Tool electrode

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. 

Figure 5.18 Scanning electron microscopy image of a microcantilever, electromachined into a stainless steel sheet by ultrashort voltage pulses (100 ns, 2 V, 1 MHz repetition rate) in 3 M HCI + 6 M HF. The tool electrode was a tiny loop of a 10 pm thick Pt wire. (Reproduced with permission from Ref. [80].)...

We will note how the shadow is in a state of continual movement. The patterns are caused by eddy currents around the heater as the air warms and then rises. After just a quick glance, it s clear that the movement of the warmed air is essentially random. By extension, we see that, as an electroanalytical tool, electrode heating is not a good form of convection, because of this randomness. Conversely, a hydrodynamic electrode gives a more precisely controlled flow of solution. In consequence, the rate of mass transport is both reproducible and predictable. 

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

The process is conducted in the working chamber (electrochemical cell) of the machine, where a workpiece (WP) and a tool electrode (TE) are placed. The WP is connected to the positive pole of a power supply, and the TE serves as the cathode. The interelectrode distance is typically 0.02-0.8 mm. The electrolyte (usually an aqueous solution of an inorganic salt, 15% NaNC>3 or NaCl, for example) is... 

SACE makes use of electrochemical and physical phenomena to machine glass. The principle is explained in Fig. 1.1 [128]. The workpiece is dipped in an appropriate electrolytic solution (typically sodium hydroxide or potassium hydroxide). A constant DC voltage is applied between the machining tool or tool-electrode and the counter-electrode. The tool-electrode is dipped a few millimetres in the electrolytic solution and the counter-electrode is, in general, a large flat plate. The tool-electrode surface is always significantly smaller than the counter-electrode surface (by about a factor of 100). The tool-electrode is generally polarised as a cathode, but the opposite polarisation is also possible. 

Figure 1.1 Principle of SACE technology the glass sample to be machined is dipped in an electrolytic solution. A constant DC voltage is applied between the tool-electrode and the counter-electrode. Reprinted from [128] with permission from Elsevier.

However, things are not as simple as they seem. The gas film around the tool-electrode is not always stable. Microexplosions may occur destroying the machined structure locally. During drilling of holes, the local temperature can increase to such an extent, resulting in heat affected zones or even cracking. 

Kulkarni et al. [75] showed by various measurements that after each discharge, the temperature of the workpiece increases above the melting temperature and sometimes even above the vaporisation temperature of the machined material. They estimated that about 77-96% of the energy supplied to the process is used to heat the electrolyte and tool-electrode and only 2-6% is used for heating up the workpiece. However, it should be emphasised that the experiments by Kulkarni et al. were performed on metallic workpieces that have very different heat conductivities compared with materials that are machined traditionally using electrochemical discharges (e.g., glass or ceramics). 

The heat power PE will only partially be transferred to the machined substrate (i.e., P0 = ePE)- The fraction of the heat flux transmitted to the substrate will depend on, besides the geometry of the problem, the ratio of the coefficients of thermal conductivity of the electrolyte (typically around 0.6 W m 1 K1 for NaOH) and the substrate. Another important parameter is the heat conductivity of the tool-electrode. As the tool-electrode has the highest heat conductivity, most part of the heat is evacuated through it. 

Figure 5.5 shows the expected material removal rate as a function of the normalised heat power k for various tool-electrode radii b. In order to determine the relation between k and the machining voltage U, one has to compare these results with experimental data. 

Figure 5.6 (a) Experimental glass machining speed dz/dt for a 0.4 mm stainless steel tool-electrode as a function of the machining voltage U. (b) Normalised heat power k as a function of the machining voltage U [65],... 

Figure 5.7 Machining speed dz/dt as a function of the excess heat power Ae for alumina at various tool-electrode radii b as predicted by Equation (5.21).

Figure 5.8 shows the numerical solution for this equation (see [88] for details of the various boundary conditions). The electrochemical discharges are modelled as a uniformly distributed heat source at the bottom surface of the cylindrical tool-electrode and the gas film, with a heat flux ( > given by ... 

The diameters of the microholes were chosen according to experimentally known over-cut (see Section 6.2.6). For simplification, it was further assumed that the lateral space between the tool-electrode and the glass is filled with the gas film. 

From Fig. 5.8d one can observe that the temperature of the workpiece at the bottom of the tool-electrode is lower than that at the edges, near the gas film (the difference is more than 100°C). Figure 5.8a-c gives a clear picture of the machining zone. The depth of the machining zone is higher near the edge of the tool-electrode than under the electrode. 

As more OH radicals are present in the case of an active anode than for an active cathode, chemical etching is also more important. Consequently, the surfaces are smoother than those obtained by cathodic machining [63,120]. However, when using anodic polarisation, the tool-electrode will be anodically dissolved resulting in high tool wear. 

During SACE, the heat source produced by the electrochemical discharges has to be in close vicinity to the workpiece. Typically, a distance of a maximum of 25 pm from the workpiece is required in the case of glass [31]. To achieve this goal, several basic feeding mechanisms of the tool-electrode can be applied. 

---