Experimental setup and diagnostics The first section of this chapter presents the EDM Filter device used in this work, along with the machining parameters. The various plasma diagnostics are then described in the following sections. Finally, in the last section we address the specific difficulties related to the experimental study of EDM plasmas.
3.1 Electrical discharge machining device Figure 3.1 presents different views of the machining equipment at the CRPP. We use a small and versatile die-sinking EDM machine, equipped with a generator of the Roboform type from Charmilles Technologies. The electrode is cylindrical, with a diameter of 3 or 5 mm. In order to better control the localization of the sparks, its tip is conical. The servo-controlled movement of the electrode is only vertical. The workpiece used in our experiments is generally a flat cylinder, 5 cm in diameter. In order to flush the particles contaminating the electrode gap during machining, the dielectric can be pumped, and re-injected into the gap with a shower. No dielectric cleaning is performed during this closed circuit circulation.
The dielectric, the electrode and the workpiece can easily be changed. We use:
² deionized water (typical conductivity 1.5 ¹S/cm), mineral oil (FluX Elf 2, oil for EDM, viscosity 6) or liquid nitrogen as dielectric. With liquid nitrogen, the workpiece is placed in a dewar to avoid boiling as much as possible;
- electrodes in copper, tungsten, graphite and zinc;
- workpieces in W300 steel (AISI Type H11).
The machining process is completely controlled by the generator. It supplies the discharge voltage and current, regulates them, and controls the servomotor for the electrode displacement. The generator uses principally the measurement of the gap voltage to regulate and control the process. EDM Filter
Figure 3.2 shows the discharge parameters which can be set with the generator: ignition voltage V , discharge current I, discharge on-time, off-time (pause between the end of a discharge and the voltage rise for the next one), electrode polarity. It is impossible to control the pre-breakdown duration, i.e. the time lag between the voltage application and the breakdown, because it depends on the electrode gap and on physico-chemical properties of the dielectric. Note that the pre-breakdown duration is also called “ignition delay time” or “breakdown time lag”. The value of the voltage during the discharge can not be set by the generator either. Its value depends on electrode materials, but is typically around 20¡25 V. The values that we can choose with our generator for
V , I, on-time and off-time are given in table 3.1.
The machining mode schematically presented in figure 3.2 is called Isopulse, because every discharge has the same on-time, independently of the pre-breakdown duration. This mode is the standard machining mode used in this work. By adding a capacitor in parallel to the gap, it is possible to use the generator in a capacitive mode, generally used for surface finishing [106]. The sparks are produced by successive discharges of the capacitor. In this mode, the polarity is always chosen negative. The discharge on-time and current are not controlled, and thus can slightly vary from a discharge to the other, in contrary to the Isopulse mode. Typically, with a 10 nF capacitor, the discharge duration is around 1.5 ¹s and the current around 6 A.EDM Filter
