Drilling, gravity-feed

These distributors are fabricated of pipe lengths tied to a central distribution header (usually) %vith orifice holes drilled in the bottom of the various pipe laterals off the header. This style of distributor can be fed by pressure or gravity for clean fluids. The gravity feed is considered better for critical distillation application when uniformity of the flow of the drip points (or flow points) through out the cross-section of the tower is extremely important, and is excellent for low flow requirements such as below 10 gpm/ft2 [131]. 

In gravity-feed drilling a constant force is applied on the tool-electrode in order to ensure close contact between the heat source (the electrochemical discharges in the gas film) and the workpiece. Although this method is particularly simple and gives excellent results, the major drawback is the mechanical contact between the tool and the workpiece. 

During gravity-feed drilling as the tool-electrode is constantly in contact with the workpiece, drilling can be followed by a measurement of the progress... 

Figure 6.1 Principle of SACE gravity-feed drilling.

Table 6.1 Tool-Electrode Wear for Gravity-Feed Drilling [76]...

Figure 6.2 Typical evolution at various voltages of SACE glass gravity-feed drilling using a cylindrical tool (cathode) of 0.4 mm diameter with a force of 0.8 N acting on it. The electrolyte (30 wt% NaOH) level above the workpiece is about 1 mm. Reprinted from [131] with the permission of the Journal of Micromechanics and Microengineering.

As glass drilling by gravity-feed is the most studied mechanism to date, this technique is discussed in detail in the remainder of this section. 

The discharge regime in glass gravity-feed drilling is characterised by a high drilling speed of typically 100 J.m/s [131]. It takes place in the first 100-200 lm. 

Figure 6.5 Example of SACE glass gravity-feed drilling with a 0.4 mm stainless steel tool-cathode at 31 V in the case of high inter-electrode resistance. In situation (a) the gas film needs to be built up more often than in situation (b). This results in an overall slower machining for situation (a) than situation (b). Reprinted from [130] with the permission of the Journal of Micromechanics and Microengineering.

Figure 6.7 (a) Standard deviation a and (b) roundness error of microholes machined using SACE gravity-feed drilling in glass. Reprinted from [84] with the permission of the... 

This effect is pronounced especially during gravity-feed drilling and is responsible for the hydrodynamic regime as discussed in Section 6.2.2. Note that during drilling in the hydrodynamic regime, the quality of the microholes deteriorates. 

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.

As in the case of tool-electrode vibration, the electrolyte flow can be promoted by tool-electrode rotation. An example combining gravity-feed drilling with tool-electrode rotation is shown in Fig. 7.6. A tungsten carbide flat sidewall tool-electrode (Fig. 7.3b) with pulsed voltage supply was used [136]. The drilling time for the fixed depth of 450 p,m increases with the tool-electrode rotation rate due to the reduced heat power. The entrance diameter shows an inverse volcano dependence on the tool-electrode rotation rate. This effect was attributed by the authors to the competition between the promotion of the electrolyte flow and the increased drilling time [136]. 

Figure 7.6 Effect of tool-electrode rotation in gravity-feed drilling. Reprinted from [136] with the permission of the Journal of Micromechanics and Microengineering.

Figure 7.9 Glass gravity-feed drilling with an electrolyte modified by adding a surfactant, (a) The critical voltage is reduced below 15 V. (b) Successive drillings at 20 V with a cylindrical stainless steel electrode of 0.5 mm diameter. The dispersion of the mean diameter is less than 5 pm. Reprinted from [129] with permission from Elsevier.

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