New Technology for Processing Ultra-Large Precision Hard Tooth Spiral Bevel Gears

1. The current situation of spiral bevel gear processing technology in China 

In the early days of our country, a batch of Grisson Phoenix series milling gear machines and a small number of Klingenberg (Olicon) gear-cutting helical bevel gears were imported. These machines could process small-sized helical bevel gears with a diameter of less than 1 meter. However, after carburizing and quenching, they could not be further finely processed and could only be reduced in tooth surface roughness through grinding. The accuracy of the hard tooth surface helical bevel gears has always remained at a low level (8th grade accuracy or lower). The contact area control adopts the trial cutting - rolling inspection method. That is, first, based on the gear parameters (modulus, number of teeth, material hardness, etc.) and according to experience, select the appropriate tools (tool disk, tool number) to conduct trial cutting on the small gear and the large gear respectively. Then, on the rolling inspection machine, install the rolling inspection according to the assembly parameters, obtain the contact area diagram. If the contact area does not meet the predetermined requirements, change the tool disk, tool number or wheel position, and repair the small gear. Change the shape, size and position of the contact area until the predetermined requirements are met. The "trial cutting - rolling inspection" process generally requires at least two repetitions, and usually requires more than three repetitions. The contact area in the tooth length direction does not exceed 40%. This method has a relatively cumbersome process, a long cycle, poor controllability of contact area quality, high dependence on experience, and the highest level that the contact area can reach is limited, and interchangeability cannot be achieved. 

In recent years, the German company Klingenberg has begun to export CNC machine tools for spiral bevel gears using the scraping-tooth process, equipped with a new CNC system, and has relaxed the export restrictions for hard-toothed processing machines with a diameter of over 1 meter. Domestic imported machine tools can currently machine spiral bevel gears with a diameter of up to 1.6 meters and a grade of 6. However, the processing method still uses the traditional tool disk milling method. Although due to the advancement of tools, it can machine hard-toothed surfaces with a hardness of 58HRC, the control of the contact area still adopts the traditional trial-cutting method. Additionally, the export of ultra-large hard-toothed spiral bevel gear processing machines with a diameter of over 2 meters is still restricted. In China, such gears are mainly imported. With the gradual advancement of the construction of large-scale vertical grinding lines in our country, the manufacturing technology of precision hard-toothed spiral bevel gears with a diameter of over 2 meters has become a bottleneck for industrial upgrading. Such technological transformation is urgently needed.

2.The implementation method of new technologies for processing super-large precision hard-toothed spiral bevel gears 

In recent years, with the development of numerical control technology, Western countries have gradually relaxed their export restrictions on five-axis linkage (or more than five-axis) technology. China has successively introduced a batch of high-performance five-axis linkage machining centers. The successful introduction of the gantry-type highly rigid five-axis linkage machining center has made the manufacturing of ultra-large hard toothed spiral bevel gears possible. 

The manufacturing of spiral bevel gears based on five-axis linkage follows the basic idea of "modeling → tooth surface contact simulation analysis (TCA) →CAM programming → precision CNC cutting". The maximum diameter and precision of gear processing depend on the processing range and precision level of the machine tool. The method of the modeling software is to strictly follow the processing principle of Krugenberg toothed bevel gears， on the basis of the control items of traditional processing， add the shaping control items along the tooth profile and tooth direction (generally the bulge quantity control)， after a series of calculations and verifications， a three-dimensional model is generated under the premise of meeting the technical requirements， and the generated three-dimensional model of the bevel gear pair is imported into the TCA software， according to the assembly relationship and parameter settings to simulate the meshing process， using the finite element method to conduct rigid contact analysis on the tooth profile surface， and obtain the contact area graphic. If the contact area does not meet the technical requirements， return to the modeling software， by modifying the machine model type， tool disk diameter， tool tooth module， tooth profile shaping quantity， tooth direction shaping quantity， offset coefficient， installation distance， tooth system， etc. parameters， obtain a new three-dimensional surface model， and then import it into the TCA software for analysis， to obtain a new contact area graphic. This process is repeated until a contact area graphic that meets the requirements is obtained. At this time， the three-dimensional model serves as the final model for numerical control processing. 

The 3D model obtained from the modeling/TCA software is imported into the CAM software. Using methods such as contour lines and five-axis profiling processing for CAM tool path programming, through the 3+2 process flow, the CAM program is compiled and imported into the machine TNC. Before connecting the CAM tool path code to the previous CAM system, the workpiece is positioned and the tool is set. After the simulation trial cutting is confirmed to be correct, the actual processing can be carried out.

3. Core Technology 

The core technologies of CNC five-axis联动 spiral bevel gear processing include two aspects: the tooth surface contact analysis TCA technology and the CAM tool path programming technology. The TCA technology has up to eight control items， and the parameter values can change continuously， without being limited by the parameters of traditional processing methods. By using the multi-parameter software control technology of TCA， the contact spots far exceed the level that traditional methods can achieve. In the tooth length direction， it can reach up to 70%， and in the tooth profile direction， it can reach 50%. At the same time， the gear model corrected by TCA analysis is processed using a five-axis联动 machining center， without the need for repeated trial cutting and inspection by traditional methods. The processing efficiency is increased by more than twice. By using this technology， contact zone pre-control and complete interchangeability between different gears can be achieved， which is something that traditional processing methods cannot do. In the CAM tool path programming technology， the process that determines the processing accuracy and the quality of the tooth profile contour is the fine processing after carburizing and quenching. In the finishing process， the five-axis profiling machining programming method is adopted. In terms of tool selection， cutting thickness， depth， span， vibration prevention， tool edge length， tool path strategy， etc.， control is carried out to prevent defects such as crawling， excessive vibration， and milling vibration patterns. At the same time， the processing efficiency is improved. With this processing technology， currently， hard tooth surface Krangenberg spiral bevel gears with a diameter of 3 meters can be processed， with a precision of DIN5 level and a tooth surface roughness of Ra0.8-1.6.

4.Conclusion

By adopting five-axis linkage CNC machining technology, it is possible to manufacture super-large spiral bevel gears with a maximum diameter of 3 meters, precision DIN5, tooth surface roughness Ra0.8-1.6, and hardness 62HRC. Combined with the matched TCA technology, it can achieve pre-control of the contact area, with the maximum contact area in the tooth length direction reaching up to 70% and in the tooth profile direction reaching up to 50%. And it can achieve complete interchangeability between gears. 

This technical concept can be applied to the processing of spur gears without reaming grooves, high-precision drum-shaped gears, high-precision large-sized worm gears, and non-circular gears. It represents a revolutionary technology for processing transmission components.