The synchronizer is an important component in the automotive gearbox to achieve speed change, and the gear ring is an important component of the synchronizer. The accuracy and quality of the gear ring will directly affect the actual effect of speed change. With the rapid development of the automotive industry, the original copper synchronizer gear rings are gradually being replaced by steel synchronizer gear rings. Our synchronizer products are made of steel materials, which are highly demanded and require mass production. This article mainly studies the precision forging process of steel synchronizer gear rings. The raw material is 20CrMnTi, and the state before precision forging is rough rolled ring blank; Develop a precision forging process plan: rough turning billet → hot precision forging → turning flash → annealing → shot blasting → polymer treatment → cold precision pressing. By determining the specifications of the hot precision forging billet (including 3D simulation), precision forging mold design and manufacturing, precision forging process testing, etc., ensure that defects such as folding, cracking, insufficient tooth filling, and surface roughness do not occur during the precision forging process. Through process verification, the precision forging process of the steel synchronizer gear ring is feasible; The 3D simulation results are consistent with the actual verification results.
1. Determine the specifications of hot precision forging blanks
The steel synchronizer gear ring studied in this study is made of 20CrMnTi hot-rolled round steel through cutting, heating, pre-forging, punching, and ring rolling. The blank is obtained after rough turning, and the outer circle has 3 convex keys and 48 small teeth. The precision and surface roughness requirements for the convex keys and small teeth are high: the width deviation of the convex keys is ± 0.1mm, and the symmetry tolerance is 0.15mm; The convex and concave rounded corners of the small teeth are as small as R0.2mm, and the runout tolerance of adjacent teeth is 0.05mm; Surface roughness requirement for non-machined surfaces Ra3.2 μm. After precision forging, these parts are no longer machined.
The theoretical weight of the finished forging product is 0.27kg. According to empirical data, the width of the inner and outer flash is 6-8mm, and the thickness is 1.5-2.5mm. Based on this, the total weight of the cutting material is calculated to be 0.39kg; The best choice for positioning the blank in the mold is the tooth root circle φ At 113mm, the outer diameter of the blank can be adjusted according to the φ 112mm determination; Reference forging inner hole size φ 94mm, initial selected billet inner diameter is φ 88mm φ 90mm φ 92mm φ There are four types of 94mm billets. Based on the total weight and inner and outer diameter dimensions of the billets, the thicknesses of the four types of billets are calculated to be 13.2mm, 14.2mm, 15.5mm, and 17.1mm, respectively.
After 3D software simulation (Figure 1), it was found that the flash sizes of the four types of billets mentioned above are different, and the product can be formed. The simulation results show that the inner and outer flash of the 2 # specification billets is relatively uniform after forming, which is theoretically the most suitable and needs to be verified by subsequent processes.
Figure.1 3D Simulation Results
2. Design and manufacturing of precision forging molds
2.1 Key points of hot precision forging mold design
According to the cold precision forging diagram, the gap around the side is reduced by 0.1-0.15mm, and the thickness is increased by 0.5-1mm to obtain the hot precision forging diagram. The heat shrinkage rate is 1.5%, which can determine the size of the hot precision forging mold cavity. This process research adopts open die forging, designing inner and outer flash grooves, and designing a top material rod at the flash position in the lower mold. Using the original mold base size of the equipment to design the external dimensions of the hot precision forging mold, the cavity mold material in direct contact with the hot material is made of 1.2367 with good red hardness, which can ensure product accuracy.
2.2 Key points of cold precision pressing mold design
The cavity size of the cold precision forging die is consistent with the cold precision forging drawing, and the ejection structure is similar to the hot precision forging die. The outer dimensions of the die are also designed according to the original equipment mold frame, and the tooth-shaped lower die is made of LD material with high hardness and good wear resistance.
2.3 Key Points for precision forging mold manufacturing
The mold cavity is rough milled by a high-speed CNC machining center, manufactured by electrical discharge machining after heat treatment, and finally polished by special polishing equipment.
3. Precision forging process test
3.1 Mold preheating
Use a dedicated heating device to preheat the mold to 250-300 ℃, which is convenient for filling the tooth shape fully and does not easily cause abnormal mold fractures.
3.2 Billet heating
Mark 20 pieces of each of the 4 rough rolled billets and quickly heat them to 1150-1200 ℃ in a dedicated medium frequency induction heating device.
3.3 Hot precision forging
The existing 630t CNC Screw press is used for hot precision forging (the simulated forming force is 327-453 tons), and the heated blank is accurately placed horizontally at the location of the root circle of the hot precision forging die. The outer circle surface of the blank is fitted with the root circle of the die to achieve blank positioning to avoid defects such as folding and insufficient tooth shape during precision forging. During the hot precision forging process, the mold release agent is also required to cool and lubricate the mold.
After on-site process verification, the hot precision forging process test results for the steel synchronizer gear ring are consistent with the 3D software simulation results. The 3D modeling diagram and the hot precision forging product are shown in Figure 2. Among the four specifications of billets, specification 2 # has the best forming effect, with even inner and outer flash sizes, which can ensure full filling of all product parts without forging defects such as folding and cracking and does not waste materials. Among the other three specifications of billets, the test result for specification 1 # shows that the outer circular flash is small, there are a few teeth that are not full, the inner flash is large, and there is a lot of material waste; Specifications 3 # and 4 # have larger rounded edges.
Figure.2 3D Modeling Drawing and Physical Product of Hot Precision Forging
3.4 Car trimming
After the hot precision forging blank is naturally cooled, the flash at the inner hole and outer circle will be removed. The flash in the thickness direction will not be removed temporarily, and it needs to be used as the fine pressing allowance for the subsequent cold pressing. The subsequent machining machine will remove the flash in the thickness direction.
3.5 Annealing softening
The annealing process softens the blank for easy cold precision forming.
3.6 Shot blasting cleaning
Blast clean the annealed blank on the existing hanging shot blasting machine to remove the oxide skin on the surface of the blank. Select fine steel balls with a diameter of less than 0.4mm to ensure the surface roughness of the product during cold precision pressing.
3.7 Polymer treatment
Apply polymer lubrication treatment to the blank after shot blasting to improve the surface roughness of the product after cold precision pressing and the lifespan of the cold precision pressing mold.
3.8 Cold precision pressing
Place the polymer-treated blank smoothly into the cold precision pressing mold, use the existing 1600 ton oil press to accurately set the pressure, and conduct a cold precision pressing test (actual demand is about 1000 tons). After each precision pressing product, use a high-pressure air gun to blow and clean the mold cavity. After testing, the product’s dimensional accuracy and surface roughness after cold precision pressing meets the requirements of the drawing.
4. Conclusion
The forging process of hot precision forging + cold precision pressing can achieve precision forging of the steel synchronizer gear ring.
The 3D software simulation results are consistent with the hot precision forging process test results of the steel synchronizer gear ring, which can fully guide the design of the steel gear ring mold, shorten the product development cycle, and reduce development costs.
Author: Wang Zixiao
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