Introduction to the Working Principle of a Polishing Machine

If you can understand the working principle of a machine when in use, you will achieve twice the result with half the effort. Therefore, before operating a polishing machine, gaining a general understanding of its working principle will provide you with a sense of openness while using it.

The key to operating a polishing machine is to achieve the highest possible polishing rate to quickly remove the damage layer generated during grinding. At the same time, the polishing damage layer should not affect the final observed microstructure, i.e., avoid creating false structures. The former requires the use of coarser abrasives to ensure a higher polishing rate for removing the grinding damage layer, but the polishing damage layer is also deeper; the latter demands the use of the finest materials to keep the polishing damage layer shallow, though the polishing rate is lower.

The best way to resolve this contradiction is to divide the polishing process into two stages. The purpose of rough polishing is to remove the grinding damage layer, which should achieve the highest polishing rate. The surface damage caused by rough polishing is a secondary consideration but should be minimized as much as possible. The next step is fine polishing (or final polishing), which aims to eliminate the surface damage from rough polishing and minimize polishing-induced damage. During the polishing process, the sample surface must be kept absolutely parallel to the polishing disc and evenly light-pressed onto it, while preventing sample ejection or new scratches caused by excessive pressure. The sample should also rotate and move back and forth along the radius of the disc to avoid localized wear of the polishing cloth. Continuous addition of abrasive suspension is necessary to maintain a certain moisture level in the polishing cloth. Excessive moisture weakens the polishing effect, causing hard phases in the sample to appear raised and leading to "tailing" phenomena in non-metallic inclusions in steel or graphite phases in cast iron. Insufficient moisture, on the other hand, generates heat due to friction, raising the sample temperature, reducing lubrication, dulling the surface, and even causing black spots or surface damage in light alloys. To achieve the goal of rough polishing, the disc rotation speed should be relatively low, preferably not exceeding 600 rpm. The polishing time should be longer than the duration required to remove scratches, as the deformation layer must also be eliminated. After rough polishing, the surface becomes smooth but dull, with uniform and fine scratches visible under a microscope, awaiting removal by fine polishing.

During fine polishing, the spindle speed can be appropriately increased, and the polishing time should be sufficient to remove the damage layer from coarse polishing. After fine polishing, the grinding surface becomes bright and mirror-like, with no visible scratches under bright-field microscopy. However, grinding marks can still be observed under phase-contrast illumination. The quality of polishing significantly affects the microstructure of the specimen, drawing increasing attention from relevant experts. Extensive research has been conducted on the performance of polishing machines both domestically and internationally, leading to the development of numerous new models and generations of polishing equipment. These advancements have evolved from manual operation to various semi-automatic and fully automatic polishing machines.