# Effect to the Surface Composition in Ultrasonic Vibration-Assisted Grinding of BK7 Optical Glass
## 3 Results and Discussions  
### Analysis of the Effects in Surface Morphologies and Surface Roughness  
#### 3.1.1 Effect of Grinding Parameters on Surface Morphologies and Surface Roughness  
To observe the details of morphologies on machined surfaces, 100 times zoomed images of the morphologies at spindle rotation speed of 1000 rpm and 15,000 rpm are shown in Figure 1(a,b), respectively. Lots of brittle-fractured pits in large sizes was obviously observed on the machined surface at the spindle rotation speed of 1000 rpm. It can be seen that the density of brittle-fractured pits on the surface produced at a spindle rotation speed of 15,000 rpm is much higher than that generated at a spindle rotation speed of 1000 rpm. The amount of brittle-fractured pits in large sizes produced at \(n\) = 15,000 rpm is also found to be much less than that produced when \(n\) = 1000 rpm. According to Equation (1), the fluctuation of protrusion heights of brittle-fractured pits is much bigger than that of other areas on the machined surface, and, the existing of these areas could be the possible reason that surface roughness always is in big value when the spindle rotation speed is small. Figure 2: Machined surface morphologies of optical glass at different spindle rotation speed. After the obvious increasing of spindle rotation speed, the sizes of brittle-fractured areas overall decrease, and the machined surface becomes smooth. Meanwhile, the brittle-fractured pits in large sizes nearly disappear. In order to further observe the influence of spindle rotation speed on the proportion of different material removal modes, especially on the changing of powdering removal, another two images of same surface areas zoomed by 1000 times are shown in Figure 2b,d. From Figure 2b, slight powdering removal is found on the machined surface, and mainly consisted of fracture lumps in micron level and scraps in sub-micron level. These defects all covered on the machined surface but did not propagate onto the surface. After the spindle rotation speed changing to 15,000 rpm, the scraps in sub-micron level nearly disappear and the amounts of fracture lumps also decrease to a certain level, but they can still be found on the machined surface. Hence, a conclusion can be drawn, obvious change in spindle rotation speed can strongly influences not only the surface roughness, but also the proportion of powdering removal in sub-micron level, but cannot make the fracture lumps in micron level change obviously. The influence law of feed rate on surface roughness is shown in Figure 3. It can be seen that the surface roughness has a rise as the feed rate increases. A possible reason for this change is that the increase of feed rate leads to the decrease of critical cutting depth of BK7 optical glass material [34]. Another reason is that the number of grinding times of a single abrasive grain during unit time decreases, to maintain the material removal rate in a certain level, the glass material will be removed in bigger volumes. While there is no change on other conditions, the above two reasons both will lead to the increase of grinding force and the proportion of brittle removal, surface roughness thus becomes bigger. It also can be seen from Figure 3, while the feed rate changing from 10 mm/min to 150 mm/min, the surface roughness only increased by around 75 nm. This means, although surface roughness is widely used to assess machined surface quality, for optical glass material, the surface roughness may not be such sensitive to cutting parameters than that in machining of other materials. On the contrary, the proportion of material removal modes may be more efficient to justify the surface quality. Thus, the effect of feed rate on the proportion of material removal modes should also be investigated.