Schematic form of the applied voltage-current pulse (top), definition of the (frontal) gap size between the electrode and the workpiece, with indication of the different plasma/gas-bubble regions created inside the dielectric liquid (e.g., water droplet) (middle), and photograph of a typical crater (of area ) generated by the plasma on the workpiece (bottom).
Classification of μEDM electrical discharges, with different time evolutions of the electric voltage and current, as a function of material removal and typical morphology of the craters. The inset photographs (each having a width of ) illustrate the type of craters obtained in each case, for typical , d = 0–15 μm and .
The μEDM prototype machine. Top: drawing showing the basic structure (with part of the flushing system), the position control devices, and the force sensor; Bottom: (a) power system; (b)–(d) three types of electric circuits to generate different voltage-current pulses for discharge excitation: (b) R: analogue resistive pulse generator, (c) RC: analogue capacitive pulse generator, and (d) Rv: variable resistance pulse generator.
Schematic representation of the data acquisition system (top) and the method for automatic pattern recognition of electrical spark discharges (bottom).
Voltage (black curves and scales) and current (light blue), as a function of time, during the generation of the spark discharge, using (a) the R circuit and (b) the RC circuit. The labels Vd and Id signal the discharge voltage and current, respectively, during the discharge time td .
Typical time-oscillation response of the force sensor (gray curve and scale) to the voltage (black) and current (light blue) time evolutions with an R-circuit spark discharge. The point P signals the peak where the force measurement is taken.
Typical spectrum of the Hα line with μEDM plasma produced in water.
Schematic electric circuit of the μEDM system. V 0 is the open-circuit voltage imposed (yielding breakdown above a critical value Vb , for which a discharge current Id is established), R is the total resistance of the power circuit, and Zp is the plasma resistance responsible for a voltage drop Vd .
Measured values (points), as a function of the effective stress-time, of the maximum gap-size that produces streamers through Shell Macron oil at and , for a positive copper electrode and a negative stainless steel workpiece. The curve is a fit to the points, obtained using Eq. (10) .
Influence of the gap size and the electric-pulse duration on the type of electrical discharges (results obtained at and ). The U-line and the L-line limit the regions for discharges belonging to groups 1–3 (see Fig. 2 ), and were obtained as best fits to the operation limiting values (d, ) defined by the vertical arrows.
Measurements (points) and calculations using Eq. (10) (curves), as a function of the breakdown voltage, of the maximum gap-size that ensures dielectric breakdown (for a positive copper electrode and a negative stainless steel workpiece) at and the following values of (in μs): 3 (squares and solid curve), 500 (circles and dashed curve).
Influence of the gap size and the breakdown voltage on the type of electrical discharges at for (a) and (b) . The U- and L-lines are as in Fig. 10 .
Measured (symbols) and calculated (curves) electron density, as a function of (a) the discharge time at and , (b) the discharge current at and , and (c) the gap size at and . Measurements are made for streamers produced in a water droplet with aluminium electrodes (negative electrode and positive workpiece). Calculations use assuming: , the area of the water droplet within which plasma is created (solid curves); , the plasma area (dashed curves); , the area of the gas-phase bubble created inside the water droplet [dotted curve in (a)].
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