Crankshaft turning
The crankshaft is a core component of power machinery such as internal combustion engines and compressors. Its complex structure consists of the main journal, connecting rod journal, crank arm, and other components. Each journal has a certain degree of eccentricity, and it is subjected to periodic alternating loads and torque during operation, thus requiring extremely high CNC machining precision and surface quality. CNC Machining the crankshaft is a key step in ensuring its performance. It involves CNC machining multiple journals with different axis lines, controlling phase angles, and ensuring rigidity. This requires specialized process equipment and appropriate CNC machining methods to meet design requirements.
Process analysis and blank selection before crankshaft turning significantly impact CNC machining quality. Crankshaft blanks are typically manufactured using forging or casting. Forged steel crankshafts feature dense microstructure and excellent mechanical properties, making them suitable for high-speed, heavy-load applications. Ductile iron crankshafts offer lower cost and excellent casting properties, making them suitable for medium- and low-speed engines. Dimensional accuracy and stock allowances must be optimized. CNC Machining allowances for the main and connecting rod journals are typically 3-5mm, while the crank arm thickness allowance is 2-3mm to ensure that subsequent CNC machining can remove surface defects and internal porosity. Before turning, the blanks must be inspected for cracks, inclusions, and other defects. They must also be tempered to maintain a hardness of 220-280 HBW to improve cutting performance and overall mechanical properties. Furthermore, a rational CNC machining route must be developed based on the crankshaft’s structural characteristics (e.g., single-turn vs. multi-turn). Typically, rough turning is performed followed by finish turning, and CNC machining the main journals before the connecting rod journals is performed to minimize the impact of clamping errors on CNC machining accuracy.
CNC Machining the crankshaft main journal is essential for ensuring rotational accuracy, requiring specific control over dimensional accuracy, roundness, and cylindricity. The main journal, the supporting portion of the crankshaft and mating with the bearing, typically has a diameter tolerance of IT6-IT7, with roundness and cylindricity errors controlled within 0.005-0.01mm, and a surface roughness Ra of 0.8-1.6μm. When turning the main journal, a double-thimble clamping method is typically used, with center holes at both ends used for positioning to ensure the coaxiality of each main journal. For long crankshafts, a steady rest or steady rest is required for auxiliary support to prevent bending and deformation during CNC machining. For rough turning, carbide tools are used with a cutting speed of 80-120 m/min, a feed rate of 0.2-0.3 mm/r, and a cutting depth of 2-3 mm to quickly remove CNC machining stock. For finish turning, ultrafine-grain carbide or CBN tools are used with a cutting speed of 150-200 m/min, a feed rate of 0.1-0.15 mm/r, and a cutting depth of 0.5-1 mm to ensure surface quality. High-pressure cutting fluid is used during turning to cool and lubricate the cutting area, minimizing tool wear and thermal deformation of the workpiece.
CNC Machining the crankshaft connecting rod journal is challenging due to the eccentricity between the journal and the strict phase angle requirements between the journals. The connecting rod journal mates with the connecting rod big end and transmits torque. Its dimensional tolerances are IT6-IT7, with a roundness error of no more than 0.005mm and a parallelism error within 0.01mm/m. CNC Machining the connecting rod journal requires the use of a dedicated eccentric fixture or the C-axis function of a CNC lathe to achieve eccentric CNC machining. For multi-turn crankshafts, an indexing mechanism is also required to ensure the phase angle accuracy of each connecting rod journal (generally ±15′). During clamping, the machined main journal is used for positioning. By adjusting the eccentricity of the eccentric fixture (equal to the eccentricity between the connecting rod journal and the main journal), the connecting rod journal axis is aligned with the lathe spindle axis. During rough turning, use high cutting parameters to remove most of the stock. During finish turning, strictly control cutting forces to avoid deformation caused by insufficient crank arm rigidity. A reverse cutting method (feeding the tool from the spindle journal toward the crank arm) can be used to offset some of the bending moment with cutting forces. For large crankshafts, floor-type lathes or specialized crankshaft lathes equipped with specialized tools and support devices can be used to ensure stability during the CNC machining process.
CNC Machining of crankshaft crank arms primarily involves CNC machining their sides and ends, ensuring perpendicularity and positional accuracy with the main journal and connecting rod journals. The crank arm connects the main and connecting rod journals, and the perpendicularity error between the sides and the main journal axis must be controlled within 0.02mm/m, while the flatness error of the end faces must not exceed 0.03mm. When turning the crank arm, a disc turning tool or a profile cutter can be used, positioned with the main journal or pre-machined surface, and excess material removed using radial feed. For rough turning, a cutting depth of 1-2mm and a feed rate of 0.2-0.3mm/rev are recommended for improved efficiency. For finish turning, a cutting depth of 0.3-0.5mm and a feed rate of 0.1-0.15mm/rev are recommended to ensure surface quality. For crank arms on multi-turn crankshafts, the thickness difference between each arm must not exceed 0.1mm to ensure dynamic balance during crankshaft rotation. After turning, the crank arm must be deburred to prevent scratching other parts during assembly.
Precision inspection and quality control of crankshaft turning are the final steps to ensure its performance. After CNC machining, the diameter, roundness, and cylindricity of the main journal and connecting rod journal need to be inspected. These can be measured using tools such as micrometers and roundness meters. Phase angle accuracy requires testing with a dedicated dividing head or a three-dimensional coordinate measuring machine. Surface roughness can be measured using a roughness meter. For high-speed crankshafts, dynamic balancing tests are also required to remove imbalances and avoid severe vibration during operation. Common quality issues and solutions: If the journal roundness is out of tolerance, it may be due to wear on the center hole or unstable support. The center hole needs to be re-grinded or the support device needs to be adjusted. If the phase angle error is too large, the accuracy of the indexing mechanism needs to be checked and calibrated. If chatter marks appear on the surface, it may be caused by unreasonable cutting parameters or tool wear. The cutting speed and feed rate need to be adjusted, or the tool needs to be replaced. Through strict inspection and quality control, the crankshaft turning quality can be ensured to meet the design requirements, providing a guarantee for the reliable operation of power machinery.
