Study On The Process And Mechanism Of Piezoelectric Self-adaptive Micro Electrical Discharge Machining

Micro-electrical discharge machining (micro-EDM) is a non-contact machining process; the macro cutting force is small, which eliminates the mechanical stresses between workpiece and electrode. And micro-EDM can machine any hardness conductive materials even nonconductive materials; it has great advantages and broad applications in micro machining fields for fabricating micro axis, micro holes and micro three-dimensional structures. Micro discharge energy and micro discharge gap cause the un-stable discharge state during micro-EDM process, which leads to the low machining efficiency and high electrode wear ratio. Such shortcomings restrict the application of micro-EDM in micro machining fields. Thus, new micro EDM techniques with high efficiency, high stabibility and low electrode wear are urgent need to be developed.On the basis of great lots of relevant literatures and data, micro-EDM techniques are reviewed to point out the latest research and future development tendency, a new piezoelectric self-adaptive micro-EDM(PSMEDM), based on inverse piezoelectric effect, was developed in this paper. The discharge gap regulation fitting and discharge energy fitting are integrated by piezoelectric actuator, realizing the self-regulation of discharge gap depending on the discharge state. The working principle and structures are simple. The system can improve the machining efficiency and reduce the electrode wear and provides a good method for fabricating high-aspect-ratio micro holes and allotypic holes, the technique has a broad prospect.The working principle, process and characteristics of PSMEDM are different from the conventional discharge machining. In order to comprehend the machining nature, the physical mechanism of PSMEDM was researched in this paper. Based on the single discharge crater, the diameter regression model of single discharge crater was set up, and the model of MRR was deduced. The mechanism of PSMEDM was further studied from the aspects of the breakdown of the dielectric, the spread of the discharging channel, the conversion and distribution of the discharging energy, the removal of the machining debris, the deionization as well as the integrated effect of the impulsive discharges. The results indicate that the removal of the machining debris during PSMEDM is concurrent results of the thermal expansion and magnetic fluid dynamic force. The PSMEDM can realize self-elimination of short circuits, which can improve the control efficiency and decrease the response requirements of servo control system. And the self-adaptive adjustable motion of the electrode during machining process is favor of removing of machining debris and deionization of working fluid, improving the discharge environment and stability of discharge state, and the discharge frequency is increased, the material removal rate is improved.Based on the analysis of special characteristics about PSMEDM, a set of PSMEDM system is designed and developed. The system includes mechanical part and electrical part. The mechanical part consists of granite basement, macro and micro servo system, precise rotary spindle, in-process fabrication and detection of micro electrode. The electrical part consists of micro generator, detecting circuit for discharge process and contact sensing circuit.The servo control system is an important part in micro-EDM system. An excellent servo control system stabilizes the machining process. A combined device with piezoelectric ceramic and DC servo motor has been developed as a servo system in this study. The PSMEDM system not only can realize EDM with long trip but also has high resolution and positioning accuracy. The piezoelectric actuator regulates the discharge gap depending on the discharge state, and insures the discharge gap retain within the best range.The fabrication and measurement in process of micro electrode is a bottleneck problem which restricts the development of micro-EDM technology at long time. After analyzing the merits and drawbacks of a variety of fabrication methods of micro-electrode, the block electrical discharge grinding with tangent feeding was adopted to fabricate micro-electrode in process. The measurement of electrode in process is completed through the combination of the contact perception function and numerical control function in this machine system.Higher MRR. lower EWR and better surface quality are the goals that micro-EDM pursues. Based on orthogonal experiment, S/N analysis method was adopted to investigate the effects of parameters such as open voltage, capacitance, resistance, rotate speed, and initial speed, upon the individual performance characteristic. Through the arrangement of such kind of experiment, a full knowledge of the implementations on each of the performance characteristics like machine time, electrode wear and surface roughness can be obtained. Further the parameters for single performance characteristic are optimized. It is found that different optimization objective results in a quite different combination of process parameters, and sometimes even lead to a quite poor performance for the other characteristic. In such cases, the theory of grey relational analysis was used in this study. S/N analysis method was adopted to dispose the experiment data, then the grey relational analysis was used to evaluate the performance with multiple characteristics; that is, the optimization of the parameter settings with multiple performance characteristics can be transformed into the maximization of the grey relational grades and the biggest grade for each parameter dictates the best arrangement of parameters corresponding to it. Thus, the optimization with multiple performance characteristics for PSMEDM system can be gained. And the validating experiment results dictate that this method can improve the machine efficiency and surface quality, decrease the electrode wear.Experimental researches are the best methods to evaluate the micro-EDM system. The performance of this PSMEDM system has been evaluated through micro EDM tests in this study. The performance of machining the micro-shafts, micro-holes and micro3D structures is validated. The experiments show its excellent machining performance and wide practicability of the machining system.