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In the manufacturing industry, surface roughness is an important indicator for measuring the microscopic geometric errors on a part’s surface. It is directly related to the product’s functionality, aesthetics, service life, and production costs. Different processing methods achieve varying levels of surface roughness due to their process characteristics and equipment limitations. This article will provide a detailed overview of the surface roughness that can be achieved through various processing methods, and explore its influencing factors and practical applications. I. Relationship between processing methods and surface roughness Surface roughness reflects the microscopic geometric shape errors on a part’s surface, and it is typically expressed using the arithmetic average deviation of the profile, Ra. Ra refers to the arithmetic average of the absolute values of the profile deviations over the sampling length. Due to their different processing characteristics, various processing methods yield varying results in terms of the surface finish of parts, which in turn determines the differences in surface roughness. II. Surface roughness of common processing methods 1. Casting Casting is a processing method in which liquid metal is poured into a mold, and after it cools and solidifies, the part is taken out to obtain the desired shape and size. During the casting process, factors such as the fluidity of the liquid metal, shrinkage during solidification, and gas evolution can result in a certain level of roughness on the surface of the part. Generally, the surface roughness Ra of castings ranges from 6.3 to 25 μm, and it may even be rougher. 2. Extrusion is a manufacturing method in which a metal material is placed in an extrusion die, and plastic deformation is induced by external forces to obtain parts of the desired shape and size. During the extrusion process, both the fluidity of the metal material and the precision of the die determine the surface roughness of the part. Generally, the surface roughness Ra of extruded parts can reach 0.8~3.2 μm, depending on the precision of the extrusion die and the extrusion process parameters. 3. Drawing is a manufacturing method in which metal materials are stretched and deformed through dies to obtain parts of the desired shape and size. During the drawing process, the fluidity of the metal material and the lubrication conditions of the die have a significant impact on the surface roughness of the part. Generally, the surface roughness Ra of drawn parts can reach 0.4~1.6 μm, making them suitable for parts with high requirements regarding surface roughness. 4. Machining: Machining is a processing method in which a tool is used to remove excess material from the surface of a metal, thereby obtaining a part with the desired shape and dimensions. Machining includes various methods such as milling, planing, broaching, drilling, reaming, boring, and reaming. The surface roughness of machined parts depends on factors such as the precision of the tool, cutting parameters, lubrication conditions, and the accuracy of the machine tool. Generally, the surface roughness Ra of machined parts can reach 0.2~3.2 μm, making them suitable for components with specific requirements regarding surface roughness. Milling: Milling is a method of cutting workpieces using a rotating milling tool. The surface roughness of milling depends on factors such as the precision of the milling tool, cutting parameters, and the hardness of the workpiece material. Generally, the surface roughness Ra of rough milling is between 3.2 and 2.5 μm, while that of finish milling can reach 0.8 to 3.2 μm. Planing: Planing is a method of performing linear reciprocating cutting on a workpiece using a planer tool. The surface roughness of the planed surface depends on factors such as the precision of the planing tool, cutting parameters, and the hardness of the workpiece material. Generally, the surface roughness Ra of planed surfaces ranges from 6.3 to 25 μm, but fine planing can achieve a lower roughness. Milling: Milling is a method of cutting workpieces using milling cutters, and it is primarily used to shape internal holes or grooves. The surface roughness of the turned surface depends on factors such as the precision of the turning tool, the cutting parameters, and the hardness of the workpiece material. Generally, the surface roughness Ra of turned surfaces ranges from 3.2 to 12.5 μm. Drilling: Drilling is a method that uses a drill bit to cut into a workpiece in order to create a circular hole. The surface roughness of the drilled hole depends on factors such as the precision of the drill bit, cutting parameters, and the hardness of the workpiece material. Generally, the surface roughness Ra of drilled holes ranges from 12.5 to 50 μm, but precision drilling can achieve a lower roughness. Reaming: Reaming is a method of enlarging an existing hole using a reamer or broach. The surface roughness of the reamed hole depends on factors such as the precision of the reaming tool, the cutting parameters, and the hardness of the workpiece material. Generally, the surface roughness of the reamed hole can reach the level of a drilled hole or even be lower. Boring: Boring is a method that uses a boring tool to machine the inner hole of a workpiece, in order to achieve higher precision and surface finish. The surface roughness of the bored hole depends on factors such as the precision of the boring tool, cutting parameters, and the hardness of the workpiece material. Generally, the surface roughness Ra of bored holes can reach 0.8~3.2μm. Reaming: Reaming is a method that uses a reamer to finely machine the inner hole of a workpiece, in order to achieve higher precision and better surface roughness. The surface roughness of reaming depends on factors such as the precision of the reamer, cutting parameters, and the hardness of the workpiece material. Generally, the surface roughness Ra of reamed holes can reach 0.4~1.6μm. 5. Grinding is a machining process that uses abrasives and grinding tools to remove a small amount of material from the surface of a workpiece and to