Brass Wet Polishing Machine: Avoid Over-Polishing, Ensure Surface Quality

In the field of metal processing, polishing is a key process to improve product surface quality and aesthetics. However, in daily polishing processes, a troublesome problem often plagues practitioners, that is, "over-polishing". Over-polishing not only fails to improve product quality but also leads to poor surface quality with problems such as "orange peel" and "pitting corrosion", which are more prominent in the wet polishing of relatively soft metals such as brass.

"Orange Peel" and "Pitting Corrosion" Caused by Over-Polishing

Simply put, "orange peel" refers to irregular rough textures on the workpiece surface, just like the surface of an orange peel. It is a common problem in brass wet polishing. When we use a polishing wheel to polish brass workpieces, improper operation can easily cause the "orange peel" phenomenon. There are many reasons for its occurrence. On the one hand, overheating of the mold surface is an important factor. During polishing, the high-speed friction between the polishing wheel and the workpiece surface generates a large amount of heat. If heat dissipation is not timely and the mold surface temperature is too high, the organizational structure of the brass material will change, leading to an increase in surface roughness and the formation of "orange peel". On the other hand, excessive carburization may also cause "orange peel". In some cases, carbon elements in the polishing environment may penetrate into the surface of brass workpieces, changing their chemical composition and physical properties and making the surface rough and uneven. In addition, excessive polishing pressure and long polishing time are also the main causes of "orange peel". Excessive polishing pressure will subject the workpiece surface to excessive extrusion and friction, and a long polishing time will further aggravate this adverse effect, ultimately leading to the appearance of "orange peel".
"Pitting corrosion" refers to the appearance of dot-like micro-pits on the workpiece surface after polishing. This is also a serious problem caused by over-polishing. Brass materials may contain some non-metallic impurities, usually hard and brittle oxides. When the polishing pressure is too high or the polishing time is too long, these impurities will be pulled out from the steel surface, thus forming micro-pits on the surface, i.e., "pitting corrosion". Especially when the purity of the brass material is insufficient and the content of hard impurities is high, the problem of "pitting corrosion" is more likely to occur. In addition, if the workpiece surface is rusty or the black skin is not completely removed, "pitting corrosion" is also easy to occur during polishing.

In-Depth Analysis of the Causes of "Orange Peel"

Overheating of the mold surface is a problem that cannot be ignored in brass wet polishing. When the polishing wheel rotates at a high speed and contacts the surface of the brass workpiece, a large amount of mechanical energy is converted into thermal energy due to friction. If the polishing equipment does not have a good heat dissipation system or the supply of cooling fluid is insufficient during polishing, the temperature of the mold surface will rise rapidly. Taking a common high-speed polishing machine as an example, after continuous operation for a period of time, if the cooling fluid flow is insufficient, the mold surface temperature may rise to more than 100℃. For brass, a metal with a large thermal expansion coefficient, this can easily lead to deformation and damage of the surface structure, thus producing "orange peel".
Excessive polishing pressure is also one of the key factors leading to the formation of "orange peel". Steels with different hardness have different bearing capacities for polishing pressure. Relatively soft brass can bear a relatively small polishing pressure. In actual operation, if operators excessively increase the polishing pressure in pursuit of a fast polishing effect, the surface of brass workpieces will be subjected to excessive extrusion and friction. For example, when using a polishing wheel for polishing, the appropriate pressure under normal circumstances may be 5 - 10N, but if the operator increases the pressure to 20N or even higher, the material on the workpiece surface will be excessively removed, the surface will become rough, and ultimately "orange peel" will form.
Long polishing time can also cause the "orange peel" problem. Polishing is a process of gradually improving the workpiece surface quality, but when the polishing time exceeds a reasonable range, it will be counterproductive. Prolonged polishing will make the workpiece surface subject to continuous friction and wear, and the originally smooth surface will be gradually damaged. Research shows that for general brass polishing processes, the appropriate polishing time may be 10 - 20 minutes, but if the polishing time is extended to more than 30 minutes, the probability of "orange peel" will increase significantly.

Effective Measures to Eliminate "Orange Peel"

When "orange peel" is found on the workpiece surface, many people may instinctively increase the polishing pressure and extend the polishing time in an attempt to improve the surface quality. But in fact, this approach often makes the surface quality worse. The correct method is to first remove the defective surface by grinding with an abrasive with a slightly coarser grain size than the sand number used previously. For example, if 200-mesh sandpaper was used before, 180-mesh sandpaper can be used for grinding at this time. During grinding, attention should be paid to controlling the force to be lower than the previous one to avoid further damage to the surface. After grinding, polishing is carried out with a relatively light force.

Knowledge on the Selection of Polishing Wheels

In brass wet polishing, selecting a suitable polishing wheel is crucial. Polishing wheels of different materials are suitable for different polishing needs. For brass polishing, linen wheels are a common choice. Linen wheels are relatively soft in texture and can contact the brass surface gently during polishing, reducing the risk of "orange peel" caused by excessive friction. They can effectively remove fine defects on the brass surface, improve the surface flatness and make the brass surface present a delicate luster.
For precision polishing with high surface quality requirements, non-stitched whole cloth wheels are more suitable. This type of polishing wheel is made of dense and soft cotton cloth, which can better fit the surface of brass workpieces and achieve fine grinding. It generates relatively little heat during polishing, which can effectively avoid the "orange peel" problem caused by overheating, making it particularly suitable for the polishing of brass workpieces with extremely high surface roughness requirements.
For the surface polishing of some plated brass workpieces, stitched polishing wheels are a good choice. It can effectively polish the workpiece surface without damaging the plating layer, making the plating surface smooth and bright while ensuring the overall quality of the workpiece.
The problem of over-polishing in brass wet polishing requires our high attention. By deeply understanding the causes of "orange peel" and "pitting corrosion", taking effective elimination measures and correctly selecting polishing wheels, we can effectively improve the polishing quality of brass workpieces, avoid surface quality problems caused by over-polishing, and provide a strong guarantee for the high-quality development of the metal processing industry.