Clamping method for turning crankshaft
The crankshaft is a core component in the engine. Its complex structure, consisting of the main journal, connecting rod journal, crank arm, and other components, also has a large eccentricity, making clamping during turning challenging. A reasonable clamping method not only ensures machining accuracy but also improves machining efficiency, avoiding workpiece deformation and machining errors caused by improper clamping. The clamping method for crankshaft turning depends on the crankshaft’s structure, size, and machining stage. Commonly used methods include double-thimble clamping, eccentric chuck clamping, special fixture clamping, faceplate clamping, and combined clamping.
The double-thimble clamping method is a commonly used method for turning crankshafts, suitable for roughing and semi-finishing small and medium-sized crankshafts. This method uses the center holes at each end of the crankshaft as a positioning reference. The crankshaft is supported by front and rear ejectors, and the main shaft drives the crankshaft in rotation. To accommodate the crankshaft’s eccentric structure, an eccentric center hole is provided at the connecting rod journal end of the crankshaft. The eccentricity of the eccentric center hole is equal to the eccentricity of the connecting rod journal. This allows for precise machining of the connecting rod journal by simply adjusting the ejector pin position. The advantages of the double-thimble clamping method include high positioning accuracy, guaranteed coaxiality of each journal, and simple clamping, making it suitable for mass production. However, this method requires high precision for the center hole. If the center hole is worn or misaligned, machining errors will increase. Furthermore, for crankshafts with poor rigidity, vibration is easily generated by cutting forces, affecting the machined surface quality. Therefore, a steady rest or steady rest is required to enhance the rigidity of the crankshaft.
The eccentric chuck clamping method is suitable for machining crankshafts with large eccentricities, and is particularly widely used when machining connecting rod journals. An eccentric chuck is a specialized fixture whose chuck body can be offset relative to the spindle axis by a distance equal to the crankshaft’s eccentricity. During clamping, the crankshaft’s main journal is clamped in the eccentric chuck. By adjusting the eccentric chuck’s offset, the connecting rod journal’s axis is aligned with the lathe’s spindle axis, enabling turning of the connecting rod journal. The advantages of the eccentric chuck clamping method include secure clamping, the ability to withstand high cutting forces, and high machining precision, making it suitable for machining medium- to large-sized crankshafts. However, eccentric chucks require high manufacturing precision, and the adjustment process is complex. The chuck offset must be precisely adjusted according to the crankshaft’s eccentricity, otherwise the resulting connecting rod journal eccentricity will be excessively inaccurate. Furthermore, the eccentric chuck’s considerable weight increases the load on the lathe spindle, so the lathe’s load capacity must be carefully considered when using it.
Dedicated fixture clamping is a clamping method designed for specific crankshaft models. Suitable for high-volume production, it significantly improves clamping efficiency and machining accuracy. Dedicated fixtures typically incorporate multiple positioning and clamping points tailored to the crankshaft’s structural characteristics to ensure stability and positioning accuracy during machining. For example, for multi-cylinder crankshafts, dedicated fixtures can simultaneously locate and clamp multiple main journals and crank arms, preventing displacement or deformation during machining. The advantages of dedicated fixture clamping include fast clamping speed and high positioning accuracy, effectively reducing downtime and improving production efficiency. However, dedicated fixtures are costly to design and manufacture, and lack universal applicability. Changes in crankshaft models require redesign and remanufacture. Therefore, they are suitable for machining large, fixed-model crankshafts. Regular maintenance and calibration are essential to ensure positioning accuracy and clamping reliability.
The faceplate clamping method is suitable for machining crankshafts with complex shapes, large eccentricities, and no center hole. The faceplate is a circular flat plate mounted on the lathe spindle. It features multiple T-slots and allows for workpiece clamping via bolts and a pressure plate. During clamping, one end of the crankshaft is fixed to the faceplate, while the other end is supported by the tailstock center. By adjusting the pressure plate on the faceplate, the axis of the crankshaft’s main journal can be aligned with the lathe spindle axis, or the axis of the connecting rod journal can be aligned with the spindle axis, to meet different machining requirements. The faceplate clamping method offers the advantage of high flexibility and adaptability to various crankshaft configurations, making it an effective clamping method for crankshafts without a center hole. However, the faceplate clamping method is complex in alignment, requiring the use of tools such as a dial indicator for precise crankshaft position adjustment. This process is time-consuming and inefficient, making it suitable for single-piece, small-batch production. Furthermore, because the faceplate’s center of gravity is misaligned with the spindle axis, centrifugal force is generated during rotation, causing lathe vibration and compromising machining accuracy and safety. Therefore, counterweights are required on the faceplate to balance the centrifugal force.
The combined clamping method is a clamping method that combines the advantages of multiple clamping methods. It is suitable for crankshaft processing with complex structures and a large number of processing steps. For example, the double-thimble clamping method is used in the roughing stage to quickly remove most of the excess; in the semi-finishing and finishing stages, the eccentric chuck clamping method is used for the processing of the connecting rod journal to improve processing accuracy; for the ends or special parts of the crankshaft, special fixtures are used for clamping. The advantage of the combined clamping method is that it can select the most suitable clamping method according to different processing stages and processing parts, taking into account both processing efficiency and processing accuracy. However, the combined clamping method requires frequent changes in the clamping method, which increases auxiliary time. Therefore, when using it, it is necessary to arrange the processing steps reasonably and reduce the number of clamping times. In addition, when changing the clamping method, the position of the crankshaft needs to be realigned to ensure that the position accuracy of each processing surface meets the requirements.
