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Polishing is a machining process that utilizes mechanical, chemical, and electrochemical processes to reduce the surface roughness of workpieces, with the aim of obtaining a shiny and smooth surface. Polishing is the process of modifying the surface of a workpiece using polishing tools, abrasive particles, or other polishing media.
Polishing cannot improve the dimensional or geometric accuracy of the workpiece, but rather to obtain a smooth surface or mirror gloss. Sometimes, polishing is also used to eliminate gloss (extinction).
Polishing type
1. Mechanical polishing.
Mechanical polishing is a polishing method that removes protruding parts through cutting and plastic deformation of the material surface to obtain a smooth surface. Generally, grinding stones, wool wheels, and sandpaper are used for polishing, mainly by manual operation. When polishing the surface of special parts such as rotating objects, auxiliary tools such as turntables can be used. If high surface quality is required, ultra precision polishing can be chosen for polishing. Ultra precision polishing, when the tool is tightly pressed against the surface of the workpiece in a polishing solution containing abrasives, a specialized grinding tool is used for high-speed rotation.
By using this ultra precision polishing technology, the surface roughness can reach Ra0.008 ¦Ì m, which is also the highest among various polishing methods. Usually, this technology will be used for optical lens molds.
2. Chemical polishing.
Chemical polishing, when the material is in a chemical medium, will cause the micro convex parts on the surface of the material to dissolve before the concave parts, thereby obtaining a smooth surface.
The main advantage of this method is that it does not require complex equipment, can polish workpieces with complex shapes, and can also polish multiple workpieces simultaneously. The efficiency is very high. The core issue of chemical polishing is the preparation of polishing solution. Using chemical polishing, the surface roughness can reach 10 ¦Ì m.
3. Electrolytic polishing
The basic principle of electrolytic polishing is the same as chemical polishing. Electrolytic polishing selectively dissolves small protrusions on the surface of the material, making the surface smooth. Compared with chemical polishing, electrolytic polishing can eliminate the influence of cathodic reactions and achieve better results.
The electrolytic polishing process can be divided into two steps.
(1) Macro leveling. The diffusion of dissolved products into the electrolyte reduces the geometric roughness of the material surface, Ra>1¦Ìm¡£
(2) Low light leveling. Anodic polarization increases surface brightness, Ra<1¦Ìm¡£
Electrolytic polishing is suitable for non-ferrous metals and other low hardness single-phase alloys, such as aluminum alloys, austenitic stainless steel, high manganese steel, etc. However, electrolytic polishing is not suitable for metal metallographic samples with uneven chemical composition, severe microsegregation in metal samples, and non-metallic inclusions. Samples embedded with plastic should not be subjected to electrolytic polishing, as it can cause severe localized corrosion.
Electrolytic polishing is an electrochemical dissolution process without mechanical force. So electrolytic polishing will not cause deformation of the metal surface and will avoid the deformation layer generated on the sample surface during mechanical polishing, thus truly displaying the metallographic structure.
4. Ultrasonic polishing.
Ultrasonic polishing is the process of placing a workpiece in an abrasive suspension and placing it together in an ultrasonic field. The abrasive then uses the oscillation effect of ultrasonic waves to polish the surface of the workpiece. Ultrasonic processing has low macroscopic force and will not cause deformation of the workpiece, but it is difficult to manufacture and install fixtures. Ultrasonic treatment can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution, separating the dissolved products from the surface of the workpiece and making the corrosion or electrolyte near the surface uniform. The cavitation effect of ultrasound in liquid can also suppress the corrosion process and promote surface brightness.
5. Fluid polishing.
Fluid polishing uses high-speed flowing liquid and abrasive particles carried by the liquid to clean the surface of the workpiece, achieving the purpose of polishing. There are some commonly used methods. Such as abrasive jet machining, liquid jet machining, fluid dynamic grinding, etc. Fluid dynamic grinding is driven by hydraulic pressure, which causes the liquid medium carrying abrasive particles to flow back and forth at high speed on the surface of the workpiece. The medium is mainly made of special compounds with good flowability at lower pressures and mixed with abrasives. Silicon carbide powder can be used as an abrasive.
6. Magnetic grinding and polishing.
Magnetic grinding and polishing is the process of using magnetic abrasives to form grinding brushes under the action of a magnetic field to polish workpieces. This method has high processing efficiency and good quality. The processing conditions are easy to control. The working conditions are very good. If suitable abrasives are used, the surface roughness can reach Ra0.1 ¦Ì m.
