## 4 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multirow{2}{*}{\begin{tabular}{c} **Spindle Speed** \\ **(rpm)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Feeding Rate** \\ **(\(\mu\)m/min)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Grinding Depth** \\ **(\(\mu\)m)** \\ \end{tabular} } & \multicolumn{2}{c}{
\begin{tabular}{c} **Surface Roughness** \\ **(\(\mu\)m)** \\ \end{tabular} } \\  & & & & **Hot-Pressing Tool** & **Gel Tool** \\ \hline
1 & 700 & 8 & 12 & 5.08 & 2.67 \\
2 & 700 & 8 & 16 & 4.07 & 2.20 \\
3 & 700 & 10 & 12 & 5.33 & 2.84 \\
4 & 700 & 10 & 16 & 5.63 & 3.09 \\
5 & 900 & 8 & 12 & 4.13 & 2.17 \\
6 & 900 & 8 & 16 & 3.96 & 1.95 \\
7 & 900 & 10 & 12 & 4.65 & 2.54 \\
8 & 900 & 10 & 16 & 4.52 & 2.38 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Machining results. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multicolumn{2}{c}{**Tool Wear Rate (\(\mu\)m/h)**} & \multicolumn{2}{c}{**MRR (\(\mu\)m/h)**} \\ \cline{2-5}  & **Hot-Pressing Tool** & **Gel Abrasive Tool** & **Hot-Pressing Tool** & **Gel Abrasive Tool** \\ \hline
1 & 0.76 & 0.19 & 0.57 & 0.43 \\
2 & 0.84 & 0.24 & 0.62 & 0.48 \\
3 & 0.95 & 0.25 & 0.63 & 0.51 \\
4 & 1.07 & 0.28 & 0.66 & 0.54 \\
5 & 1.12 & 0.31 & 0.63 & 0.56 \\
6 & 1.36 & 0.36 & 0.71 & 0.59 \\
7 & 1.72 & 0.39 & 0.77 & 0.61 \\
8 & 1.94 & 0.46 & 0.76 & 0.66 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Tool wear rate and MRR. Figure 15: Sapphire machined by gel abrasive tool: **(a)** before; **(b)** after. ### Comparative Processing Experiments  
The dressing of the abrasive tool greatly influences its performance during the polishing process, affecting not only the surface shape and sharpness of the grinding tools, but also the wear, grinding force, grinding temperature, and surface integrity of the workpiece [31]. The surface morphology of the damaged abrasive tool and the degree of exposure of the abrasive particles change, and these changes are copied to the surface of the workpiece. As a result, the surface quality of the workpiece is affected. To explore the change in surface morphology of the gel abrasive tool during processing, a spindle speed of 900 rpm and feeding rate of 8 um/min were selected, and grinding was performed in 10 strokes. When the feeding depth reached 16 um, the damage of the abrasive tool during the grinding process was observed using SEM. It can be seen from Figure 18 that, when the grinding depth of the abrasive tool reached 32 um, the surface presented a glazed layer. At 64 um, the phenomenon of glazing became much more severe, the pores of the abrasive tool were blocked by extensive debris due to long-term processing, and the number of exposed abrasive particles on the surface was greatly reduced. When the grinding depth reached 80 um, the surface of the abrasive tool was glazed. This phenomenon needs to be addressed. The pores on the surface of the abrasive tool were blocked by wear debris, and the blocked pores lost the ability to hold debris and store coolant. Because the frictional heat accumulated during the machining process was difficult to eliminate in time, it likely damaged the abrasive tool. Therefore, when the grinding depth reached 64 um, it was necessary to dress the abrasive tool. The surface SEM image shows that the exposed abrasive particles on the surface were covered by wear debris. The abrasive tool was trimmed using an electroplated diamond disc. Figure 19 shows the comparison of the abrasive tool before and after modification. It can be seen that the glaze layer on the surface of the abrasive tool disappeared. Figure 16: Flatness of sapphire after machining using different tools: (**a**) gel abrasive tool; (**b**) hot-pressing abrasive tool. Figure 17: Surface topography of the sapphire after machining using different tools and after CMP: (**a**) gel abrasive tool; (**b**) hot-pressing abrasive tool.