Optimized Bilateral Turning Process for Electric Vehicle Transmission Components

Due to the complex nature and low processing efficiency of the input shaft in new energy vehicles following heat treatment, we utilize a double-head turning process. This method allows us to complete the machining of the outer circle, end face, inner hole, and center hole at both ends in a single clamping. By doing so, we reduce the need for frequent adjustments of the reference points, which helps ensure product accuracy.

We summarize the impact of the double-head turning process on the positional accuracy of the inner spline, as well as the inner hole and outer circle of the bearings at both ends. Additionally, we verify the process’s stability and efficiency through batch production, providing valuable insights for the high-precision and efficient machining of gear shaft components.

 

PART 1 Introduction

The input shaft, a key component of new energy vehicles, is recognized as one of the most challenging parts to manufacture within the pure electric vehicle reducer assembly. This is due to its stringent internal spline accuracy requirements, precise internal and external circular dimensions, geometric tolerance demands, and typical thin-walled structure.

Additionally, the rapid growth of the passenger car new energy vehicle market has intensified competition regarding automobile performance and pricing. As a supplier of gear shaft components for new energy vehicles, our company is actively pursuing new equipment and processes to enhance our market competitiveness. We have streamlined the processing route for the input shaft after heat treatment by employing double-head CNC turning technology, which integrates the grinding, turning, and grinding processes into a single step.

Through this innovative approach, we have verified the effects of the double-head turning scheme on processing efficiency and positional tolerance. We also summarized how factors such as blank accuracy, heat treatment methods, and clamping techniques influence product precision.

By controlling the quality of the blanks, optimizing heat treatment parameters, and maintaining accurate positioning benchmarks before and after heat treatment, we can ensure high product quality while improving processing efficiency and reducing costs. An illustration of the internal structure of the double-head CNC lathe is shown in Figure 1.

PART 2 Double-head turning process plan

After the heat treatment of the input shaft, it is necessary to process not only the gears through grinding but also the outer circle, end face, and inner hole of the bearings at both ends. The traditional post-heat treatment process follows these steps: grinding the holes, grinding the outer circle and end face of one end, grinding the outer circle and end face of the other end, turning the inner hole and end face of one end, and finally turning the inner hole of the other end. This sequence involves five different pieces of equipment, each requiring distinct clamping methods. Because the benchmark is frequently changed, maintaining product accuracy becomes challenging. Additionally, the constant movement between processes increases the risk of damage and rust.

In contrast, the double-head CNC lathe employs a mobile support block to axially position the parts. It utilizes left and right expansion sleeves of the fixture to clamp the outer circle of the shaft shoulder and the top circle of the gear, thereby securely positioning the parts. Once the parts are clamped, the mobile support block automatically retracts. At this point, the turrets on both sides of the lathe can simultaneously process the outer circle, end face, and inner hole at both ends. A diagram of the clamping plan is shown in Figure 2.

 

Currently, the multi-station rotary turrets on both sides of the double-head lathe are utilized to simultaneously process the outer circle, end face, and inner hole at both ends.

 

PART 3 Comparison of processing efficiency

The comparison of the process routes for the input shaft after heat treatment is presented in Table 1. The first five processes of the original route have a total time of 320 seconds. In contrast, the double-head turning process has a processing time of 150 seconds, resulting in a 113% increase in efficiency. Initially, the complete machining cycle after heat treatment took 510 seconds, while the current cycle time has been reduced to 340 seconds, reflecting a 50% increase in processing efficiency after heat treatment.

 

The double-end turning scheme demonstrates a significant improvement in processing efficiency compared to the traditional processing route. However, given the stringent precision requirements for the input shaft, it is essential to verify the stability of the double-end turning process. This verification will assess its impact on key components of the input shaft, including the radial runout of the inner spline, outer circle, and inner hole.

 

PART 4 Impact on position tolerance

To ensure a high qualification rate for subsequent hard-drawn splines and gear grinding, we implement a double-end turning scheme. It is crucial that the precision requirements for each processing component are met, with particular emphasis on strictly controlling the radial runout of the outer circle. The outer circle of the two bearings serves as the reference measurement for the gear component; any variation in its radial runout directly impacts the inclination deviation of the grinding gear, which in turn affects the gear’s lifespan and the levels of noise, vibration, and harshness (NVH). Therefore, maintaining strict control over the radial runout during the double-end turning process is essential. The geometric tolerance control requirements for this process are illustrated in Figure 3.

 

PART 5 Impact on the radial runout of the internal spline

The radial runout of the internal spline is a crucial characteristic of the input shaft. To ensure product quality, 100% inspection is required, meaning the manufacturing process must guarantee that all components are fully qualified. Currently, the parts are machined using a double-ended approach, which involves clamping at the tooth top circle and the outer circle of the shaft shoulder. The machining of the outer circles of the two bearings relies on the alignment of the tooth top circle and the outer circle of the shaft shoulder, while the radial runout of the internal spline is determined by the outer circles of the two bearings.

If the coaxiality among the tooth top circle, the outer circle of the shaft shoulder, and the internal spline is inadequate, it will negatively impact the radial runout of the internal spline. To maintain consistent quality in the internal spline, it is essential to strictly control the quality of the blanks during the double-ended turning operation. This includes managing the geometric tolerances of the spline minor diameter, the tooth top circle, and the outer circle of the shaft shoulder.

Currently, there are two processes that occur before heat treatment: finishing one end and then finishing the other. During the finishing of the second end, the machining process is supported by the tooth end, with positioning and clamping done at the tooth top circle. The fine turning process scheme is illustrated in Figure 4. In this setup, both the outer circle of the shaft shoulder and the inner hole of the spline are machined in a single operation, ensuring that the coaxiality between the two is within 10 micrometers (μm). Additionally, the inner hole of the spline uses the tooth top circle as its positioning reference, maintaining a coaxiality of within 15 micrometers (μm) with the top circle.

