip adhesion and ablation.In addition,under the same experimental parameters,the micro-cavity milling time using ultrasonic translational electrode is shortened by more than 20% compared with that using rotary electrode.By the established moving heat source model,the temperature distribution near the discharge point on ultrasonic translational electrode surface was studied.The reason why the ultrasonic translational electrode can produce lower surface temperature,smaller depth of melting zone and heat affected zone was explained,and the influences of machining parameters on temperature distribution near discharge point were analyzed.The sliding phenomenon of discharge crater on cathode surface was analyzed by single pulse discharge experiment,finding that the discharge pit is roughly circular with rotating electrode,while the discharge crater is elongated in certain direction with ultrasonic translational electrode,and the average maximum depth of discharge pit is reduced by 24% compared with the former.Micro-hole array machining experiments showed that the axial wear length has a quadratic function relationship with micro-hole number,and the tapered zone length has an exponential function relationship with micro-hole number,which are similar with the wear characteristics of rotating electrode.The milling experiments results showed that the electrode wear rate with ultrasonic translational electrode is less affected by the diameter of electrode,which is consistent with the simulation results.From the perspective of gap discharge characteristics,the influencing mechanisms of ultrasonic translational electrode on surface roughness and residual stress were studied.Under the same discharge energy,it was found that the surface roughness obtained with ultrasonic translational electrode is lower,and this is mainly due to the uniform single pulse discharge energy and the larger bottom discharge gap.Meanwhile,due to the improvement of cooling effect and uniformity of discharge location,the residual stress of surface obtained with ultrasonic translational electrode is smaller than that obtained with rotational electrode.For single groove milling,the mathematical model between erosion depth coefficient with discharge energy,electrode diameter,scanning speed and milling length was established,and the influences of various factors on erosion depth coefficient were studyed by variance analysis.Results of arrayed micro-structure machining experiments showed that the entrance and exit of arrayed mciro-hole machined with ultrasonic translational electrode had better shape accuracy,and their diameter consistency was respectively improved by 22.0% and 2.8% compared with rotating electrode.In addtition,the ultrasonic vibration can reduce the gathering of debris and secondary discharges in viscous working fluid,which is beneficial to improve the size consistency of micro surface texture. |
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