After fine grinding with grinding fluid, the surface of optical quartz lens still has a crack layer about 2-3m thick. The method to eliminate this crack layer is polishing. The polishing mechanism is the same as that of grinding, except that the tool material used is different from that of polishing fluid. The materials used for polishing include flannelette (cloth), polishing leather (polyurethane) and asphalt (pitch). It is usually necessary to achieve high-precision polishing surface, and the commonly used material is high-grade polishing asphalt.
The optical lens is polished with asphalt. The fine surface of asphalt drives the polishing liquid to grind the lens surface to generate heat, so that the glass melts and flows, melts the rough vertices, fills the valley bottom of the crack, and gradually removes the crack layer. At present, cerium oxide (CeO2) is the main polishing powder used for polishing glass lenses, and the proportion of polishing solution varies according to the polishing period of the lens. Generally, in the initial stage of polishing, a higher concentration of polishing solution is used when combining with the polishing mold. After the lens surface is bright, a thinner concentration of polishing solution is used to avoid orange peel phenomenon (lens surface atomization).
The motion mechanism used for polishing is the same as that used for grinding. Except that the polishing tools are different from the working fluid, the environmental conditions required for polishing are also more stringent than those used for grinding. General precautions during polishing are as follows: there shall be no impurities on the surface of polishing asphalt and polishing solution, otherwise the mirror will be scratched. The polished asphalt surface shall be consistent with the lens surface, otherwise it will jump during polishing, so it will bite the polishing powder and scratch the lens surface.
Before polishing, it must be determined whether there are scratches or punctures left on the lens surface after grinding. Whether the size and material of polishing tools are appropriate. Whether the soft hardness and thickness of asphalt are appropriate. In the process of polishing, attention must be paid to the condition and accuracy inspection of the lens surface at any time. For the inspection of lens surface defects, because the inspection process is judged by personal vision and methods, the inspector should have a deep understanding of the specifications of scratches and sand holes, and often compare the standard samples of scratches and sand holes to ensure the correctness of the inspection.