Machining technology of six-turn crankshaft
As a core component of multi-cylinder internal combustion engines and compressors, six-turn crankshafts are characterized by complex structures, poor rigidity, and high machining precision. Their machining process involves multiple types of work and complex procedures, requiring strict quality control at every stage to ensure the crankshaft’s performance. The six-turn crankshaft’s structure features six crank throws (composed of the main journal, connecting rod journal, and crank arm), each with a strict phase angle relationship (typically 60°). Therefore, the machining process requires guaranteed dimensional accuracy, form and position accuracy, and phase angle accuracy for each journal.
Blank selection and forging are the first steps in the six-turn crankshaft manufacturing process, directly impacting subsequent machining quality and the crankshaft’s mechanical properties. Because six-turn crankshafts are subject to heavy alternating and impact loads, materials typically used are 45 steel, 40Cr, or ductile iron. 40Cr, after quenching and tempering, achieves high strength and toughness, making it suitable for medium- and high-speed internal combustion engines. The blank is formed by die forging, using a hot die forging press or hammer forging. This distributes the metal along the crankshaft’s contour, refines the grain size, and improves the material’s density and mechanical properties. After forging, the blank undergoes normalizing to eliminate forging stresses, reduce hardness (to a range of 180-220 HBW), and improve machinability. Non-destructive testing (such as magnetic particle inspection) is also required to inspect the blank for forging defects, such as cracks and folds, to ensure acceptable blank quality.
The primary task of the rough machining phase is to remove significant machining excess and initially establish the basic structure of the crankshaft. Rough machining operations include milling the end faces, drilling the center hole, rough turning the main journal and connecting rod journals, and rough milling the crank arms. A dedicated face milling machine is used to mill the end faces, ensuring they are parallel and perpendicular to the axis, providing a reference for subsequent center hole machining. A dedicated center drill is used to drill the center hole, ensuring the required depth and diameter to facilitate subsequent double-thimble positioning. Rough turning of the main journal and connecting rod journals is performed on a CNC lathe or a dedicated crankshaft lathe. Because six-turn crankshafts have multiple eccentric connecting rod journals, an eccentric fixture or CNC indexing device is used to ensure accurate phase angles for each connecting rod journal. Cutting parameters for rough turning include a cutting speed of 80-100 m/min, a feed of 0.3-0.5 mm/r, a depth of cut of 3-5 mm, and a journal diameter 5-8 mm larger than the design size. The crank arm is rough-milled using a vertical milling machine to remove the excess on both sides of the crank arm to ensure that the crank arm thickness meets the process requirements.
Semi-finishing and heat treatment are key steps in improving the precision and performance of six-turn crankshafts. Semi-finishing includes semi-finishing the main and connecting rod journals, drilling oil holes, and milling keyways. For semi-finishing the journals, carbide tools are used at a cutting speed of 100-120 m/min and a feed rate of 0.15-0.25 mm/r. The journal diameter is machined to 1-2 mm larger than the design size, with a surface roughness of Ra 3.2 μm. Oil holes are drilled using a deep-hole drill. The position and angle of the oil holes must be precisely controlled to ensure that the lubricant reaches all friction surfaces. After drilling, burrs are removed and chamfers are applied. Heat treatment includes tempering and surface hardening. Tempering involves heating the crankshaft to 850-860°C, holding the temperature, then oil cooling, and finally tempering at 550-600°C to a hardness of 220-250 HBW, ensuring the crankshaft’s excellent overall mechanical properties. Surface hardening of the main and connecting rod journals is performed using induction hardening, achieving a quenching depth of 2-3mm and a surface hardness of 50-55HRC, enhancing journal wear resistance. Straightening is performed after heat treatment using a pressure straightening method to maintain crankshaft straightness within 0.1mm/m.
The finishing stage is the core process for ensuring the ultimate precision of the six-turn crankshaft. It primarily involves fine grinding of the main and connecting rod journals, as well as precision milling of the locating surfaces. This process is performed on a dedicated crankshaft grinder using a grinding wheel. The main journal is positioned using the center holes at both ends, while the connecting rod journal requires an eccentric fixture. A CNC system controls the grinding wheel feed and workpiece indexing, ensuring dimensional accuracy (IT6 grade), roundness (0.005mm), and cylindricity (0.008mm) for each journal. Grinding parameters include a grinding wheel speed of 30-40m/s, a workpiece speed of 100-150r/min, a feed rate of 0.01-0.02mm/r, and a surface roughness of Ra 0.4μm. Precision milling of the locating surfaces is performed on a CNC milling machine to ensure perpendicularity and flatness between the locating surfaces and the main journal, providing a reference for subsequent assembly. After finishing, the crankshaft needs to be dynamically balanced to remove the imbalance and achieve a balancing accuracy level of G2.5 to avoid vibration during high-speed rotation.
Final inspection and surface treatment are the final steps in the six-turn crankshaft processing. Final inspection items include the dimensional accuracy, form and position accuracy (such as the parallelism and coaxiality of the main and connecting rod journals), surface roughness, hardness, and phase angle of each journal. Precision measuring instruments (such as micrometers and dial indicators) and coordinate measuring machines are used for testing to ensure that all indicators meet design requirements. Surface treatment includes cleaning, deburring, and rust prevention. Cleaning uses an ultrasonic cleaner to remove surface oil and chips. Deburring is done manually or with specialized tools to remove sharp edges and burrs. Rust prevention involves applying anti-rust oil or phosphating to prevent rust during storage and transportation. Six-turn crankshafts that pass inspection are packaged, labeled with the model, specifications, and production date, and stored for assembly. Through rigorous processing and quality control, six-turn crankshafts meet the equipment’s requirements for transmission accuracy, load capacity, and service life.
