Formed coupling is a shaft-hub coupling formed by a shaft with a non-circular section and a hub hole with a corresponding profile (Figure 1). This connection does not use keys or splines, so it is also called a keyless connection, or a profile connection.
The shaft head and hub bore can be cylindrical (Fig. 1a) or conical (Fig. 1b). In addition to the static connection, the cylindrical shape can also be used for the dynamic connection of axial movement under no load; the conical shape is easy to assemble and disassemble, and can also bear the axial force in one direction.
Because the formed connection is easy to disassemble and assemble, it can ensure good alignment; there are no keyways and sharp corners on the connection surface, which reduces stress concentration, so it has high bearing capacity, can transmit large torque, and has high fatigue strength, which can replace keys or splines key join. For the taper formed connecting shaft (hole), it can achieve no clearance fit, better performance and more reliable work. This ideal design has always been due to the complex processing technology, insufficient research on the process, and no good processing equipment, cutting tools, and measuring tools. So far, its wide application has been greatly restricted. Therefore, it must be used as a general basic part. To popularize and apply, the key is to solve its processing method correctly.
Among the existing machining methods, the typical non-circular forming shaft machining can be realized by the imitation method and the mechanical motion synthesis method. Non-circular holes can be processed with forming inserts. These methods require either hard molds, or special machine tools or series of special tools. The manufacturing flexibility is poor and the production efficiency is low. Therefore, the use of CNC machining has become a trend. Wu Yijie from Zhejiang University conducted a milling test on a triangular axis on an XK5040 three-coordinate CNC milling machine.