Precision Machining Techniques for EV Main Reduction Gears

This paper discusses the challenges involved in machining the inner bore and end face of typical main reduction gears used in new energy vehicles after heat treatment. By refining the process planning and improving the fixtures, we address issues related to low machining efficiency and inconsistent quality. We also summarize the effects of various process plans on machining cycles and product accuracy and verify the stability and efficiency improvements achieved in batch production. This research serves as a reference for high-precision and high-efficiency machining of typical gears in new energy vehicles.

 

01 Introduction

With the rapid development of the new energy passenger vehicle market, competition regarding vehicle performance and pricing has become increasingly intense. To gain a competitive edge in this demanding market, it is crucial to continuously explore the potential of new energy gear shaft manufacturing processes. Our goals are to maximize efficiency, reduce costs, and maintain consistent quality.

A typical gear shaft used in new energy vehicles is illustrated in Figure 1. As a key component in the reducer assembly of these vehicles, the main reduction gear has a well-established manufacturing process. The traditional machining steps include rough and fine turning, drilling, gear hobbing, milling, heat treatment, and grinding of the inner bore and end face. However, in practice, the turning process of this component after heat treatment has faced challenges such as high tool wear, lengthy machining cycles, and the need to strike a balance between machining quality and efficiency.

This paper addresses the challenges encountered during the turning of new energy main reduction gears following heat treatment. Several improvement strategies have been tested in both prototyping and batch machining processes, and we have summarized the advantages and disadvantages of each approach. The most suitable machining strategy has been selected based on the specific circumstances. This method not only shortens the cycle time and enhances efficiency but also reduces costs while ensuring quality. The findings provide practical insights for the machining of typical disc gears for new energy vehicles and contribute to the rapid advancement of gear machining technology in this sector.

Precision Machining Techniques for EV Main Reduction Gears1

02 Turning Process Sample Machining Plan

After heat treatment, the main reduction gear requires finishing of the inner bore, end face, and teeth, which involves two machining processes: turning and grinding. The clamping plan for these processes is illustrated in Figure 2. The inner bore and end face machined during the turning process serve as references for positioning and support in the gear grinding process.

Excessive axial runout of the end face relative to the inner bore can directly impact the accuracy of gear grinding. Likewise, excessive radial runout of the pitch circle in relation to the inner bore can lead to an insufficient effective involute due to uneven distribution of the grinding allowance, potentially causing black scale on the tooth surface.

To enhance the pass rate of gear grinding, it is crucial to maintain strict control over the machining accuracy of the inner bore and end face during the turning process.

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2.1 Bar Self-Centering Chuck Processing Solution

A common clamping solution for turning operations involving internal holes and end faces is the use of a bar self-centering chuck. This device supports the end face, accurately locates, and clamps the addendum circle. This approach offers a simple fixture structure, a short manufacturing cycle, high versatility, and easy maintenance of tooling. It is often utilized in the early stages of a project to reduce the trial production cycle time and facilitate rapid prototype delivery. The setup for clamping with a bar self-centering chuck is illustrated in Figure 3.

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In practical applications, the clamping scheme described earlier consistently faces challenges in balancing machining quality and efficiency. The bar-shaped self-centering chuck has a relatively small clamping area, which inevitably leads to deformation when the part is clamped at three points. While the inner hole can achieve acceptable roundness during fine machining, it tends to revert to its original shape upon removal from the machine, resulting in a pronounced three-lobed shape. This three-lobed deformation becomes more significant as the clamping force increases, leading to progressively poorer roundness.

Figure 4 illustrates the roundness of the inner hole at different machine pressures. At pressures of 0.5 MPa and 0.7 MPa, the inner hole roundness measures 5.30 mm and 7.73 mm, respectively. Therefore, it is crucial to maintain strict control over machine pressure to avoid out-of-tolerance roundness in the inner hole.

While decreasing the machine clamping force can effectively reduce part deformation, the presence of 16 holes on the small end of the part creates significant cutting impact during intermittent cutting. Insufficient clamping force might cause the part to loosen, resulting in increased axial runout. Thus, to maintain acceptable roundness in the inner hole, it is essential to appropriately manage the machine clamping force.

When maintaining constant machine pressure, the only way to prevent parts from loosening during intermittent turning is to reduce the cutting force by decreasing the cutting depth and feed speed. However, this adjustment leads to a longer machining cycle and decreased processing efficiency.

Precision Machining Techniques for EV Main Reduction Gears4

 

Precision Machining Techniques for EV Main Reduction Gears5

 

Verification of the processing has indicated that, to maintain a consistent quality during the turning process, the clamping solution utilizing a bar-shaped self-centering chuck requires a machine pressure of 0.7 MPa. Additionally, to avoid part loosening during intermittent turning, it is necessary to reduce the cutting depth and feed rate. This reduction minimizes cutting forces and ensures stable axial runout, resulting in a cycle time of approximately 4 minutes. Consequently, the bar-shaped self-centering chuck clamping solution has low processing efficiency and is primarily suitable for small-batch prototype production.

 

2.2 Sector-shaped Self-centering Chuck Processing Solution

To improve the balance between processing quality and efficiency, the self-centering chuck design was modified from a bar shape to a sector shape, which enhances the clamping area. The sector-shaped self-centering chuck clamping solution is illustrated in Figure 5. In this design, increased machine pressure provides reliable clamping, reduces part deformation, and ensures acceptable inner hole roundness and axial runout. Additionally, CNC efficiency improvements can be made, allowing for a cycle time of 2 minutes and 30 seconds.

