As the first stage of gear precision forging, hot and warm forging process is relatively easy to control, because the forged gear has a certain margin to adjust. The cold forming process requires quite high precision. When the forged gear is squeezed through the cold forming die, the shape should meet the final requirements without further processing. For forged gear teeth, the accuracy of profile is required to be within 10 μ M, which puts forward very high requirements for the design of the die.
In the gear cold forming (finishing) process, the forged gear is gradually squeezed through the cold forming die. The internal stress and deformation of the die are related to the position of the forged gear. In particular, the change of radial pressure will determine the deformation of the die. If the appropriate die shape can be designed, the deformation of the die can be used. For example, when the forged gear is at the inlet and outlet of the die, the force on the die is relatively small, so the deformation is relatively small. When the forged gear is in the middle of the die, the deformation of the die is relatively large. This feature may be used to obtain gear teeth with drum shape. Theoretical analysis and experimental study on drum formation are made by using hollow cylindrical parts with angle.
The finite element method is used in the cold forming (finishing) process of forged gear to analyze the deformation of die, the deformation and springback of forged gear. The influence of forging gear allowance, die shape, size and structure on the final product is considered. Fig. 1 is a finite element model for analyzing the cold forming process of cylindrical spur gears. The deformation of the punch has little effect on the shape of the forged gear, so it can be treated as a rigid body in the finite element model. The deformation of the mold cavity directly affects the shape and size of the contour, so the mold is simulated according to the deformed body. In the process of cold treatment, only the surface of the gear tooth is plastically deformed, and the interior of the forged gear tooth and most areas of the gear tooth are in the state of elastic deformation. The proportion of elastic recovery is very large, and the final shape of the contour can only be predicted by elastic-plastic material model.
Figure 2 shows a die for cold forming of cylindrical spur gears. The punch and the mandrel are integrated and installed on the upper slider of the press. There is a small gap between the mandrel and the forged gear hole. It is not used for positioning, but only for guiding. Because in the process of forging gear, only the accuracy of gear teeth can be guaranteed, but the accuracy of inner hole can not be guaranteed. Gear into the mold cavity is also guided by the chamfer of the mold.
If it is used for the finishing of helical gears, the forged gears will rotate during the extrusion process. It is necessary to assemble a rolling bearing between the punch and the press so that the punch can rotate with it. In addition, when extruding the helical gear, the forces on both sides of the forged gear teeth are asymmetric and the deformation is different. The gear can be extruded again in the opposite direction. In fact, spur gears can also be treated twice, which not only has better tooth shape, but also has better surface residual stress state than one-time extrusion. Through experiments, the residual compressive stress of 20MPa ~ 50MPa can be obtained by single extrusion, and the residual compressive stress can reach about 100MPa by two treatments.
Figure 3 shows the analysis and experimental results of cold forming of a cylindrical spur gear. The profile of cold extruded gear is measured by coordinate measuring machine and compared with the results predicted by finite element method. The figure shows the profile of forged gear, die, measured finishing gear and finite element calculation. It can be seen that the theoretical analysis is in good agreement with the actual measurement. The springback of the contour can be accurately estimated from the analysis, so as to accurately design the size and shape of the die. Another advantage of cold forming is to improve the surface quality of gear teeth. Experiments show that its finish reaches RA = 1 μ M below, meeting the requirements of gear tooth processing.