CNC Machining of quenching sleeve
Quenched sleeves are quenched sleeves with high hardness and wear resistance. They are widely used in mechanical transmissions, bearing supports, and other applications. Due to their high hardness (typically above HRC50), turning them is challenging, leading to rapid tool wear, poor surface finish, and difficulty maintaining precision. Therefore, turning quenched sleeves requires the use of appropriate tool materials, cutting parameters, and machining processes to improve efficiency and quality.
The choice of tool material is crucial for turning hardened sleeves. Tool materials with high hardness, high wear resistance, and good heat resistance must be selected. Commonly used tool materials include cubic boron nitride (CBN) tools, ceramic tools, and carbide tools. CBN tools are the preferred tool material for machining hardened steel. They have a hardness of up to HV3000-5000, excellent wear resistance, and can maintain excellent cutting performance at high temperatures. Cutting speeds can reach 80-200 m/min, making them suitable for finish and semi-finish turning of hardened sleeves. Ceramic tools also have a high hardness (HV1800-2200), good wear and heat resistance, and are more affordable than CBN tools. They are suitable for rough and semi-finish turning of hardened sleeves. However, ceramic tools are brittle, have poor impact resistance, and are prone to chipping during cutting. Therefore, they are not suitable for intermittent cutting or when the blank surface is uneven. Among cemented carbide tools, tungsten-cobalt-titanium (YT) and tungsten-cobalt (YG) tools are not suitable for machining hardened steel, while ultrafine-grained cemented carbide tools (such as YM051 and YM052) have higher hardness and wear resistance and can be used to machine lower hardness quenching sleeves (HRC50-55), but the cutting speed is low, generally not exceeding 50m/min.
The proper selection of cutting parameters, primarily including cutting speed, feed rate, and back-cut depth, has a significant impact on the turning quality and efficiency of quenched sleeves. Cutting speed is a key factor influencing tool life and surface quality. For CBN tools, the typical cutting speed for quenched sleeves is 80-150 m/min. Excessively high cutting speeds can cause tool temperatures to rise sharply, exacerbating wear. Excessively low cutting speeds can reduce machining efficiency and easily cause built-up edge, impacting surface quality. The feed rate should be determined based on machining accuracy and surface quality requirements. For fine turning, the feed rate is generally 0.05-0.1 mm/r to ensure a low surface roughness (Ra 0.8-1.6 μm). For rough turning, the feed rate can be increased to 0.1-0.2 mm/r, but the maximum feed rate should not exceed 0.2 mm/r, as this increases cutting forces and can lead to tool chipping. The selection of back cutting depth needs to consider the hardness and allowance of the quenching sleeve. The back cutting depth is generally 0.1-0.5mm for rough turning and 0.05-0.1mm for fine turning. Due to the high hardness of the quenching sleeve, excessive back cutting depth will increase the cutting force, which is easy to cause vibration and tool damage.
The turning process for quenched sleeves should be rationally arranged based on the required precision and heat treatment conditions. A “rough turning – quenching – finish turning” process is generally adopted. Rough turning, performed before quenching, primarily removes most of the excess stock, initially establishing the part’s shape and dimensions, and preparing for finish turning after quenching. High-speed steel or carbide tools can be used for rough turning, and cutting parameters can be relaxed appropriately to improve processing efficiency. Quenching can cause deformation in sleeve parts, so straightening or aging treatment is required after quenching to eliminate internal stresses and minimize deformation during finish turning. Finish turning is crucial for ensuring the precision and surface quality of quenched sleeves. It should be performed after quenching, using CBN or ceramic tools to achieve the final dimensions and surface finish as specified in the drawing. For quenched sleeves with higher precision requirements, grinding is also required after finish turning. However, for sleeves with less stringent precision requirements or complex shapes, finish turning can directly achieve the required results. During turning, the number of clamping operations should be minimized to avoid inaccuracies caused by clamping errors.
Cooling and lubrication are crucial elements in the turning process of quench sleeves. Proper cooling and lubrication can lower cutting temperatures, minimize tool wear, and improve surface quality. Due to the high hardness of quench sleeves, significant cutting heat is generated during cutting, necessitating the use of a cooling lubricant with excellent cooling properties, such as an extreme pressure emulsion or extreme pressure cutting oil. Extreme pressure emulsions offer excellent cooling and lubrication properties, effectively reducing cutting temperatures. They also contain extreme pressure additives that form a lubricating film under high temperature and pressure, reducing friction between the tool and the workpiece. A sufficient supply of cooling lubricant should be supplied directly to the cutting area via a high-pressure nozzle to ensure adequate cooling of both the tool and the workpiece. While CBN tools offer excellent heat resistance, proper cooling can extend tool life, so dry cutting is not recommended. Furthermore, the cleanliness of the cooling lubricant is crucial and should be filtered regularly to remove impurities and chips to prevent scratches on the workpiece surface.
Controlling vibration during the turning process of quench sleeves is crucial to machining quality. Due to the high hardness of quench sleeves and the high cutting forces, vibration is easily generated, leading to surface ripples, dimensional inaccuracies, and tool damage. To reduce vibration, the lathe’s rigidity must be ensured. Regularly inspect components such as the spindle and guideways to ensure clearances are within acceptable limits and adjust or repair them as necessary. Tool rigidity is also crucial. Tool shanks with larger cross-sections should be selected, and tool extension lengths should be shortened to increase tool rigidity and reduce vibration. When clamping the quench sleeve, ensure a secure and reliable clamping. For thin-walled quench sleeves, specialized fixtures or additional supports can be used to prevent deformation and vibration. Proper selection of cutting parameters can also reduce vibration, such as using a lower feed rate and appropriate cutting speed to avoid excessive cutting forces. If vibration occurs during turning, adjust the cutting parameters or inspect the tool and clamping immediately. Continue machining only after the vibration has subsided.