The input shaft components are thin-walled parts, and the design of these parts significantly influences heat treatment deformation. Therefore, it is essential to optimize the thermal process parameters as well as the positioning and placement methods during the heat treatment process. Strict control of thermal deformation is necessary, and multiple rounds of thermal deformation tests should be conducted. Currently, the deformation of the spline after heat treatment is within 10 μm.

We tracked the radial runout of 10 internal splines of the input shaft at three stages: before heat treatment, after heat treatment, and after double-head turning, all in the state of fine turning blanks. The specific results are shown in Table 2.

By comparing the data before and after heat treatment, we found that the radial runout of the internal splines of eight parts increased following the heat treatment, with the maximum change not exceeding 10 μm. When comparing the data after heat treatment and double-head turning, we observed that the radial runout of the internal splines in five out of ten parts increased after double-head turning, with a maximum change of 9.1 μm. Conversely, five parts showed a decrease, with a maximum change of 9.6 μm. The maximum radial runout of the internal splines across the ten parts was 25.1 μm, while the radial runout of the other nine parts remained within 20 μm, thus meeting the process requirement of 30 μm for radial runout. Additionally, the product requirement of 40 μm radial runout for the internal splines was consistently achieved.

 

Following this process plan, we processed over 500 parts continuously, maintaining a stable radial runout of the internal splines within 30 μm and achieving a 100% qualification rate. The CNC machining process capability index (Cpk) reached 2.43, indicating excellent quality control. To ensure this quality, the coaxiality between the spline inner hole, the tooth top circle, and the shaft shoulder outer circle should be maintained within 15 μm. Furthermore, the heat treatment process should be optimized to control thermal deformation to within 10 μm. By adopting the double-head turning process solution, we can consistently guarantee the quality of the inner spline in subsequent processing.

 

PART 6 Impact on the radial runout of the outer circle and inner hole

The traditional processing solution for the input shaft after heat treatment uses the center hole as the reference point for both the outer circle of the bearings at each end and the outer circle of the shaft shoulder. This is achieved using an outer cylindrical grinder, which reliably maintains the radial runout of the outer circle relative to the center hole at 5 μm.

However, the inner hole of the part is machined using a CNC horizontal lathe. The clamping setup for turning one end after heat treatment, as illustrated in Figure 5, features the tooth end as the supporting surface. The single-side width (L1) measures 3.5 mm, while the clamping surface length (L3) is only 5 mm. Additionally, since the distance (L2) between the supporting surface and the clamping surface exceeds 95 mm, improper clamping during the machining process can lead to radial runout of the processed inner hole in relation to the two outer circles. This discrepancy fails to meet product specifications.

 

 

To ensure that the radial runout of the inner hole is within acceptable limits, it is essential to first ensure the correct positioning of the outer circle of the shaft shoulder and the outer circle of the bearing at the clamping position. This requires that both the outer circle of the shaft shoulder and the outer circle of the bearing be processed simultaneously during the outer cylindrical grinding process.

After this, the turning process for the inner hole and end face must achieve 100% alignment of the radial runout of the bearing’s outer circle within 8μm when the parts are clamped. This alignment is crucial to ensure that the radial runout of the inner hole does not exceed 12μm. However, this approach involves an additional turning step after the outer cylindrical grinding and heat treatment, which necessitates multiple clamping and alignment cycles by the operator, resulting in low processing efficiency. Under this method, the radial runout of the inner hole ranges from 6μm to 13μm, failing to consistently meet the required specifications.

By implementing a double-head turning process, the outer circle, inner hole, and center hole of the input shaft can all be processed in a single clamping operation. This approach ensures that the processing references for the outer circle, inner hole, and center hole are perfectly aligned. Consequently, the radial runout of the outer circles at both ends relative to the two center holes is maintained within 5μm. Additionally, the radial runout of the inner hole relative to the two outer circles also remains within 5μm. This method effectively addresses the issues of low processing efficiency and unstable radial runout present in the original process.

 

PART 7  Conclusion

The double-head turning process, combined with the method of process concentration, allows for the machining of the outer circle, end face, inner hole, and center hole at both ends of the input shaft in a single clamping. This approach minimizes the need for frequent datum conversion and reduces the likelihood of positioning errors, ultimately enhancing product accuracy and production efficiency. The key points are summarized as follows:

1) The double-head CNC turning process, applied after the heat treatment of the input shaft, can replace the traditional five-step process, leading to stable batch processing.

2) To control the radial runout of the internal spline, the minor diameter of the internal spline is machined using the tooth top as the positioning reference before heat treatment. This ensures that the coaxiality remains within 15μm. Radial runout is carefully managed within 10μm during heat treatment. After this stage, the double-head turning scheme is utilized again, with the outer circles of the bearings at both ends machined using the tooth top as the positioning reference. A small batch verification involving 500 pieces was conducted, resulting in a 100% qualification rate for the radial runout of the internal spline, which can now be reliably controlled within 30μm.

3) The double-head turning scheme is also employed for controlling the radial runout of the inner hole and outer circle. By clamping and machining the outer circles at both ends, the inner hole, and the center hole all at once, we ensure that the radial runout of the inner hole relative to the two outer circles, as well as the radial runout between the two outer circles and the center holes, remains stable at around 5μm. This consistency aids in stabilizing the tooth dispersion difference during the subsequent gear grinding process within 5μm.

 

 

 

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