Turning of copper and its alloys
Copper and its alloys are widely used in electronics, aerospace, medical devices, and other fields due to their excellent electrical and thermal conductivity, corrosion resistance, and ease of machining. Turning requires appropriate process measures tailored to the material’s properties. Pure copper (red copper) exhibits extremely high plasticity and toughness, but is prone to built-up edge and chip entanglement during turning, making surface roughness difficult to control. Brass (copper-zinc alloy) offers excellent machinability, but excessive zinc content can lead to “zinc embrittlement,” resulting in chip breakage. Bronze (copper-tin alloy), on the other hand, exhibits significant variations in machinability depending on its tin content. High-tin bronze exhibits increased hardness and results in faster tool wear during turning. Processing data from an electronic component manufacturer shows that using appropriate turning processes can reduce the surface roughness of copper alloy parts from Ra3.2μm to Ra0.8μm, improving machining efficiency by 50%.
When selecting turning tools for copper and its alloys, focus on sharpness and wear resistance. Due to the high plasticity of copper, the tool rake angle should be relatively large (15°-25°) to reduce cutting forces and friction. The clearance angle should be 8°-12° to avoid excessive friction between the flank and the workpiece surface. Regarding tool materials, high-speed steel tools (such as W18Cr4V) are suitable for low-speed precision turning, achieving excellent surface quality. Carbide tools (such as YG8 and YT15) are suitable for high-speed turning. YG-type carbide has excellent wear resistance and is suitable for machining pure copper and brass, while YT-type carbide is suitable for machining high-strength copper alloys such as bronze. For high-precision, high-finish surface machining, diamond tools (PCD) can be used. Their extremely high hardness and low coefficient of friction enable mirror-finish turning with surface roughness reaching Ra0.02μm. When a precision instrument factory processed brass gears, it used YG8 carbide tools and special cutting fluid to extend the tool life by 30% and the tooth surface roughness reached Ra1.6μm.
The turning process parameters for copper and its alloys must be appropriately set according to the material type. Cutting speed significantly impacts machining quality. When turning pure copper, a higher cutting speed (100-300 m/min) should be used to avoid built-up edge. The cutting speed for brass can be controlled between 80-200 m/min, though brass with high zinc content should be reduced to prevent defects caused by zinc embrittlement. The cutting speed for bronze is generally 50-150 m/min, with lower values being used for high-hardness bronze. The feed rate should be selected based on both efficiency and surface quality. For rough turning, the feed rate should be 0.2-0.5 mm/r, while for finish turning, it should be 0.05-0.15 mm/r. The backcut depth is determined by the machining allowance: 1-5 mm for rough turning, and 0.1-0.3 mm for finish turning. When processing brass pipe fittings, an automobile parts factory increased the cutting speed to 150m/min and adjusted the feed rate to 0.1mm/r by optimizing parameters, which increased the processing efficiency by 40% while ensuring the dimensional accuracy of the pipe fittings.
Cooling and chip removal technology is particularly important when turning copper and its alloys. Due to the good thermal conductivity of copper, cutting heat is easily transferred to the tool, resulting in increased tool wear. Therefore, a cutting fluid with good cooling performance is required. Emulsions or synthetic cutting fluids are acceptable. The flow rate should be sufficient and sprayed directly onto the cutting area. When turning pure copper and high-plasticity brass, the chips are in the form of ribbons, and chip flutes or chip breakers are required to force the chips to break to prevent them from wrapping around the workpiece or tool. Bronze chips are mostly in the form of fragments, and chip removal devices need to be strengthened to prevent chips from accumulating on the machined surface and causing scratches. When a motor factory processed pure copper commutators, it used high-pressure cutting fluid (pressure 5-8MPa) in combination with spiral chip flute tools to successfully solve the chip wrapping problem and eliminate scratch defects on the machined surface.
Quality control in turning copper and its alloys requires careful attention to multiple indicators. Regarding dimensional accuracy, pure copper is prone to deformation due to its high plasticity, requiring controlled clamping force and a small finishing allowance. Brass, on the other hand, offers superior dimensional stability and can achieve IT7-IT8 accuracy. Regarding geometric tolerances, slender copper parts with an aspect ratio greater than 10 require a steady rest or center rest to prevent bending and deformation, and maintain straightness within 0.05 mm/m. Regarding surface quality, built-up edge and chip scratches must be avoided. The surface roughness of pure copper can be reduced to below Ra 0.4 μm through high-speed finishing or polishing, while brass can achieve a surface quality of Ra 0.8 μm by optimizing tool angles and cutting parameters. An aerospace company, in machining copper waveguide components, achieved a dimensional tolerance of ±0.01 mm and a surface roughness of Ra 0.05 μm by using diamond tools for high-speed finishing (cutting speed 300 m/min) combined with strict deformation control measures, meeting microwave transmission requirements. With the expansion of the application of copper alloys in high-end manufacturing, turning technology is developing towards high precision, high efficiency and greenness. For example, the application of low-temperature cold air cutting technology can reduce the use of cutting fluid while improving processing quality, providing a new solution for the precision processing of copper and its alloys.
