CNC Machining of Large Inner Tapered Bushings Made of Copper Alloy
Large copper alloy internal tapered sleeves are key components in heavy machinery, ships, and chemical equipment. They are used to achieve centering fit or sealed connections between shafts and holes. Their inner walls are conical, and the taper accuracy and surface quality directly impact assembly precision and sealing performance. Copper alloys offer excellent plasticity, thermal conductivity, and corrosion resistance, but their strength is relatively low (tensile strength 200-400 MPa). CNC Machining large internal tapered sleeves (diameter > 300 mm, length > 500 mm) presents challenges such as insufficient rigidity, easy deformation, and difficult dimensional control. Dedicated processing equipment and appropriate CNC machining methods are required to ensure that the taper error, roundness, and surface roughness of the conical surface meet the required standards.
The material properties of large copper alloy internal tapered sleeves significantly influence the turning process, necessitating tailored cutting strategies. Common copper alloys include brass (such as H62), bronze (such as ZCuSn10Pb1), and nickel silver (such as B30). Brass offers excellent cutting performance, with easy chip breakage and excellent surface quality. Bronze with a high tin content results in higher cutting forces, which can easily lead to built-up edge. Nickel silver, with its higher strength, requires higher cutting forces. The thermal conductivity of copper alloys is two to three times that of steel, allowing for easy dissipation of cutting heat. Therefore, higher cutting speeds are acceptable. However, their high plasticity (elongation of 10%-30%) makes them susceptible to tool sticking, resulting in surface tearing or burrs. These characteristics require the use of sharp tools, controlled cutting speeds and feed rates, to avoid built-up edge, and enhanced cooling and lubrication to reduce friction and tool sticking.
The clamping method for large copper alloy internal tapered sleeves requires a balance between positioning accuracy and rigid support. Due to the large size and heavy weight of the workpiece (up to hundreds of kilograms), specialized fixtures are required, such as a four-jaw single-action chuck in conjunction with a steady rest or steady rest. The four-jaw chuck is used to align and clamp the workpiece, while the steady rest or steady rest increases workpiece rigidity and prevents bending during CNC machining. Before clamping, the workpiece’s outer diameter and end faces must be rough-machined to serve as a positioning reference. During alignment, a dial indicator should be used to check radial runout of the outer diameter and axial runout of the end faces, ensuring an error of ≤0.05mm. For thin-walled internal tapered sleeves (wall thickness <10mm), elastic jaws or copper padding between the jaws and the workpiece are required to increase the contact area, reduce the clamping force per unit area, and prevent plastic deformation of the workpiece. Balancing is required after clamping, especially during high-speed turning (speeds > 300 rpm). Unbalance should be controlled to within 0.1mm to minimize the impact of vibration on CNC machining accuracy.
The turning process for large copper alloy internal tapered sleeves requires a phased approach to gradually control the tapered surface accuracy. A typical process is: forging or casting the blank → annealing (stress relief) → rough turning the outer diameter and end faces → rough turning the inner bore (reserving a 3-5mm allowance) → aging (relieve CNC machining stress) → semi-finish turning the outer diameter and end faces (as a positioning reference) → semi-finish turning the inner tapered surface (reserving a 0.5-1mm allowance) → final aging → finish turning the inner tapered surface → inspection. During the rough turning stage, carbide tools are used to quickly remove most of the excess material, with a cutting speed of 100-150 m/min, a feed rate of 0.2-0.3 mm/r, and a cutting depth of 2-5 mm. During the semi-finishing turning stage, to correct form errors, the cutting speed is increased to 150-200 m/min, a feed rate of 0.1-0.2 mm/r, and a cutting depth of 0.5-1 mm. To ensure final accuracy during the finishing stage, high-speed steel tools or coated carbide tools are used, with a cutting speed of 200-300 m/min, a feed rate of 0.05-0.1 mm/r, and a cutting depth of 0.1-0.3 mm. Continuous cutting is required during finishing, avoiding stops and preventing tool marks on the workpiece surface.
Tool selection and geometric design for turning large internal tapered sleeves in copper alloys must be optimized to improve surface quality. Rough turning tools should be made of carbide (such as YG8) with a rake angle of 15°-20°, a clearance angle of 8°-12°, and a lead angle of 45°-60° to reduce cutting forces and vibration. Finishing tools should be made of high-speed steel (such as W18Cr4V) or coated carbide (such as TiN coating), with a rake angle of 20°-25° and a clearance angle of 10°-15°. The cutting edge should be sharp and free of chipping, with a roughness of Ra ≤ 0.025μm to minimize tool sticking and surface tearing. The tool’s rake angle should be between -5° and 0° to control chip flow and prevent chips from scratching the machined surface. For wide-blade finishing tools (used for final CNC machining of conical surfaces), the blade width should be determined based on the length of the conical surface, generally 10-20mm. The blade straightness error should be ≤0.01mm/100mm to ensure the straightness of the conical surface. The toolholder should have a rigid rectangular cross-section (30×40mm or larger), with an extension length no more than three times the toolholder height to minimize toolholder deformation during cutting.
Precision inspection and quality control during the turning of large copper alloy internal tapered sleeves are critical to ensuring part performance. The taper error of the conical surface should be checked using a taper template or a universal angle gauge. For high precision requirements, a sine rule and a dial indicator should be used. The taper error should be ≤0.001mm/mm (i.e., the error should not exceed 0.001mm per millimeter of length). Roundness error should be checked with a roundness tester and should be ≤0.01mm. Surface roughness should be measured with a roughness tester, with an Ra value of ≤1.6μm. The surface roughness of sealing internal tapered sleeves should be ≤0.8μm. If the taper tolerance is exceeded after CNC machining, it may be due to inaccurate tool feed angles or machine guideway errors, requiring tool readjustment or machine calibration. If scratches or tool sticking appear on the surface, sharpen the tool, increase the cutting speed, or increase the cutting fluid concentration. If the workpiece deforms, increase the number of aging treatments or optimize the clamping method. Through strict quality control and process optimization, high-precision turning of large copper alloy inner tapered sleeves can be achieved to meet the use requirements of heavy machinery and large equipment.
