The efficiency of polishing process depends on the properties of convex layer and crack layer left by optical glass grinding. The common misconception is that the smaller the concave-convex layer of the rough ground surface, the better, which ignores the effect of the microstructure of the rough ground surface on the polishing process. Polishing die, especially thermosetting plastic die, is easy to passivate and lose polishing ability in polishing process. The concave-convex layer is beneficial to reduce or eliminate this passivation phenomenon.

The polishing process can be basically divided into two stages, the first stage to remove the concave-convex layer, the second stage to remove the crack layer.

At the beginning of the first stage, the polishing die is in contact with the concave and convex layer of the glass at its peak, the pressure is very high, and the concave valley provides good conditions for the polishing liquid to enter the whole surface, so the polishing is very fast. As the polishing process continues, the contact area increases, the pressure decreases, and the adhesion ability of the polishing liquid decreases, which slows down the polishing process.

When the polishing surface of optical glass reaches the cracked layer, the glass surface completely contacts with the polishing die surface, the polishing process tends to be stable and slow, and the polishing die begins to passivation, polishing continues, passivation intensifies, and the polishing efficiency decreases further. The degree of passivation depends on the duration of the process, which is directly determined by the depth of the crack layer. The optimum thickness of the concave and convex layer is determined by the properties of the die material and the polish used in conjunction with the material, as well as other factors such as spindle speed, pressure and the ability of polishing fluid to enter

Different rough grinding methods are used for optical glass, or the cracks obtained by the same method vary with the passivation degree of grinding tools and the lubrication state of cooling. Practice has proved that although the convex layer is small, the crack layer is deep in the workpiece processed with passivated diamond abrasives. Therefore, not only the influence of concave and convex layer on polishing should be considered, but also the depth of crack layer should be considered as an important index of rough grinding process.