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The circularity of the fan-shaped self-centering chuck remains relatively stable as the pressure of the machine tool increases. In comparison to the bar-shaped self-centering chuck, both types rely on the tooth top circle as the positioning reference after heat treatment. However, the fan-shaped chuck has a larger clamping area. This means that any slight bumps on the tooth top or deformation from heat treatment can introduce clamping errors, leading to deviations in the turning of the inner hole. As a result, the radial runout value from the pitch circle to the inner hole increases, which negatively impacts the gear grinding accuracy.

Therefore, the fan-shaped self-centering chuck is more suitable for small batch sample processing. If it is used for batch processing, careful control of heat treatment deformation is necessary, along with measures to prevent bumps during the process. Additionally, it is important to maintain high processing accuracy in the gear hobbing stage before heat treatment. To ensure positional accuracy of the tooth top and pitch circle, it is recommended to use the full cutting tooth top method.

 

03 Batch processing plan for turning process

To achieve cost advantages in the processing of self-centering chucks, it is essential to consider both processing quality and efficiency during batch production. Therefore, optimizing and improving the processes and tooling is necessary to find the most suitable solution for batch processing. This approach will help ensure quality, enhance cycle time, and reduce costs.

 

3.1 Process Optimization

The traditional method of positioning and clamping the tooth tip has been replaced with a new approach that utilizes the tooth tip circle or pitch circle for centering, end face support, and clamping. A clamping device has been added to the tooth end to enhance stability. To prevent compression deformation, the support surface has been modified from the smaller end face at the spoke to the larger end face near the tooth. The improved clamping scheme is illustrated in Figure 6. In this new arrangement, the clamping of the part relies mainly on axial force, which helps to avoid internal bore deformation that could result from radial clamping. However, to prevent axial runout caused by axial compression deformation, it is essential to strictly control both the size of the support area and the relative positioning of the support and clamping points.

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3.2 Tooling Improvements

The tooling setup consists of three evenly spaced positioning blocks that securely center the part through the elastic force of the film chuck. In addition, three evenly spaced support blocks stabilize the lower end face, while three evenly spaced pressure plates compress the upper end face. The support points align with the pressure points, ensuring reliable pressure by adjusting the machine’s settings. A complete set of precision turning end pressure fixtures is illustrated in Figure 7.

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This clamping scheme applies less radial centering force to the parts, which helps minimize deformation of the inner bore. To prevent loosening during CNC custom machining, the end faces of the parts can be securely clamped by increasing the machine pressure to 0.9 MPa. By utilizing a precision turning fixture and optimizing cutting parameters, the time required for batch processing was reduced to just 2 minutes. At this efficiency, the inner bore roundness and axial runout met the required specifications, ensuring a stable process. The inner bore roundness values for the two clamping schemes were 3.44 mm and 3.73 mm, respectively. Figure 8 shows the inner bore roundness for the end-pressure fixture.

 

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A comparison of batch processing techniques shows that after using the tip circle centering method to turn the inner hole, the radial runout of the tooth pitch circle is mainly influenced by the accuracy of hobbing prior to heat treatment and any deformation that occurs during heat treatment. This method is recommended as long as both the hobbing and heat treatment processes remain stable. The pitch circle centering method not only achieves acceptable roundness and axial runout but also maintains a stable pitch circle radial runout within 50 mm, making it highly suitable for batch processing.

 

Utilizing a complete set of finishing fixtures can enhance processing efficiency and ensure consistent quality; however, the decision to implement this method should take comprehensive market conditions into consideration. Finishing fixtures are complex in design, have a lengthy manufacturing cycle, and typically cost around 100,000 yuan, which is approximately 100 times the cost of self-centering chucks. Pitch circle centering is slightly more expensive than tip circle centering, and this clamping method requires higher positioning accuracy from robots in automated production lines. Additionally, tool life—especially for interrupted turning—decreases significantly as cutting speeds increase. Therefore, in actual machining, it is essential to select appropriate fixtures, tools, and cutting parameters based on production capacity requirements to achieve a balance among efficiency, quality, and cost.

 

04 Conclusion

This article discusses the commonly used processes and tools for machining the internal bore and end face of typical new energy gears after they have undergone heat treatment. It highlights the challenges associated with different machining solutions and, through batch processing, identifies the optimal approach for turning the internal bore and end face after heat treatment.

1) Self-centering chuck machining solution: The machine pressure is maintained between 0.6 and 0.7 MPa, allowing for roundness control within 8 mm. At this clamping force, it is necessary to reduce the feed rate to prevent part loosening and ensure acceptable axial runout. The cycle time is 4 minutes. While this solution has low machining efficiency, it is appropriate for the new product development stage.

2) TThe fan-shaped self-centering chuck clamping method provides reliable clamping while minimizing roundness deviations that can occur due to clamping deformation. This approach increases the cycle time to 2 minutes and 30 seconds. It offers high versatility, fast cycle times, and low cost. However, heat treatment deformation can significantly impact the process, so it is essential to carefully control this deformation and prevent process collisions. This method is particularly suitable for the new product development stage.

3) For the finish turning fixture end face clamping solution, the machine pressure is maintained at 0.9 MPa, with a cycle time of 2 minutes. The inner hole roundness and axial runout are within acceptable limits.

 

In terms of the tip circle centering solution, the stability of the pitch circle radial runout during batch processing is significantly influenced by the accuracy of the hobbing process and the deformation that occurs during heat treatment. This solution is ideal for production lines that have stable hobbing and heat treatment processes. The pitch circle centering solution effectively compensates for the impacts of hobbing and heat treatment deformation, providing consistent pitch circle radial runout, which makes it the preferred choice for batch processing.

 

 

 

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