## 4 Verification and Discussion  
Considering that only one line was polished under each condition in the experiments conducted in Section 3, and the bonnet tool deals with the edge only at the end of each line within an extremely short time, it cannot reflect that the bonnet tool can not be damaged after polishing the whole surface under these corresponding conditions. Hence, several verification experiments were also conducted to test the conclusion summarized above. Experiments have been done on two ground flat fused silica glasses with the size of 100 mm \(\times\) 100 mm. Figure 5: Experimental prototype. Figure 6: Experiment results of each polishing line. (a) removed form shape of each polishing line and (b) extracted sectional profiles of them. (\(\lambda\) = 632.8 nm). Firstly, the pre-polishing process was executed using the conditions as shown in Table 1, except for the feeding speed being 800 mm/min. The raster path with the pitch of 2 mm was adopted. We had optimized the structure of the SR bonnet as reported in Ref. [24]. There are two kinds of SR bonnets with two different embedded materials, which are steel sheet and steel mesh, respectively. The embedded material of the one used in the experiment above was a steel sheet, which has higher rigidity than the one embedded with steel mesh, leading to larger contact pressure. In this experiment, we also used the R80 SR bonnet embedded with the steel sheet. After 24 min polishing of the whole surface, the polyurethane pad was heavily worn as shown in Figure 7a. In addition, the R80 SR bonnet embedded with the steel mesh was also tested. Considering that it has less rigidity, the polishing spot with 25 mm size and with the H-axis speed 1000 rpm were adopted. When the bonnet tool contacted the edge of the glass, the pad material was directly removed, just like the turning process as shown in Figure 7b. Hence, the conclusion made above is not suitable for the SR bonnet embedded with the steel mesh. The R80 SR bonnet tool embedded with steel sheet was adopted in the subsequent experiments. Because the polishing pad was damaged under the conditions as shown in Table 1, the size of the polishing spot was reduced to 25 mm in the following experiments. In addition, three cycles of pre-polishing on one 100 mm \(\times\) 100 mm surface were executed with three different H-axis speeds of 1000 rpm, 1250 rpm, and 1500 rpm, respectively. The polishing time of each cycle was still 24 min. The H-axis speed started with 1000 rpm. After the first cycle, no obvious wear was found on the polishing pad. After polishing with 1250 rpm H-axis speed, it was still hard to find the obvious tool wear. However, after polishing with 1500 rpm H-axis speed, the polishing pad became worn as shown in Figure 8. It indicates that the threshold value for the H-axis speed lies between 1250 rpm and 1500 rpm under those conditions. After these three pre-polishing times, the grey of the ground surface has been cleared and it also reached an accuracy level that could be measured through the interferometer. Figure 9 shows the measured result of the polished surface. The size of the area with the effective measured data reaches \(\sim\)84.5 mm \(\times\) 83 mm. In addition, the average upstand width of the edge is only \(\sim\)8 mm. Through this experiment, the H-axis speed of 1250 rpm when the polishing spot size is 25 mm (other conditions are as shown in Table 1) can be adopted safely in the practical pre-polishing process when using the R80 SR bonnet. Figure 7: Two different SR bonnet tools with the damaged polyurethane pad. (**a**) damaged SR bonnet tool with the embedded material of stainless steel sheet and (**b**) damaged SR bonnet tool with the embedded material of steel mesh. If the above surface was pre-polished without the overhang of the tool, the upstand edge width would be 25 mm (equal to the diameter of the polishing spot as demonstrated in Figure 2).