Precautions for crankshaft turning
As a core transmission component in internal combustion engines, compressors, and other equipment, the crankshaft’s turning quality directly impacts the overall machine’s operating accuracy and service life. The crankshaft’s complex structure, comprising multiple components such as the main journal, connecting rod journal, and crank arm, is also eccentric, making it prone to vibration and deformation during turning. Therefore, a series of precautions must be strictly adhered to. From pre-processing of the roughcast to final inspection, meticulous attention to detail is crucial at every stage; even the slightest oversight can lead to defects such as dimensional deviations, substandard surface quality, and even equipment failure.
The choice of clamping method is paramount when turning crankshafts. It should be selected based on the crankshaft’s structure and size to ensure machining stability and precision. For small and medium-sized crankshafts, a dual-thimble positioning method combined with a steady rest can be used. Center holes at both ends ensure rotational accuracy, while the steady rest effectively offsets radial deflection caused by cutting forces, reducing workpiece vibration. Large crankshafts or those with multiple turns, due to their reduced rigidity, require a four-jaw single-action chuck combined with a steady rest. The steady rest is supported on the main or process journal, and precise alignment is performed to ensure that the crankshaft axis coincides with the lathe spindle axis. Careful control of clamping force is crucial during clamping. Excessive clamping force can cause plastic deformation in the crankshaft, while too little can cause vibration due to workpiece loosening. It is recommended to use a hydraulic or pneumatic chuck for uniform clamping. A dial indicator should be used to monitor the workpiece’s radial runout in real time to ensure that the clamping error is within 0.02mm.
Optimizing cutting parameters significantly impacts crankshaft turning quality and requires targeted adjustments based on the crankshaft material, tool performance, and machining stage. Common crankshaft materials are 45 steel, 40Cr, or ductile iron. 40Cr can achieve a hardness of 220-250 HBW after tempering, requiring high cutting speeds and reasonable feed rates during turning. The rough turning stage, focused on stock removal, employs a larger depth of cut (3-5mm) and a medium feed rate (0.2-0.3mm/r), with cutting speeds controlled at 80-120m/min to improve machining efficiency. The finish turning stage prioritizes precision, reducing the depth of cut to 0.5-1mm, the feed rate to 0.1-0.15mm/r, and the cutting speed to 120-180m/min. High-speed cutting reduces tool-workpiece contact time and reduces surface roughness. For ductile iron crankshafts, due to their greater brittleness, the cutting speed should be appropriately reduced by 10%-20% to avoid chip breakage and damage to the processed surface.
Tool selection and sharpening must be tailored to the machining characteristics of the crankshaft. Tool rigidity and wear resistance are crucial, especially for eccentric areas such as connecting rod journals. For rough turning, carbide tools (such as YT15 and YG8) are recommended. These tools offer high strength and wear resistance, and can withstand heavy cutting loads. For finish turning, coated carbide tools (such as TiAlN coatings) or high-speed steel tools are recommended. The low friction coefficient of coated tools reduces built-up edge, while the sharp cutting edge of high-speed steel tools ensures a surface roughness of less than Ra1.6μm. Tool geometry must be optimized, with a rake angle of 5°-10° to reduce cutting forces, a back angle of 8°-12° to minimize flank friction, and a nose radius of 0.8-1.2mm to ensure blade strength while avoiding excessive cutting resistance. The symmetry of the main and secondary cutting edges must be ensured during sharpening. The straightness error of the cutting edge should be checked under a tool microscope to ensure it does not exceed 0.01mm, and the radius of the cutting edge should be controlled within 0.02-0.05mm to prevent gnawing during processing.
Deformation control is a key challenge in crankshaft turning. Process measures are required to minimize deformation caused by internal stress release and cutting heat. Residual internal stresses remain after forging or quenching and tempering, so crankshafts should be aged before turning. Natural aging requires at least 24 hours, while artificial aging uses a temperature of 550°C for 4 hours to fully release these stresses. A phased stock removal approach is employed during machining. After rough turning, a 2-3mm stock is retained for semi-finish turning. After semi-finish turning, aging treatment is performed to eliminate machining stresses, followed by finish turning. Multiple aging cycles are then performed to minimize final deformation. Thermal deformation caused by cutting heat can be addressed by strengthening the cooling system. A dual-nozzle high-pressure cooling system is recommended, spraying cutting fluid (extreme pressure emulsion, 8%-10% concentration recommended) directly onto the cutting area at a flow rate of at least 20L/min to ensure the cutting temperature remains below 200°C. Critical dimensions should be measured immediately after finish turning, and rechecked after the workpiece cools to room temperature (approximately 2 hours) to avoid measurement errors caused by thermal deformation.
Quality inspection and error compensation mechanisms are the last line of defense for ensuring crankshaft turning accuracy, requiring a multi-step inspection system. After rough turning, the dimensional tolerances and eccentricity of the main and connecting rod journals must be checked. Dimensions are measured using a vernier caliper or micrometer, while eccentricity is checked using a dedicated gauge or a coordinate measuring machine to ensure an error of no more than 0.1mm. After semi-finish turning, form and position tolerance testing is added, using a dial indicator to check the radial runout of the main journal (≤0.03mm) and the parallelism of the connecting rod journal (≤0.02mm/m). After finish turning, a comprehensive inspection is conducted, including surface roughness (Ra ≤0.8μm), hardness (based on material requirements), and phase angle (phase angle error between adjacent turns of a multi-turn crankshaft is ≤±15′). Errors discovered during inspection can be adjusted using tool compensation. For example, if the main journal diameter is 0.01mm smaller, a radius compensation of +0.01mm can be set in the CNC system. In addition, the lathe needs to be calibrated regularly, especially the spindle rotation accuracy and guide rail straightness, to ensure that the error of the machine tool itself does not affect the crankshaft processing quality.
