In the 1990s and before, EDM was the main method of bevel gear forming die tooth profile processing, and the gear electrode was mainly planed by precision gear planer. Gear planing is a kind of gear generating and forming process. Therefore, the gear electrode planed out is an involute gear conforming to the standard, while the gear die produced by electrochemical etching faithfully copies the shape of the electrode, so it is a standard gear cavity. This is not a problem for the general precision of bevel gear forming, but for the car gear with high requirements of noise and transmission stability, sometimes a bevel gear conforming to the standard profile can not obtain the best transmission effect. In order to improve the transmission stability and reduce the transmission noise, it is generally necessary to repair the tooth profile and tooth direction of the bevel gear, as shown in FIG. 12 and 13. When there is no load or light load, the tooth profile engagement is the involute of drum repair, as shown in Fig. 12a; in the heavy load state, the elastic deformation occurs at the tooth profile meshing due to the large contact stress, which is close to the standard involute, so that the tooth profile maintains good tooth profile under heavy load, as shown in Fig. 12b. As shown in Fig. 13a, it is the standard tooth profile shape. If there is a slight error in gear assembly, the contact trace of tooth profile will be deviated, thus the transmission accuracy and stability will be reduced, and the transmission noise will be increased; as shown in Figure 13b, the tooth profile contact trace can maintain a good symmetrical shape even if there is a slight error in the assembly.

because it is difficult to produce the gear electrode for tooth profile and tooth direction drum repair, the electrode drum repair has become a technical problem in precision forging of high-precision bevel gear. A university in China once proposed to use the principle that the corrosion rate at the sharp corner of the metal object is greater than that at the flat part to conduct chemical corrosion on the standard gear electrode, so as to obtain the bevel gear electrode with tooth profile and tooth direction for drum repair. However, this method is lack of corrosion control means and the corrosion consistency is poor, so it is difficult to be applied in the batch production of bevel gear.

in the late 1990s, the appearance of high-speed milling solved the problem of drum repair of tooth shaped electrode. The precision of copper electrode can reach micron level in the modern high-precision CNC high-speed milling machining center (FIG. 14). As long as the modeling is correct, it is possible to machine the tooth shaped electrode with any drum repair amount. High alloy die steel hardened to more than 60HRC can be directly milled into gear dies by using high-performance composite coating tools and CBN tools, as shown in FIG. 15. However, the tool life of high-speed milling is very low when cutting gear cold forging dies with hardness more than 60 HRC, which seriously affects the economy of die manufacturing. EDM is the only effective method for cemented carbide gear dies at present. Therefore, EDM is still the main die making technology of gear cold forging die at present, but the generation machining of tooth shaped electrode has been basically replaced by high-speed milling.

for the warm forging and hot forging die of bevel gear with quenching hardness of about 50HRC, the high-speed milling tool has enough durability; however, the processing speed and precision of high-speed milling is far higher than that of electrical discharge machining; the surface of high-speed milling die is smooth, and it will not leave "white layer" defects and residual tensile stress that affect the service life of the die like EDM The service life of the die is obviously longer than that of the EDM die. Therefore, in the manufacturing of warm forging and hot forging die for bevel gear, high-speed milling has completely replaced EDM.