1. Analysis of the effect of vibrations on the micro- EDM process at the workpiece surface 指導老師 : 戴子堯教授 學生 : 郭正宇 日期 :2011.10.12 1/13

2. outline Introduction Experiment Results Arc analysis Conclusions 2/13

3. Introduction Vibrations can be applied on the EDM tool electrode as well as on the workpiece. Both possibilities have been examined in the past and both are shown in the literature. As a result of vibration, the following effects are described. – increase in material removal rate – possibility to machine cavities with higher aspect ratios – improvement of process stability – reduction of arcing and short circuit pulses – impact of vibration frequency and amplitude In this work, the effects of vibrations on the micro-EDM process are examined for the start-up process. This is done by recording and analysing current and voltage of single discharge pulses and the electrode feeding using high-resolution recording equipment. Possible mechanisms of action are derived from the waveform analysis. This was done in the process of micro electro-discharge machining micro bores in steel. 3/13

4. Experiment (1/2) The experiments are carried out on a standard micro-EDM machine designed to manufacture micro bores. The test setup is shown in Fig. 1. Fig. 1. Experimental setup. A micro RC-type pulse generator SARIX Gen13 was used as the energy source. A tungsten carbide (6% Co binder) rod with a diameter of 0.085 mm served as the electrode tool, and the spindle rotation was set to 1000 rpm. The discharge energy was set to 25 μJ per pulse with a repetition rate of 130 kHz. For the dielectric circuit deionised water and open flushing were used. 4/13

5. Experiment (2/2) The microholes were eroded in a flat workpiece made of 1.5920(18CrNi8) steel with a thickness of 1.0 mm. The surface of the workpiece was finished by grinding with Ra = 0.5 μm. The workpiece was excited to vibrate by a piezoceramic actuator setup, which simultaneously served as the workpiece clampingunit. In this study, vibration frequencies of 1.0 kHz, 22 kHz and54 kHz were used. The amplitude for all frequencies was 3 μm peak to peak. The vibration direction was coaxial to the machining direction. To measure single discharges, a Tektronix current probe (typeTCP312) and a PMK 50:1 voltage probe Were used. For the electrode feed, the position signals of the z-axis were recorded.The measurement signals were logged using a SPECTRUM M2i 8bit transient recorder at a sampling rate of 2 × 100 MS/s (current and voltage) and 20 kS/s (electrode feed). Tektronix TCP312, AC/DC Current Probe Measurement Systems 5/13

6. Results(1/4) Fig. 2. Exemplary electrode feed for micro-EDM drilling process and process phases. 6/13

7. Results(2/4) Fig. 3. Electrode feed conventional and vibration assisted processes. 7/13

8. Results(3/4) Fig. 4. U/I signals for the conventional process (start-up process). 8/13

9. Results(4/4) Fig. 5 shows a typical arc discharge. The duration of the phenomenon is distinctive. With a duration of 250μs, During the arcing, the current remained between 0.1 and 0.75 A.The pulse-shaped current flow is distinctive and is caused by the RC-generator that tries to charge the discharge circuit with the preset pulse frequency and sustains the arc. Fig. 5. U/I regime during arc discharge. 9/13

10. Arc analysis(1/3) The following definitions were created to determine when an ‘arc discharge’ event occurred: U < 80 V, 0.1 2 μ s. A typical section of the signal and the arc determination in the 22 kHz process are shown in Fig. 6. Fig. 6. Arc analysis, 22 kHz process (current). 10/13

11. Arc analysis(2/3) A comparison of the four processes shows that vibrations reduce the total duration of arc discharges (Fig. 7, left). The differences between the total arc duration of conventional process(281 ms) and those with induced vibrations with frequencies of 1 kHz (188 ms) and 22 kHz (34 ms) are significant. In comparison,the further reduction of arc duration for the process employing a frequency of 54 kHz (31 ms) is small. Fig. 7. Total duration and number of arc discharges. 11/13

12. Arc analysis(3/3) average arc durations (t arc ) frequencies (f vib ) 12/13

13. Conclusions 1. By the machining of micro bores with micro-EDM, a machining delay can be observed on the workpiece surface (start-upprocess). 2. By the application of vibration onto the micro-EDM process, the machining time can be reduced significantly. The reduction or elimination of the start-up process. 3. Process time decreases with increased vibration frequency and constant amplitude. 4. Causes for the start-up process are arc discharges and the resulting retracting movements of the tool electrode, which leads to an open circuit state. 5. The total duration of the arcing events is reduced by the application of vibration and decreases with increasing frequency. 6. The number of arc discharges, however, increases with the frequency. 13/13