Precision Engineering Solutions for Pantograph Pipe Machining

Today, the processing of pipe components plays a crucial role in the performance of pantographs. We have selected 3mm thick seamless aluminum tubes as the base material for processing. By analyzing the production process of the connecting pipe, developing tooling fixtures, and choosing and enhancing tools, we can improve both the efficiency of processing and the quality of the product. This approach addresses production bottlenecks while ensuring a smooth flow of assembly line operations.

 

1 Introduction

The pantograph is a crucial component of high-speed railways, locomotives, and urban rail vehicles. The upper frame, which is welded together with connecting pipes, serves as the core of the pantograph. This device is mounted on top of the locomotive and must withstand various environmental challenges, such as wind, frost, rain, and snow, while maintaining sufficient strength, rigidity, stability, and mechanical longevity, especially at high speeds.

The primary function of the pantograph is to collect electricity from the overhead contact line, providing the necessary power and energy for vehicle operation. The pantograph operates in three states: raised, held, and lowered. The reliability and stability of this component are directly linked to the overall performance and safety of high-speed railways, locomotives, and urban rail vehicles.

The upper frame, which is welded based on the connecting pipe, plays a vital role in the assembly. The connecting pipe is welded to the reducer, and its geometric tolerances, angular positioning of the keyway, bevel angles, and symmetry of the groove are all critical for accurate positioning and connection of the upper frame. Consequently, the quality of processing for the connecting pipe directly impacts the performance of the pantograph, which in turn affects the normal operation of high-speed railways, locomotives, and urban rail vehicles.

 

2 Parts analysis and processing difficulties

The connecting pipe is a thin-walled aluminum alloy pipe with a diameter of 60mm. Its structure is depicted in Figure 1, while the actual object is shown in Figure 2. The processing challenges are outlined below.

1) Machining 41° bevels that are symmetrical on both sides presents challenges in controlling clamping deformation, machining deformation, and tool vibration.

2) It is difficult to maintain the correct relationship between the symmetrical machining positions of the four closed keyways and the open keyways, as well as the angle of the keyway and the bevel’s position.

3) The wall thickness of the connecting pipe is 3mm, classifying it as a thin-walled pipe fitting. When clamping the bevel in the vise, the tool overhang can be excessive, leading to workpiece vibration. Various cutting and clamping methods may contribute to issues such as insufficient rigidity in the machining system, poor surface roughness, and deviations in machining size.

4) While machining a 6.5mm wide keyway, the drill tip often slips and fails to stay centered on the top arc surface of the workpiece. The cutting force can lead to built-up edge formation in high-temperature conditions, resulting in uneven cutting, chipping of the milling cutter, and deformation of the keyway. Consequently, it becomes challenging to ensure the correct positional dimensions and radial angle relationships between the slots, creating production bottlenecks in batch processing. Therefore, technical breakthroughs are urgently needed.

 

3 Inclined surface processing route

Figure 3 illustrates the clamping method for the clamping plate. It features two inclined surfaces at a 41° angle, which are connected by a pipe. Initially, a 41° boring machine clamp was used to process these inclined surfaces. The clamping plate bolts secure and press against the curved surface. A 30mm extended end mill is selected for machining. However, this original method presents several issues, including long clamping times, low efficiency, and the tendency for the workpiece to rotate, which can lead to angular displacement and compromise production safety.

 

 

The CNC milling method is utilized to address the challenges associated with the 41° bevel milling process, ensuring precise control over size, angle, safety, and product quality. The thin-walled pipe fittings are constructed from aluminum alloy. When processing the two 41° bevels on the 60 mm diameter pipe fittings, a 30 mm extended end mill is employed. Due to the high cutting force and long tool length, there is a tendency for vibration, which can lead to deformation of the bevel corners under the cutting force.

Figure 4 illustrates the clamping method used in CNC milling. Initially, a clamping plate bolt was employed for securing the workpiece. The primary force at play is the clamping force; however, the cutting force can cause the product to rotate during processing, resulting in displacement and negatively impacting machining quality. To mitigate this issue, a pair of diameter pipe combination clamps was developed for the 60 mm aluminum alloy thin-walled pipe fittings. This design ensures that the arc surface of the clamp block makes contact with the arc surface of the pipe fitting. By wrapping and securing the workpiece with the clamp block, the bevel can be machined effectively. The advantages of this clamping method are as follows:

 

1) The clamping method effectively prevents deformation of the pipe fittings caused by clamping force, facilitating smoother processing.
2) An overall clamping approach is utilized to ensure the workpiece is evenly secured, which effectively combats processing deformation caused by fluctuations in milling force on the tool.
3) The inclined surface is formed and clamped in one operation, ensuring accuracy in both angle and symmetry.
4) Direct pressure is applied on both vise jaws to simplify the clamping method, providing precise positioning for the workpiece.
5) This method significantly enhances processing efficiency and product quality.

