# Experimental Study on Micro-Grinding of Ceramics for Micro-Structuring
## 3 Results  
### 3.1. Machining Characteristics of Zirconia  
When a PCD tool is machined by EDM, the surface of the tool is covered with many craters generated by the EDM spark. As a result, a rough tool surface is obtained. During micro-grinding using an EDMed tool, the craters serve to remove material, which is similar to the action of abrasives of conventional grinding wheels. In conventional grinding, the larger the size of the abrasive of the tool, the higher the decrease in the grinding force. In EDM using an RC circuit, when the working capacitance increases, the crater size and tool surface roughness increase. Focusing on this point, an experiment was conducted to investigate the effect of the working capacitance on the grinding force. As shown in Figure 3a, a ceramic workpiece was ground using the side surface of the PCD tool, and the force components were measured. The PCD tools were fabricated with two different capacitances: 1000 pF and 474,000 pF. Figure 4 shows a graph of the force components according to the working capacitance used in tool fabrication. Using a tool machined at 474,000 pF, the normal force (y-direction) was reduced by 36%, compared to using a tool machined at 1000 pF. Since the radial depth was only 5 \(\upmu\)m, the change in the tangential force was very small. Next, the effect of the working capacitance on the normal force (z-direction, Fz) was investigated in the machining of micro-grooves, as shown in Figure 3b. In this experiment, tools with a diameter of 200 \(\upmu\)m were fabricated with a working capacitance in the range of 1000 to 100,000 pF. Figure 5 shows the normal force at different feed rates when micro-grooves were machined using tools with working capacitances of 1000 and 100,000 pF. When a tool manufactured with a capacitance of 100,000 pF was used, the normal force was higher than that manufactured with a capacitance of 1000 pF; in particular, when the tool feed rate was 200 \(\upmu\)m/s, the normal force was increased by up to 50%. When a large capacitance is used, as the size of the craters formed on the tool surface increases, the surface roughness of the tool also increases. The grinding force tends to decrease as the capacitance  
Figure 4: Force component comparison according to different working capacitances used in tool fabrication. increases, similar to side-grinding. However, the experimental results indicated that the grinding force increased despite the use of a larger capacitance (100,000 pF). To analyze these contrasting tendencies, an experiment was conducted to investigate the change in the tool geometry according to the working capacitance used during tool fabrication. It is noted that the tool edge between the side and bottom of a tool is not involved during the side-grinding. Regarding the size effect on micro-grinding using a micro-tool, machining with the edge of the tool participating must be considered. To identify the influence of the tool during groove-grinding with a micro-tool, the edge radius of the tool was analyzed. In EDM, the current was concentrated in a sharp part such as an edge during machining [27], and resulted in the rounding of the edge. If the discharge energy is reduced using a condenser with a small capacitance, sharper edges can be obtained with a smaller energy. In this study, the micro-tool was manufactured with the capacitance of 1000, 10,000, 100,000, and 474,000 pF to confirm the change in the edge radius of the tool. Figure 6 shows the change in the edge radius (\(r_{e}\)) according to the working capacitance. A tool with an edge radius of 3.5 \(\upmu\)m was fabricated using a capacitance of 1000 pF. Figure 6a,b show scanning electron microscopy (SEM) images of the edges of the machined tools with capacitances of 1000 and 474,000 pF, respectively.