Turning of pure copper
Pure copper (also known as red copper) is a metal material with excellent electrical and thermal conductivity and ductility. It is widely used in electrical components, pipe fittings, and other fields. However, its turning process presents unique challenges. Pure copper has a Brinell hardness of only 35-45HB and an elongation of up to 45%. This makes turning prone to tool sticking, resulting in built-up edge and increased workpiece surface roughness. Furthermore, the thermal conductivity of pure copper is over five times that of steel, rapidly transferring cutting heat to the tool and increasing tool wear. Turning pure copper requires targeted measures in terms of tool selection, cutting parameter optimization, and cooling and lubrication to ensure machining quality and efficiency.
The choice of tool material is crucial for pure copper turning. Ordinary high-speed steel tools, while sharp, lack wear resistance and are suitable for low-speed finish turning. Carbide tools are the preferred choice for pure copper turning, particularly tungsten-cobalt carbides (YG-type), such as YG6 and YG8. These offer excellent wear resistance and adhesion resistance, effectively reducing built-up edge. YG6 carbide is suitable for semi-finishing and finish turning of pure copper, with cutting speeds reaching 100-150 m/min. YG8 offers greater toughness, making it suitable for roughing and interrupted cutting, and can withstand significant impact loads. For high-precision pure copper parts (such as electrical contacts), diamond tools can be used. Their high hardness and low friction coefficient enable mirror-finishing (surface roughness Ra 0.02μm or less), but they are more expensive and are therefore suitable for mass production. Regarding tool coatings, TiN coatings increase tool surface hardness, reduce adhesion, and can extend tool life by over 30%, making them an ideal choice for pure copper turning.
Optimizing tool geometry can significantly improve pure copper turning performance. A large rake angle (15°-25°) should be used to maintain a sharp cutting edge and minimize cutting deformation and tool sticking. A wide, shallow chip flute (8-12mm wide, 0.8-1.2mm deep) should be ground on the rake face to guide chips and prevent them from wrapping around the workpiece. A 45°-60° lead angle balances radial and axial cutting forces and minimizes workpiece deformation. For thin-walled pure copper parts, the lead angle can be increased to 90° to reduce radial forces and prevent workpiece bending. A clearance angle of 8°-12° reduces friction between the tool face and the workpiece. A slightly larger clearance angle is recommended for finish turning and a smaller one for rough turning. A 5°-10° rake angle ensures chip flow toward the surface being machined, preventing scratches on the machined surface. A 0.2-0.5mm radius should be ground at the tool tip to enhance tip strength and reduce surface roughness.
The setting of cutting parameters must take into account both processing quality and efficiency. When turning pure copper, the cutting speed should avoid the medium-speed range (50-80m/min) where built-up edge is prone to occur, and low or high-speed cutting should be selected. When finishing at low speeds (30-50m/min), high-speed steel tools with a feed rate of 0.05-0.1mm/r can achieve lower surface roughness. When cutting at high speeds (150-250m/min), carbide tools with a feed rate of 0.1-0.2mm/r can reduce tool sticking and built-up edge, making it suitable for mass production. The cutting depth is adjusted according to the processing stage. For rough turning, the cutting depth is 2-4mm to quickly remove the excess; for finishing, the cutting depth is 0.1-0.3mm to ensure dimensional accuracy. For example, when turning pure copper bars (50mm in diameter), rough turning uses 150m/min, feed rate 0.2mm/r, and cutting depth 3mm, and fine turning uses 200m/min, feed rate 0.08mm/r, and cutting depth 0.2mm, which can achieve a surface roughness of Ra1.6μm.
Improved cooling and lubrication conditions can effectively suppress built-up edge and tool wear. The cutting fluid for pure copper turning requires excellent lubricity and cooling properties. Kerosene is an ideal choice, forming a lubricating film on the tool surface and reducing adhesion, making it particularly suitable for fine turning. For rough turning, an emulsion (8%-10% concentration) can be used to enhance cooling and reduce tool temperature. The supply of cutting fluid must be sufficient, using a high-pressure jet (3-5 MPa) to spray the cutting fluid directly into the cutting zone, with a flow rate of at least 15 L/min, to ensure timely removal of chips and heat. For high-precision pure copper parts, oil mist lubrication can be used. Compressed air is used to atomize the lubricating oil and deliver it to the cutting zone. This ensures effective lubrication while preventing contamination of the workpiece by the cutting fluid, making it particularly suitable for machining electrical components.
The process for turning pure copper also needs to consider the structural characteristics of the workpiece. For thin-walled pure copper parts (wall thickness <2mm), a soft-jaw chuck or axial clamping device should be used during clamping, with a clamping force controlled at 5-10 MPa to prevent workpiece deformation. A low-melting-point alloy (such as bismuth-tin alloy) can be filled into the workpiece to increase rigidity, which can then be removed by heating after processing. For slender pure copper shafts (aspect ratio >20), a toolholder is required for support. The support block should be made of brass to avoid scratching the workpiece surface, and the contact pressure between the toolholder and the workpiece should be controlled at 2-3N. After processing, pure copper parts require surface treatment, such as light polishing with fine sandpaper (800#-1000#) to remove fine surface burrs and improve surface quality. By combining these measures, efficient and high-quality turning of pure copper parts can be achieved, meeting the requirements of their use in various fields.
