Voltage, machining

R, = lecommended minimum insulation resislanee in Mil of the entire machine windings, at 40°C kV = rated machine voltage in kV. 

In order to estimate P0, one has to first estimate the heat power PE of the electrochemical discharges. Therefore, Equation (5.1) can be used. The interelectrode resistance R can be evaluated by inspecting the slope of the mean I - U characteristics in the ohmic region (the linear part from 5-15 V of the mean I- Ucharacteristics). Typical values for PE are around a few watts (for machining voltages in the range of 30-40 V). 

For practical applications, one should be able to estimate the normalised heat power k as a function of the machining voltage. A qualitative estimation can be obtained if it is assumed that each discharge transfers a similar heat quantity qE to the workpiece. The heat power P0 can be related to the mean number of discharges using Equation (4.38) ... 

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 5.8 Temperature distribution at high-depth machining for three different machining voltages according to the model of Mishra et al. [88].

Typical examples for glass drilling as a function of various machining voltages are shown in Fig. 6.2. Drilling was done with a 0.4 mm cylindrical stainless steel cathode in 30 wt% NaOH [131]. After a first phase, where the drilling speed is fast, a progressive slow down of the material removal rate is observed... 

For very high depths (more than a few millimetres), the limiting speed vUm vanishes and the model (6.2) is no longer valid. Drilling reaches a limiting depth which becomes a function of the machining voltage [21]. 

The fluctuation in the drilling time increases with the machining voltage. Due to these large fluctuations, the drilling time cannot be used as a control parameter for the drilling depth. 

The existence of the two machining regimes results in different drilling qualities as a function of the machining voltage and depth. In the case of a cathode tool, the holes drilled in glass can be classified into four different types (Fig. 6.9) [84] ... 

This type of contour is characteristic for low depths (100 pm, 28-37 V) and low machining voltages (28 V, up to 300 Jim). The entrance of the hole is well defined and characterised by a smooth surface. 

Jagged outline contours (Fig. 6.9(b)) This type of hole appears at depths between 200 and 300 pm using a machining voltage of about 30 V. The contour is no longer smooth but jagged. 

Hole with heat affected zone (Fig. 6.9(c)) For machining voltages above 30 V and depths higher than 100 pm, the hole is surrounded by a heat affected zone. The contour remains cylindrical. 

Figure 6.10 Evolution of SACE glass gravity-feed drilling in the machining voltage-drilling depth plane. Reprinted from [84] with the permission of the Journal of Micromechanics and Microengineering.

On the other hand, using high machining voltages (more than 32 V) at low tool travel speeds results in a non-smooth channel surface with significant depth variation along the channel. As a general rule, the quality of the microchannels deteriorates as the tool travel speed is decreased. This can be attributed to the poor material removal rate and the accumulation of melted material inside the microchannel immediately behind the tool. 

Jagged outline contours with smooth channel surface This contour is observed for low machining voltages (less than 32 V) with tool travel speeds lower than in the previous contour type. 

Heat affected edges with non-smooth channel surface and thermal cracks When the tool travel speed is low and the machining voltage is high (typically 32 V with speed less than 30 pm/s and 35 V with speed less than 40 lm/s) the edges are unclear and heat affected and the surface is not flat and smooth. Thermal cracks are observed (Fig. 6.12(d)). 

Figure 6.13 summarises the type of microchannel obtained for different combinations of the machining voltage and the tool travel speed. Machining voltages less than 32 V result in acceptable microchannel quality. In this voltage range, an appropriate selection of the tool speed will result in the best... 

Figure 6.13 Characterisation diagram for microchannels machined using SACE technology as a function of the machining voltage and the tool travel speed (for tool-electrode distances less than 15 J.m). Machined using a 0.5 mm stainless steel cathode in 20 wt% NaOH. Reprinted from [23] with the permission of the Journal of Micromechanics and Microengineering.

If the trajectory of a channel can be directly controlled by the motion of the tool-electrode, the depth of the channel cannot be monitored directly. Depending on the tool travel speed and the machining voltage, different depths are achieved. Another important issue is the chemical etching of the substrate. Due to this effect, the depth of the channels will not remain constant over machining time, but on the contrary increases slightly. A typical example is shown in Fig. 6.16, which illustrates the depth of a channel machined at 28 V... 

The material removal rate and the machining over-cut increase with the machining voltage [100,105] and electrolyte concentration [105,133], which also increases the probability of wire breaking [105]. Polarising the wire as a cathode generally results in higher material removal rates than for a wire polarised as an anode. 

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