Tool-electrode shape

Increasing the electrolyte supply to the machining zone can be achieved by using appropriate tool-electrode shapes or by promoting the flow of the electrolyte by tool-electrode motions. These strategies are discussed below. Note that until now no attempt has been made to study the potential benefit of using hollow electrodes, which would allow injecting the electrolyte from inside the tool. 

A wide range in hole si2es can be drilled. Diameters as small as 0.05 mm to ones as large as 20 mm have been reported (5). Drilling by ECM is not restricted to round holes. The shape of the workpiece is deterrnined by that of the tool electrode, thus a cathode drill having any cross section produces a corresponding shape on the workpiece. 

Prediction of Workpiece Shape and Tool-electrode Design. 823... 

Figure 7.2 Typical evolution at various voltages of SACE gravity-feed drilling using a needle shaped tool-electrode with a force of 0.8 N acting on it. The electrolyte level above the workpiece was about 1 mm. Reprinted from [131] with the permission of the Journal of Micromechanics and Microengineering.

Figure 7.3 SACE gravity-feed drilling with a flat sidewall shaped tool-electrode ...

Figure 7.4 Comparison of microholes drilled by gravity feed at 40 V using (a) a 0.2 mm cylindrical tool-electrode (b) a flat sidewall (0.1 mm thickness) shaped tool-electrode. In both cases a tool-electrode rotation of 500 rpm is used. Reprinted from [136] with the permission of the Journal of Micromechanics and Microengineering.

The combination of tool-electrode rotation, tool shape, and optimal pulse duty ratio results in this excellent result. 

Tool-electrodes can be fabricated in several ways. An excellent and flexible solution is wire electrical discharge grinding (WEDG), which is well suited for microfabrications [85]. An alternative way is to use anodic etching of tungsten. As shown by Lim et al. [82] very thin cylindrical electrodes (down to 50 pm) with controlled diameters can be fabricated. The important point is to use a high concentration of the electrolyte (typically 5 M KOH) and relatively high current densities around 10 mA/mm2. Thus, the diffusion layer around the electrode can be controlled in order to achieve various tool shapes by anodic... 

Figure 8.1 Discharge activity around (a) a cylindrical and (b) a needle shaped tool-electrode [55].

Different types of EMMs require microtools with different features such as shape and size. For through-mask EMM, the shape of the tool electrode is simple because a mask will mainly restrict the machining area and in turn control the final shape evolution, with a very low aspect ratio. However, for maskless EMM, the shape of the tool electrode will localize the machining area to generate the desired microfeature with a higher aspect ratio. Hence, the shape and size of the microtool are vital factors in the case of maskless EMM. Depending on the types of EMM as discussed in Chapter 4 and shown in Fig. 4.1, EMM tools can also be of various types. 

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