CNC Machining technology of lathe spindle
The lathe spindle is a core component of a lathe. Its precision and performance directly impact the machine’s machining accuracy and operational reliability. Lathe spindles must withstand significant torque and radial forces while maintaining high rotational accuracy. Therefore, their processing is complex, requiring multiple steps and processing to achieve the required dimensional accuracy, form and position tolerances, and surface quality. The lathe spindle manufacturing process typically includes roughing, rough machining, semi-finishing, heat treatment, finishing, and final inspection before assembly.
Blank manufacturing is the first step in lathe spindle machining, and its quality has a significant impact on subsequent processing and the spindle’s ultimate performance. Forgings are typically used for lathe spindle blanks because the forging process refines the grain size and improves the material’s mechanical properties, particularly strength and toughness, to meet the spindle’s requirements for high-speed rotation and load bearing. There are two main forging methods: open die forging and die forging. Die forging is commonly used for small and medium-sized lathe spindles, ensuring dimensional accuracy and shape consistency, reducing the amount of excess required for subsequent machining. Open die forging is often used for large lathe spindles because the larger size of large spindles makes die forging dies more difficult and expensive to manufacture. After forging, the blank undergoes normalizing. Normalizing aims to eliminate internal stresses generated during the forging process, refine the grain size, improve the material’s machinability, and prepare it for subsequent machining. Normalizing process parameters are determined based on the material’s composition. For example, for 45 steel spindle blanks, the normalizing temperature is typically 850-880°C, followed by a period of holding at this temperature and air cooling to achieve a uniform pearlite structure.
The primary task of the roughing stage is to remove most of the stock, initially establishing the basic shape and dimensions of the spindle and laying the foundation for semi-finishing and finishing. Roughing typically includes turning the outer diameter, drilling the center hole, and milling the keyway. When turning the outer diameter, different diameters should be cut in stages based on the spindle’s structural characteristics. The stock removal during each turning should be evenly distributed to minimize cutting force fluctuations and workpiece deformation. Drilling the center hole ensures center positioning during subsequent machining and ensures spindle machining accuracy. The size and accuracy of the center hole should meet design requirements and is typically performed using a dedicated center drill. Milling the keyway, for mounting components such as gears and pulleys on the spindle, requires high position and dimensional accuracy, necessitating a milling machine. A dedicated fixture should be used to ensure parallelism and symmetry between the keyway and the spindle axis. While high precision is not required during the roughing stage, uneven machining stock removal should be minimized to avoid insufficient or excessive stock removal during subsequent machining. At the same time, stress relief treatment should be carried out after rough machining to eliminate the internal stress generated during the machining process and prevent the spindle from deforming during subsequent machining and use.
The semi-finishing stage, building on the roughing process, further improves the spindle’s dimensional accuracy and surface quality, preparing for finishing. Semi-finishing typically includes semi-finishing turning, rough grinding, and spline milling. During semi-finishing turning, the dimensional tolerance and surface roughness of the outer diameter must be controlled to provide a uniform machining allowance for rough grinding. The surface roughness of the outer diameter after semi-finishing turning is generally required to reach Ra 3.2μm. Rough grinding removes the remaining allowance after semi-finishing turning using a grinding wheel, further improving the dimensional and form accuracy of the outer diameter and reducing roundness and cylindricity errors. The surface roughness of the outer diameter after rough grinding can reach Ra 1.6μm. Spline milling is performed on spindles with splines. The machining accuracy of the splines directly affects the fit between the spindle and the spline sleeve. Spline milling is typically performed on a spline milling machine using a dedicated spline milling cutter to ensure the tooth profile accuracy, indexing accuracy, and symmetry of the splines. The semi-finishing stage also requires processing of some important end faces on the spindle, such as the shoulder end face of the spindle. These end faces need to be kept perpendicular to the spindle axis to ensure the assembly accuracy of the parts. End turning tools or end milling cutters are usually used for processing.
Heat treatment is a key process in the lathe spindle processing. Its purpose is to improve the mechanical properties of the spindle, especially the surface hardness and wear resistance, while ensuring that the core has sufficient toughness. Common heat treatment processes for lathe spindles include tempering and surface quenching. Tempering is usually carried out after semi-finishing. For 45 steel spindles, the tempering process is high-temperature tempering after quenching. The quenching temperature is 840-860℃ and the tempering temperature is 580-600℃. Through tempering, the spindle can obtain a uniform tempered bainite structure with good strength and toughness to meet the load requirements of the spindle. Surface quenching is to improve the hardness and wear resistance of the spindle working surface. Induction heating surface quenching is usually used. Induction heating has the advantages of fast heating speed, controllable quenching layer depth, and small deformation. Surface hardening is primarily performed on the spindle’s journal, tapered bore, and other surfaces that mate with bearings or withstand friction. After quenching, the surface hardness should reach HRC 50-55. The depth of the quenched layer is determined by the spindle’s size and operating requirements, typically 1.5-3mm. After surface hardening, low-temperature tempering is required to eliminate quenching stresses and stabilize microstructure and dimensions. The tempering temperature is typically 180-200°C.
The finishing stage is a critical step in ensuring the final accuracy and surface quality of the lathe spindle. Its purpose is to ensure that all spindle precision indicators meet design requirements. Finishing typically includes processes such as fine grinding of the outer diameter, fine grinding of the taper hole, and lapping. Fine grinding of the outer diameter involves grinding the spindle’s outer cylindrical surface with a high-precision grinding wheel to achieve extremely high dimensional and form accuracy. The outer cylindrical dimension tolerance after fine grinding can be controlled to IT5-IT6 levels, with roundness and cylindricity errors not exceeding 0.001-0.003mm, and surface roughness reaching Ra0.8-0.4μm. Fine grinding of the taper hole is performed to ensure the precise fit between the spindle taper hole and the center or tool taper shank. The taper accuracy and surface roughness requirements of the taper hole are relatively high, and a dedicated taper grinding wheel or template device is required during fine grinding to ensure that the taper error of the taper hole does not exceed the specified value. For spindles with extremely high precision requirements, grinding is also required. Grinding can further improve surface roughness to Ra0.1-0.025μm, while also correcting minor shape errors, further improving spindle rotation accuracy. Grinding is usually performed by hand or mechanical grinding, using abrasives and grinding tools. The grinding tool material should be softer than the spindle material, such as cast iron or copper, to ensure the grinding effect.
Final inspection before assembly is the final step in the lathe spindle machining process. Its purpose is to ensure that all spindle specifications meet design requirements and provide qualified parts for assembly. Inspection items mainly include dimensional accuracy inspection, form and position tolerance inspection, surface roughness inspection, and hardness inspection. Dimensional accuracy inspection uses tools such as micrometers and vernier calipers to measure the spindle’s outer diameter and taper bore dimensions. Form and position tolerance inspection uses instruments such as dial indicators, roundness gauges, and cylindricity gauges to measure the spindle’s roundness, cylindricity, and coaxiality. Surface roughness inspection uses a roughness meter or sample comparison method. Hardness inspection uses a Rockwell hardness tester to measure the hardness of the spindle’s surface and core to ensure compliance with design requirements. For spindles that fail inspection, the reasons must be analyzed and appropriate rework measures must be implemented, such as additional processing or reheat treatment. Only spindles that pass inspection can enter the assembly stage to ensure the overall performance and machining accuracy of the lathe.
