Processing technology of screw and crankshaft
As key components in mechanical transmission, the processing technology of lead screws and crankshafts directly affects the transmission accuracy and operational stability of the equipment. The lead screw is the core component that converts rotational motion into linear motion and is widely used in machine tool feed systems, precision workbenches, etc. The crankshaft is the “heart” of the engine, converting reciprocating motion into rotational motion through the connecting rod and bearing periodic impact loads. Although the two have different functions, they both require high dimensional accuracy, form and position accuracy, and surface quality. The thread accuracy of the lead screw must reach C3-C5 level, and the roundness error of the crankshaft main journal and connecting rod journal should be controlled within 0.005mm. The practice of a certain machine tool factory has shown that the optimization of the lead screw processing technology can increase the positioning accuracy of the machine tool from 0.01mm/m to 0.005mm/m, significantly improving the dimensional consistency of the processed parts.
The machining process for lead screws must prioritize thread accuracy and wear resistance. Lead screws are typically made of 45 steel, 40Cr, or 9Mn2V. For high-precision lead screws, alloy tool steels such as CrWMn are used. The typical process is: forging → annealing → rough turning → quenching and tempering → semi-finish turning → stabilization → thread finishing → quenching → rough grinding → aging → thread finishing → super-finishing. The quenching and tempering treatment creates a uniform tempered bainite structure and maintains a hardness of 220-250 HB. After quenching, the lead screw surface hardness must reach 58-62 HRC to ensure wear resistance. Stabilization and aging treatments eliminate internal stresses and prevent deformation after machining. Thread machining is crucial for lead screw manufacturing. Precision lead screws typically use whirl milling for rough thread production, followed by fine grinding on a thread grinder to maintain a lead error within 0.01mm/300mm. When a precision instrument factory processed ball screws, it added a super-finishing process after fine grinding the threads, reducing the thread surface roughness from Ra0.8μm to Ra0.1μm, thereby reducing the friction loss between the ball and the thread raceway.
Crankshaft machining requires addressing complex shapes and uneven stress distribution. Crankshafts are typically made of ductile iron (such as QT800-2) or medium-carbon steel (such as 45 steel), with larger crankshafts utilizing alloy structural steel (such as 42CrMo). The machining process follows: casting or forging → annealing or normalizing → rough turning → quenching and tempering → semi-finish turning → flaw detection → quenching → rough grinding → aging → fine grinding → dynamic balancing. Crankshafts feature multiple journals, eccentricity, and oil holes. Rough turning requires the use of specialized fixtures to ensure accurate center-to-center distances between the main journals and connecting rod journals. After semi-finish turning, magnetic particle inspection or penetrant testing is performed to check for cracks and other defects. Induction heating is used for hardening the main and connecting rod journals, achieving a surface hardness of 50-55 HRC and a hardened layer depth of 2-5 mm. When machining crankshafts, an engine factory used the turning center of a CNC lathe to complete the turning of multiple journals in one clamping, so that the parallelism error between the main journal and the connecting rod journal was controlled within 0.01mm, laying a good foundation for the subsequent grinding process.
The key machining processes for lead screws and crankshafts each focus on different technologies. Lead screw thread machining methods include turning, milling, and grinding. Turning is suitable for low-precision lead screws, using multiple passes of a thread turning tool. Milling (especially cyclone milling) is highly efficient and suitable for mass production of medium-precision lead screws. Grinding is the final finishing method for high-precision lead screws, ensuring the lead and profile accuracy of the threads through the synchronized motion of the grinding wheel and workpiece. Crankshaft journal machining primarily relies on grinding. Rough grinding removes the post-quenching oxide layer, while fine grinding ensures dimensional accuracy and roundness. For high-speed engine crankshafts, the journals are also polished or super-finished to reduce surface roughness. An automotive engine manufacturer uses a dedicated crankshaft grinder, utilizing a CBN grinding wheel for high-speed grinding. This achieves a roundness error of 0.002mm and a surface roughness of Ra 0.05μm for connecting rod journals, reducing engine vibration and noise during operation.
Quality inspection and process optimization of lead screws and crankshafts are carried out throughout the entire manufacturing process. Screw inspection items include: thread lead error (using a lead gauge), thread half angle (using a universal tool microscope), pitch diameter (using a thread micrometer), and full-length straightness (using a plate and dial indicator). Crankshaft inspection items include: main journal and connecting rod journal diameters (using an outside micrometer), roundness and cylindricity (using a roundness gauge), parallelism between journals (using a length gauge), and dynamic balancing (using a dynamic balancing machine). Regarding process optimization, a heavy industry company simulated the crankshaft forging process through finite element analysis, optimizing the forging temperature and deformation, resulting in a more uniform internal structure and a 20% increase in fatigue life. A machine tool manufacturer, by monitoring stress during lead screw processing and adjusting the aging treatment temperature and time, improved the long-term accuracy of the lead screw by 30%. With the application of intelligent manufacturing technology, the processing of lead screws and crankshafts is gradually becoming digital and intelligent. Through real-time monitoring and adaptive control of the processing process, the stability of product quality is guaranteed, providing reliable core parts for high-end equipment manufacturing.
