Effective Methods for Eliminating Surface Discoloration in Turbine Flange Machining

For a specific eccentric volute, the initial fixture scheme was established through a process review, and the method for selecting the fixture’s positioning was introduced. However, after sample verification, a significant issue arose with a high incidence of “black skin” in the subsequent process. Through analysis and investigation, it was determined that the cause of the “black skin” was the repeated positioning issues with the fixture. To address this problem, the fixture scheme was optimized by improving the positioning method, successfully resolving the “black skin” issue.

 

1 Introduction

Currently, the country’s emission regulations are very stringent. The volute is widely utilized in the automotive industry due to its unique capability to boost performance and reduce emissions. However, the irregular shape and thin-walled characteristics of the volute make its casting and processing quite challenging. Additionally, designing fixtures for this type of product is also complex. This article focuses on the design and optimization process of a fixture for an eccentric volute, providing valuable references for the future design of similar product fixtures within the industry.

 

2 Product Analysis

Figure 1 displays a three-dimensional side view of a volute, highlighting the various processing positions, which include the inlet flange, turbine end flange, outlet flange, and the sleeve hole. To optimize the processing procedures, the non-processed surfaces are minimized for repeated positioning, emphasizing large flat surfaces for this purpose.

The first processing step is aligned as closely as possible with the actual assembly sequence. For this product, it has been decided to process the intake flange first. In subsequent processes, the “one face and two pins” positioning method will be employed to complete the processing of the entire product.

The overall processing sequence is as follows:
- OP10: Process the intake flange
- OP20: Process the turbine end flange
- OP30: Process the exhaust flange
- OP40: Process the shaft sleeve hole and the back of the intake flange.

Solution to the black skin problem in turbine flange processing1

 

The product datum is illustrated in Figure 2. The initial process involves positioning the blank. The processing datum is aligned closely with the drawing, adhering to the general six-point positioning principle [1, 2]. Hydraulic clamping is utilized to ensure the stability and reliability of the clamping force. The specific design scheme is outlined as follows.

Solution to the black skin problem in turbine flange processing2

1) Positioning scheme (see Figure 3): The central cylinder of the turbine end restricts movement and rotation along the X and Z axes. Additionally, the back surface of the inlet flange limits rotation around the Y axis. The internal end surface of the turbine end, along with the position of the inlet flange pit, further constrains movement along the Y axis. Together, these factors establish constraints for all six degrees of freedom.

Solution to the black skin problem in turbine flange processing3

2) Clamping scheme (see Figure 4): Given the product’s complex shape, thin walls, and overall low rigidity, we incorporated some auxiliary supports during the fixture design process. These supports were put in place to enhance the rigidity of the entire fixture system, ensuring that the product size remains stable during processing and that the surface roughness meets the specified drawing requirements.

Solution to the black skin problem in turbine flange processing4

3 Process tracking and troubleshooting

During processing, it was observed that black skin appeared on the outlet flange (the affected area is highlighted in red in Figure 5). On the first day, 100 pieces were produced, and 8 of those exhibited black skin, resulting in a high scrap rate. The locations of the defects were very close together. On the second day, another 100 pieces were produced, with 7 displaying black skin, still indicating a high scrap rate and similarly concentrated defect locations. After two days of trial production, it became clear that the current milling process could not support mass production and required immediate improvement. Following a thorough evaluation by relevant professionals, the following corrective measures were proposed.

Solution to the black skin problem in turbine flange processing5

 

1) Analysis of Possible Causes and Countermeasures:
1. A significant error was detected in the size of the blank positioning part, as confirmed by 3D scanning.
2. The positioning of the fixture is inadequate, leading to error accumulation. A single product was repeatedly positioned, and the height of the fixed point was measured using a laser probe.

 

2) Troubleshooting Results:
1. After confirming through 3D scanning, it was determined that the size of the blank positioning part was within the tolerance range, ruling out this cause.
2. The same product was repeatedly positioned, and the height of the fixed point was measured with a laser probe (refer to Figure 6). The statistical data is outlined in Table 1.

Solution to the black skin problem in turbine flange processing6

After analyzing the statistics, it was noted that the height of the fixed point fluctuated significantly after each product clamping, with some variations reaching up to nearly 0.6 mm. In some cases, the height was above the theoretical position, while in others, it was below.

3) Investigation conclusion: The fixture system has issues that need optimization to ensure controllable changes in the fixed point position after repeated product clamping.

 

4 Fixture optimization

After our investigation, we identified issues with the positioning and clamping of the fixture. During a group review, we concluded that the vortex end cylinder was too short, and that the product outlet flange was located too far from the positioning point. A slight deviation at the positioning end could lead to significant misalignment at the outlet flange, which resulted in the occurrence of black skin during processing.

To address these issues while considering cost effects, we decided to add a positioning point and a clamping mechanism at the outlet flange to improve the original fixture. The new design is illustrated in Figure 7. In the future, we will assess the verification results to determine whether a complete redesign of the fixture structure is necessary.

Solution to the black skin problem in turbine flange processing7

 

5 Effect verification

Conduct repeated positioning verification for a single product and use a laser probe to measure the fixed point’s height. The statistical data is presented in Table 2.

Solution to the black skin problem in turbine flange processing8

 

After analyzing the statistics, it was determined that following the clamping of each product, the height of the fixed point still exhibited fluctuations. However, the maximum deviation after optimizing the fixture was less than 0.3 mm, which is within the acceptable tolerance range.

For production verification, the workshop ran continuous operations for two shifts. The scrap statistics revealed that out of 200 pieces produced, 198 were finished products and 2 were scrapped. One piece was scrapped due to a sudden power outage during lathe processing, while the other was discarded because of excessive polishing of its appearance. There were no issues with black skin.

 

6 Conclusion

This article outlines the design and optimization process of a fixture for thin-walled products with an eccentric structure. A fundamental principle to follow in fixture design is the 6-point positioning principle. It’s essential to integrate the entire process, from initial modeling to final product processing. This includes avoiding the positioning at the parting line and the grinding area of the blank, while aiming to choose positioning points that are more remote. This approach helps to achieve an even distribution of blank errors across all processing positions. Accumulating errors at any specific processing stage can lead to product failure.

To address the challenges faced during actual production, it is crucial to employ specific measurement methods rather than proceeding without a plan. The cases discussed in this article represent common scenarios encountered in practical processing. I hope this article inspires colleagues in the industry.

 

 

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