J. Micromech. Microeng. 18(2008)105009 CR Alla Chaitanya and K Takahata Oscilloscope Current probe mete ektronix CT-2 20kQ Fluid 50ns EDM fluid Copper electrode Ultrasonic (b)Perforation Figure 6. A set-up used for the characterization of electrode actuation and uEDM tests Electrode Figure 8.(a) Measured pulses of disch Decreasing velocity 90 V with an inset of single pulse clos of spark light captured through the el g16 contact of the electrode to the substrate. The resist at the anchors were measured to show no detectable in thickness after immersing the devices in the EDM 2-3 days, suggesting that the swelling effect due 12 absorption of oil from their sidewalls is negligible With the application of machining voltage and the injection of the fluid to the electrodes, sequential pulses of micro spark discharge were successfully generated and Fluid flow velocity(m/s) sustained at flow velocities of 3.9 m- and 3. 4 ms-I for the Figure 7. Built-in capacitance versus fluid flow velocity measured fixed-fixed and cantilever electrodes, respectively(figure 8) with a fabricated device with the design shown in figure 2(a). The typical peak current and pulse duration were measured to be 2-3 A and 50 ns, respectively, in the set-up used. Figure 9(a)shows the stainless-steel workpiece machined to form the RC circuit with built-in capacitance, as shown figure 6. The electrical discharge pulses were monitored using the cantilever device(figure 2(b)at 90V for about 15 min. The pattern of the cylindrical structures in the sing an ac current probe, which has a minimal loading on machined area corresponds to that of the holes of the the discharge circuit. A variable-speed motor pump was used electrodes. The machined structures were characterized pressure to the electrodes for their actuation. The ultrasonic using a WykoTM NT1100 optical profiler(figure 9(b).The measurement indicates a removal depth of 20 um and an rave was applied to the fluid bath during the process to assist average surface roughness of 520 nm in the machined areas in the dispersion of the byproducts. As described earlier with Figure 9(c) shows an optical image at one of the holes in equation (D), the discharge energy Epsc, or machining quality the electrode. The image was taken after machining but depends on the built-in capacitance of the device, which is a before removing the electrode structure from the workpiece dynamic parameter as it is partially determined by the movable indicating a discharge gap of about 10 um between the electrode(in addition to the fixed anchors ). The behavior of machined cylindrical structure and the perimeter of the the capacitance was characterized using an HP 4275A LCR electrode hole meter with varying How rate of the EDM fluid, as shown in Figure 7 shows the built-in capacitance of the fixed-fixec 6. Analysis and electrode structure versus the flow rate in both increasing and decreasing directions, measured while applying no voltage to consistency with theoretical estimations, The pressure measured in air, the increased static capacitance was expected onto the electrode immersed in a fluid by a flow of the due to operation in a liquid ambient. The plot in figure 7 that is injected from a circular nozzle perpendicular to the indicates a nonlinear rise of the capacitance, i.e., decrease of electrode plane can be represented by [14 the capacitive gap with the flow velocity as well as a highly elastic behavior of the electrode structure during the actuation. The capacitance was observed to become immeasurable at a where H is the normal distance between the exit of the nozzle flow velocity of 5. ms and greater, indicating the physical and the electrode, d is the diameter of the nozzle, p is theJ. Micromech. Microeng. 18 (2008) 105009 C R Alla Chaitanya and K Takahata Figure 6. A set-up used for the characterization of electrode actuation and μEDM tests. Figure 7. Built-in capacitance versus fluid flow velocity measured with a fabricated device with the design shown in figure 2(a). to form the RC circuit with built-in capacitance, as shown in figure 6. The electrical discharge pulses were monitored using an ac current probe, which has a minimal loading on the discharge circuit. A variable-speed motor pump was used to inject the EDM oil at a controlled rate to apply fluidic pressure to the electrodes for their actuation. The ultrasonic wave was applied to the fluid bath during the process to assist in the dispersion of the byproducts. As described earlier with equation (1), the discharge energy EDSC, or machining quality depends on the built-in capacitance of the device, which is a dynamic parameter as it is partially determined by the movable electrode (in addition to the fixed anchors). The behavior of the capacitance was characterized using an HP 4275A LCR meter with varying flow rate of the EDM fluid, as shown in figure 6. Figure 7 shows the built-in capacitance of the fixed–fixed electrode structure versus the flow rate, in both increasing and decreasing directions, measured while applying no voltage to the electrode. Compared to the 7.2 pF built-in capacitance measured in air, the increased static capacitance was expected due to operation in a liquid ambient. The plot in figure 7 indicates a nonlinear rise of the capacitance, i.e., decrease of the capacitive gap with the flow velocity as well as a highly elastic behavior of the electrode structure during the actuation. The capacitance was observed to become immeasurable at a flow velocity of ∼5.4 m s−1 and greater, indicating the physical /A (a) (b) Figure 8. (a) Measured pulses of discharge current at a voltage of 90 V with an inset of single pulse close-up, and (b) an optical image of spark light captured through the electrode’s holes. contact of the electrode to the substrate. The resist spacers at the anchors were measured to show no detectable change in thickness after immersing the devices in the EDM oil for 2–3 days, suggesting that the swelling effect due to the absorption of oil from their sidewalls is negligible. With the application of machining voltage and the injection of the fluid to the electrodes, sequential pulses of micro spark discharge were successfully generated and sustained at flow velocities of 3.9 m s−1 and 3.4 m s−1 for the fixed–fixed and cantilever electrodes, respectively (figure 8). The typical peak current and pulse duration were measured to be 2–3 A and 50 ns, respectively, in the set-up used. Figure 9(a) shows the stainless-steel workpiece machined using the cantilever device (figure 2(b)) at 90 V for about 15 min. The pattern of the cylindrical structures in the machined area corresponds to that of the holes of the electrodes. The machined structures were characterized using a WykoTM NT1100 optical profiler (figure 9(b)). The measurement indicates a removal depth of ∼20 μm and an average surface roughness of 520 nm in the machined areas. Figure 9(c) shows an optical image at one of the holes in the electrode. The image was taken after machining but before removing the electrode structure from the workpiece, indicating a discharge gap of about 10 μm between the machined cylindrical structure and the perimeter of the electrode hole. 6. Analysis and discussion It is worth evaluating the measurement results and their consistency with theoretical estimations. The pressure applied onto the electrode immersed in a fluid by a flow of the fluid that is injected from a circular nozzle perpendicular to the electrode plane can be represented by [14] p = 50 (H/d)2 ρv2 2 (4) where H is the normal distance between the exit of the nozzle and the electrode, d is the diameter of the nozzle, ρ is the 5