Mechanical Polishing
Polishing is a precision machining process that uses mechanical, chemical, or electrochemical methods to remove minute amounts of material from a workpiece’s surface to improve surface finish and reduce surface roughness. It is widely used in mold manufacturing, precision instruments, aerospace, and other fields, achieving precision levels down to nanometers and surface roughnesses as low as Ra0.01μm or less. The main difference between polishing and grinding is that polishing removes smaller amounts of material (typically 0.01-0.1mm) and relies primarily on the micro-cutting action of the abrasive or the plastic flow of the surface material to achieve surface finish, rather than cutting with a rigid tool. Depending on the processing principle, polishing can be categorized into various types, including mechanical polishing, chemical polishing, electrolytic polishing, and ultrasonic polishing. Each method has its own unique scope of application and processing results.
Mechanical polishing is the most commonly used polishing method. Through the friction between a polishing tool (such as a polishing wheel or polishing belt) and the workpiece surface, microscopic surface bumps are removed to achieve a smooth surface. Mechanical polishing tools typically consist of a base and an abrasive. Base materials include cloth, felt, sponge, leather, and other materials, while abrasives can be categorized as diamond, silicon carbide, or aluminum oxide. Abrasives of varying grit sizes are selected based on the polishing stage. For rough polishing, larger grits (such as 800#-1200#) are used to remove machining marks left by the previous process. For fine polishing, smaller grits (such as 3000#-8000#) are used to achieve an extremely high surface finish. Mechanical polishing process parameters include polishing pressure, polishing speed, and polishing time. Too low a pressure results in low removal efficiency, while too high a pressure can easily scratch or deform the workpiece surface. The polishing speed is generally 10-30 m/s. Too high a speed generates significant heat, affecting surface quality. Polishing time is determined based on surface requirements, typically 5-10 minutes for rough polishing and 10-30 minutes for fine polishing. Mechanical polishing is suitable for a variety of metal and non-metal materials, especially complex curved surfaces. However, it is labor-intensive, has low processing efficiency, and surface quality is significantly affected by operator skill.
Chemical polishing utilizes the dissolving action of a chemical solution on the workpiece surface, preferentially dissolving microscopic surface protrusions, thereby achieving a smooth and shiny surface. Chemical polishing requires no complex equipment; the workpiece is immersed in a chemical polishing solution, and the polishing effect is achieved by controlling parameters such as temperature and time. This method is suitable for workpieces with complex shapes that are difficult to mechanically polish. The composition of the chemical polishing solution is determined by the workpiece material. For example, chemical polishing solutions for stainless steel typically contain nitric acid, hydrofluoric acid, and phosphoric acid, while those for aluminum alloys often contain phosphoric acid, sulfuric acid, and nitric acid. The advantages of chemical polishing include high processing efficiency, the ability to process multiple workpieces simultaneously, and freedom from workpiece shape restrictions. However, its disadvantages are the environmental impact of the chemical solution, and the resulting surface quality is inferior to that of mechanical polishing, with a surface roughness typically ranging from Ra 0.1-0.5μm. To improve chemical polishing effectiveness, electrolytically assisted chemical polishing can be used. By applying a weak current, electrolytically assisted chemical polishing accelerates surface dissolution, reducing surface roughness to below Ra 0.05μm.
Electropolishing is a process based on electrochemical principles. A workpiece is placed in an electrolytic cell, acting as an anode. Direct current is applied to the workpiece surface, causing anodic dissolution, resulting in a smooth surface finish. During electropolishing, microscopic protrusions on the workpiece surface are preferentially dissolved due to the high current density, gradually becoming flat and smooth. Simultaneously, an oxide film forms on the surface, protecting it from excessive corrosion. The effectiveness of electropolishing depends on parameters such as the electrolyte composition, current density, temperature, and time. For example, a phosphoric acid-sulfuric acid electrolyte is commonly used for electropolishing stainless steel, with a current density of 10-30A/dm², a temperature of 50-80°C, and a time of 5-15 minutes. Electropolishing offers advantages such as high efficiency, uniform surface quality, and freedom from workpiece shape restrictions. It is particularly suitable for polishing materials such as stainless steel and aluminum alloys, achieving surface roughnesses of up to Ra 0.02-0.1μm. However, electropolishing requires specialized electrolytic equipment and is costly to process. Furthermore, uneven surface dissolution can lead to poor polishing results for materials such as high-carbon steel and cast iron.
Post-polishing quality inspection and post-processing are crucial steps in ensuring the polishing effect. Quality inspection primarily includes testing for surface roughness, gloss, and surface defects. Surface roughness can be measured using a surface roughness meter. For mirror-polished surfaces, an atomic force microscope (AFM) is used for nanometer-level measurement. Surface gloss can be measured using a gloss meter to assess the surface’s reflectivity. Surface defects (such as scratches, pits, and depressions) are visually inspected or microscopically to ensure the absence of obvious defects. Post-polishing treatment depends on the material and application requirements. Metal workpieces require cleaning and rust prevention after polishing to remove residual abrasives and chemicals and prevent rust. Precision parts also require stress relief to eliminate residual stress generated during the polishing process and prevent deformation during use. Furthermore, for high-precision polished workpieces such as optical components, surface flatness, parallelism, and other form and positional accuracy must be tested to ensure they meet application requirements. Rigorous quality inspection and post-processing ensure the final polishing result, ensuring the workpieces have excellent appearance quality and performance.
