Principles and characteristics of rolling processing
Rolling is a non-cutting plastic working method. Rolling tools (such as rollers and balls) apply pressure to the workpiece surface, causing the surface metal to plastically flow, thereby changing the surface’s microgeometry and mechanical properties. Rolling doesn’t remove material, but instead uses the plastic deformation of the metal to enhance surface quality and improve precision. It is widely used in the finishing and surface treatment of metal parts and is a highly efficient, high-quality process.
The basic principle of rolling is to exploit the plastic deformation properties of metal. When the rolling tool applies a certain pressure to the workpiece surface, the surface metal undergoes plastic flow under the pressure, flattening surface peaks and filling valleys with surrounding metal, thereby reducing surface roughness. Simultaneously, plastic deformation refines the surface metal’s grain size, increases dislocation density, and forms a dense fibrous structure, improving the surface metal’s hardness and strength. Rolling pressure is a key parameter influencing the machining effect. Too low a pressure will result in insufficient plastic deformation, failing to achieve the desired surface quality and strengthening effect. Too high a pressure can cause cracks, peeling, or dimensional deviations on the workpiece surface. Therefore, the appropriate rolling pressure should be selected based on the workpiece material’s plasticity, hardness, and surface condition. Generally, for materials with good plasticity (such as mild steel and aluminum alloys), the rolling pressure is 50-200 MPa; for materials with higher hardness (such as medium carbon steel and alloy steels), the rolling pressure is 200-500 MPa.
Rolling processing has a significant effect on improving surface quality and can significantly reduce the surface roughness of workpieces. The surface roughness of unrolled workpieces is generally Ra3.2~12.5μm. After rolling processing, the surface roughness can be reduced to Ra0.02~0.8μm, and the surface presents a mirror effect. This is because during the rolling process, the plastic flow of the surface metal fills the microscopic pits on the surface, eliminating the tool marks and burrs left by the cutting process, making the surface smoother and flatter. The smooth surface can reduce the friction coefficient of the parts during use, reduce wear and energy consumption, and is especially suitable for the processing of moving parts (such as journals and piston pins). In addition, rolling processing can also improve the geometric shape accuracy of the surface, such as roundness and cylindricity, and correct the slight shape errors of the workpiece through uniform plastic deformation, thereby improving the fitting accuracy of the parts.
Rolling significantly improves the mechanical properties of a workpiece’s surface, enhancing its performance and lifespan. After rolling, the hardness of the workpiece’s surface metal typically increases by 20% to 50%. For example, the surface hardness of 45 steel after rolling can reach HB250-350 (compared to approximately HB180-220 without rolling). This increased hardness significantly enhances the wear resistance of parts, extending their service life by 2-5 times. Rolling also generates residual compressive stress on the workpiece’s surface, which offsets the tensile stresses generated during use and improves fatigue strength. For example, the fatigue strength of an automotive engine crankshaft can increase by 30% to 50% after rolling. Rolling also seals micropores and cracks on the workpiece’s surface, improving corrosion resistance and reducing rust in humid or corrosive environments.
Rolling is efficient and economical, making it suitable for mass production. Rolling is a continuous process with high efficiency. Generally, it only takes a few seconds to tens of seconds to roll the outer cylindrical surface of a shaft-type part, which is much more efficient than grinding. Rolling does not require the consumption of cutting fluids and cutting tools. Only the rolling tools need to be replaced regularly, which results in low processing costs. This can significantly reduce production costs, especially for parts produced in large quantities. Rolling can be performed on ordinary lathes, milling machines, and drilling machines. It does not require special equipment, requires low equipment investment, and is easy to implement automated production. In addition, rolling is a cold process that does not generate cutting heat, thus avoiding the impact of thermal deformation on workpiece accuracy, making it suitable for parts with high processing precision requirements.
Rolling also has certain limitations, requiring careful consideration when applying it. Rolling is only suitable for metal materials with good plasticity, such as low-carbon steel, medium-carbon steel, alloy steel, aluminum alloy, and copper alloy. Brittle materials (such as cast iron and hardened steel) are generally unsuitable due to their poor plasticity and prone to cracking during rolling. Rolling also requires high pre-processing quality for the workpiece. The original surface roughness should be no greater than Ra12.5μm. Otherwise, excessive surface peaks will cause excessive plastic deformation during rolling, potentially damaging the workpiece or the rolling tool. After rolling, the workpiece’s dimensions will vary slightly (typically 0.005-0.03mm), so an appropriate allowance (usually 0.05-0.1mm) must be reserved during pre-processing to ensure that the final dimensions meet the requirements. Furthermore, rolling is not suitable for complex surfaces. For parts with deep grooves, narrow slits, or complex curves, the rolling tool struggles to reach all surfaces, resulting in poor results.
