Turning of hardened bearing sleeves
Quenched bearing sleeves are key components in mechanical transmissions, requiring high hardness and wear resistance on both inner and outer surfaces. They are typically made from bearing steel (such as GCr15) after quenching, achieving a hardness of HRC 60-65. This presents significant challenges in turning. The difficulties in turning quenched bearing sleeves primarily stem from the high material hardness, high cutting forces, and rapid tool wear. The high material hardness creates a dramatic increase in pressure and temperature at the tool edge during cutting, making it difficult for ordinary high-speed steel tools to withstand. Even carbide tools experience wear rates far higher than those used in machining unquenched steel. Furthermore, quenched bearing sleeves require a small machining allowance, typically between 0.5 and 1 mm, and demand high dimensional accuracy and surface quality. The roundness error of the inner hole must be controlled within 0.005-0.01 mm, and the surface roughness must reach Ra 0.8-1.6 μm. This places even higher demands on machining technology and tool performance.
Tool selection for turning hardened bearing sleeves is crucial. Tool materials with high hardness, high wear resistance, and good heat resistance are essential. Commonly used tool materials include cubic boron nitride (CBN) tools, ceramic tools, and ultrafine-grain carbide tools. CBN tools are ideal for machining hardened bearing sleeves. With hardness reaching HV3000-5000, they offer excellent wear resistance and can maintain good cutting performance at high temperatures, making them suitable for high-speed finish turning of the internal and external surfaces of hardened bearing sleeves. Ceramic tools (such as Al2O3-based ceramics and Si3N4-based ceramics) also offer high hardness (HV1800-2500) and are more affordable than CBN tools. However, they are more brittle and are suitable for continuous cutting, but not for intermittent cutting or machining materials with hard spots. Ultrafine-grain carbide tools (such as WC-Co alloys with grain sizes less than 1μm) offer high hardness and toughness and are suitable for semi-finishing and finish turning of hardened bearing sleeves. Their cutting speeds and wear resistance are intermediate between those of ceramic tools and CBN tools.
Tool geometry significantly impacts the turning quality and efficiency of hardened bearing sleeves. The goal should be to reduce cutting forces, lower cutting temperatures, and maximize tool life. The rake angle is generally between -5° and 0°. A smaller rake angle, or even a negative rake angle, strengthens the tool’s cutting edge and prevents chipping when cutting high-hardness materials. The clearance angle is 6° to 10° to reduce friction between the tool’s flank and the workpiece surface while ensuring heat dissipation. The lead angle ranges from 45° to 90°. A 90° lead angle produces low radial cutting forces, making it suitable for machining thin-walled hardened bearing sleeves and preventing workpiece deformation. A 45° lead angle provides better heat dissipation, making it suitable for machining thick-walled bearing sleeves. The tool nose radius is generally between 0.2 and 0.5 mm. A smaller nose radius reduces cutting forces and heat, but increases surface roughness. The tool should be appropriately selected based on the required machining accuracy. Furthermore, the tool’s cutting edge should be properly passivated, typically with a radius of 0.01 to 0.03 mm, to improve its impact resistance.
The cutting parameters for hardened bearing sleeves must be optimized based on the tool material and machining requirements to maximize efficiency while ensuring machining quality. Regarding cutting speed, CBN tools can be used for hardened bearing sleeves at 80-150 m/min; ceramic tools at 60-100 m/min; and ultrafine-grain carbide tools at 30-60 m/min. The feed rate should be kept low, generally 0.05-0.15 mm/r. This reduces cutting forces and surface roughness, helping to ensure machining accuracy. The depth of cut is determined based on the machining allowance; it should be 0.3-0.5 mm for semi-finish turning and 0.1-0.2 mm for finish turning to avoid excessive depth of cut, which can lead to increased tool wear or workpiece deformation. Adequate cooling and lubrication are essential during machining. Extreme-pressure emulsions or extreme-pressure cutting oils are typically used. A high-pressure cooling system sprays the cutting fluid directly onto the cutting area, effectively lowering cutting temperatures, minimizing tool wear, and preventing burns on the workpiece surface.
The turning process and operating techniques for hardened bearing sleeves are also crucial for ensuring machining quality. Before machining, the hardened bearing sleeve blank must be inspected to measure its hardness and deformation. Excessive deformation requires prior straightening or increased machining allowance. When clamping the workpiece , a rigid fixture should be used, such as a three-jaw self-centering chuck with a center. For thin-walled bearing sleeves, soft jaws or specialized fixtures can be used to avoid excessive clamping force that could cause workpiece deformation. The machining sequence should follow the principle of “interior first, exterior second, roughing first, finishing second.” The inner hole should be machined first, and then the outer diameter should be machined using the machined inner hole as a reference to ensure coaxiality between the inner and outer diameters. During finish turning, all surfaces should be machined in a single clamping to minimize clamping errors. During machining, tool wear should be closely monitored and workpiece dimensions should be measured regularly. If tool wear or dimensional deviations are detected, the tool should be replaced or the cutting parameters adjusted promptly. After machining, the workpiece should be cleaned and inspected, and dimensional accuracy, geometric tolerances, and surface quality should be checked to ensure compliance with design requirements. High-quality turning of quenched bearing sleeves can be achieved through reasonable tool selection, cutting parameter setting and process control.