polish it. Grinding can achieve very high surface precision and surface roughness. Generally, the surface roughness Ra of ground parts can reach 0.025~0.8 μm, or even lower. Grinding processes include various methods such as grinding external circles, grinding flat surfaces, and honing. External cylindrical grinding: External cylindrical grinding is a method that uses an abrasive wheel to grind the outer surface of a workpiece. The surface roughness of external cylindrical grinding depends on factors such as the precision of the grinding wheel, grinding parameters, and the hardness of the workpiece material. Generally, the surface roughness Ra of ground outer circles can reach 0.025~0.8μm. Grinding surface: Grinding the surface is a method that uses an abrasive wheel to grind the flat surface of a workpiece. The surface roughness of the ground surface also depends on factors such as the accuracy of the grinding wheel, grinding parameters, and the hardness of the workpiece material. Generally, the surface roughness Ra of a polished surface can reach 0.025~0.8μm. Honing: Honing is a method that uses a honing tool and honing stone to remove a small amount of material from the surface of a workpiece and to polish it. Honing can achieve high surface precision and surface roughness, and is often used in the machining of precision parts. The surface roughness Ra of honed surfaces can reach 0.01~0.2 μm. 6. Other processing methods: In addition to the common processing methods mentioned above, there are also some special processing methods, such as roll polishing, superfinishing, cylindrical grinding, polishing, chemical grinding, electrochemical grinding, electrical discharge machining, etc. Due to the process characteristics and equipment limitations, these processing methods achieve different levels of surface roughness. Rolling polishing: Rolling polishing is a method that uses rolling tools to induce slight plastic deformation and polishing of the workpiece surface. Rolling polishing can achieve high surface precision and low surface roughness, and is often used in the processing of precision parts. The surface roughness Ra of roll polishing can reach 0.05~0.4μm. Ultra-precision machining: Ultra-precision machining is a processing method that uses super-hard abrasives and precision machine tools to remove minute amounts of material from the surface of a workpiece and to polish it. Superfinishing can achieve extremely high surface precision and surface roughness, and is often used in the machining of high-precision parts. The surface roughness Ra of superfinishing can reach 0.001~0.01 μm. Cylindrical grinding: Cylindrical grinding is a method that uses grinding tools and abrasives to slightly remove material and polish the cylindrical surface of a workpiece. Cylindrical grinding can achieve high surface precision and low surface roughness, and is commonly used in the machining of precision parts. The surface roughness Ra of cylindrical grinding can reach 0.025~0.4 μm. Polishing: Polishing is a method that uses polishing tools and polishes to remove a small amount of material from the surface of a workpiece and to polish it. Polishing can achieve a high level of surface gloss and low surface roughness, and is often used in the processing of parts that require extremely high surface quality. The surface roughness Ra of the polished surface can reach below 0.001 μm. Chemical grinding: Chemical grinding is a method that uses chemical reactions to slightly remove material and polish the surface of a workpiece. Chemical grinding can achieve high surface precision and surface roughness, but it is suitable for specific materials and process conditions. Electrochemical grinding: Electrochemical grinding is a method that combines electrolytic action with grinding action to slightly remove material and polish the surface of a workpiece. Electrochemical grinding can achieve high surface precision and low surface roughness, and is often used for processing difficult-to-machine materials. Electrical discharge machining: Electrical discharge machining is a method that uses the energy generated by electrical discharge to remove small amounts of material from a workpiece and to polish it. Electrical discharge machining can achieve high surface precision and surface roughness, but it is suitable for specific materials and process conditions. III. Influencing Factors and Improvement Measures There are many factors that affect the surface roughness of machined parts, including the machining method, cutting parameters, tool precision, machine accuracy, workpiece material, and lubrication conditions. To improve the surface roughness of the machined surface, the following measures can be taken: selecting appropriate machining methods and cutting parameters. Based on factors such as the material, shape, and size of the workpiece, as well as the requirements for surface quality, appropriate machining methods and cutting parameters are selected, such as cutting speed, feed rate, and depth of cut. Improve the precision of cutting tools and machine tools. Choose high-precision cutting tools and machine tools to ensure the accuracy of the relative position between the tool and the workpiece during cutting, as well as the stability of the cutting forces. Optimize lubrication conditions. Select an appropriate cutting fluid or lubricant to reduce friction and heat during cutting, minimize tool wear and workpiece deformation, thereby improving the surface roughness of the machined part. Advanced processing technologies and equipment are employed. With the advancement of technology, more and more advanced processing techniques and equipment are being applied in the manufacturing industry, such as laser processing, ultrasonic processing, ion beam processing, etc. These technologies offer advantages such as high precision, high efficiency, and high surface quality, which can significantly improve the roughness of the machined surface. Strengthen quality control and testing. During the processing stage, strengthen quality control and inspection to identify and resolve issues promptly, ensuring that the surface roughness meets the design requirements.