During the processing phase, it was observed that due to variations in the lengths of the blanks, it is necessary to leave a margin on the right side. Therefore, during rough processing, the right inclined surface is machined using reverse milling, while the left inclined surface is machined with forward milling. In the fine processing stage, the right inclined surface is machined with forward milling and the left inclined surface with reverse milling. This approach aims to prevent excessive material removal and avoid issues such as overcutting and gnawing during the work on the inclined surfaces.

In finishing, the cutting allowance for the workpiece undergoing reverse milling reduces from thick to thin, while for the workpiece undergoing forward milling, the allowance increases from thin to thick. The combination of forward milling and reverse milling techniques is employed to prevent deformation of the thin-walled tube due to changes in cutting force.

 

4 Keyway processing process route

Figure 5 illustrates the grinding of the milling cutter’s cutting edge alongside the finished pipe fitting. During the processing of closed and open keyways in the aluminum alloy workpiece, significant deformation occurs. High-speed machining generates high cutting temperatures, which in turn accelerates tool wear. This increased wear necessitates frequent sharpening and replacement of the tools, ultimately impacting processing efficiency and raising tool costs.

There are eight keyways that need to be processed on the connecting pipe, each requiring specific angle relationships. A universal dividing head, referenced as T11160A, will be used for clamping. Based on the drawing requirements, the following points are analyzed: ① How to ensure the positional relationship between the bevel and the keyway. ② How to determine the angle relationship between the bevel and the keyway. ③ The position and size requirements among the keyways.

 

5 Processing method

5.1 Positioning method of bevel and keyway

When clamping the pipe fitting, start by securing the end of the bevel on the connecting pipe that has already been processed. Position it appropriately and place it into the clamping direction of the dividing head for angle alignment. The three screw holes in the dividing head are primarily used to support the bevel. Align the bevel angle of the pipe fitting with these screw holes, using them as a rough reference for positioning.

As shown in Figure 6, the dividing head should clamp the end of the connecting pipe that requires processing. The machine tool worktable serves as the reference plane. To ensure accurate vertical positioning of the bevel at 90 degrees, utilize a right-angle ruler to calibrate it, making sure the bevel fits snugly against the right-angle edge of the ruler.

After analyzing the CNC machined components and fixture, the processing method will involve first machining in a clockwise direction, followed by a counterclockwise rotation. This approach is designed to eliminate any processing gaps associated with the dividing head, ensuring that the radial angle of the processing position remains accurate.

5.2 Comparative analysis and optimization of tools

The keyway cutter processing is illustrated in Figure 7. After thorough verification, it has been determined that a keyway cutter with a diameter of 6.5 mm is necessary for the operation. The cutting edge of the keyway cutter is shaped to resemble a drill tip at the main cutting edge to effectively grind the bottom tooth of the keyway. Given that the processing thickness of the pipe keyway is only 3 mm, the milling cutter is first used to drill a bottom hole through the pipe, and then the keyway is milled. This approach helps ensure the rigidity of the tool during processing. The processing steps and their analysis are outlined below.

 

1) When processing thin-walled pipe fittings, the keyway milling cutter experiences force primarily at the center, where the chip groove is relatively shallow. Because aluminum alloy is a soft material, it tends to stick during milling. To prevent the keyway milling cutter from deforming the workpiece, the cutter should be ground into a drill tip shape.

2) When using a keyway milling cutter on the pipe surface, the circular arc interpolation feed method is employed. However, this method can lead to issues with centering, resulting in uneven force on the milling cutter. Such instability can cause the radial angle position of the keyway to shift, potentially leading to scrap.

3) Prior to keyway processing, it is advisable to grind the tool into a drill tip shape for improved centering, which ensures the correct position and size of the keyway’s radial angle. The integrated drilling and milling method reduces tool change time, enhances processing efficiency, and maintains product quality. This process involves using the same milling cutter to first drill the bottom hole, followed by milling the keyway directly.

4) The keyway milling cutter is shaped into a drill tip and features a chip groove to effectively minimize sticking.

5) During processing, the operation is adjusted from milling to drilling to ensure stable centering.

6) After the composite tool drills the bottom hole, it directly penetrates 3mm into the keyway for milling, making the CNC manufacturing process both quick and convenient.

 

5.3 Solution to the processing problem of thin-walled pipe parts

During processing, the thin wall of the pipe is influenced by cutting forces, which can lead to deformation of the groove width, resulting in unwanted openings and contractions. This compromises product quality. After multiple tests, we determined that a f6.5mm keyway milling cutter is the best option. By using a well-considered ratio of cutting parameters, we can effectively control the deformation of the product’s keyway opening and improve processing stability. The optimized cutting parameters are detailed in Table 1.

After processing, the optimized cutting parameters enhanced processing efficiency and quality, effectively addressing the challenges of thin-walled pipe parts.

 

6 Fixture production and tool improvement

1) By optimizing and improving the process, we enhanced processing efficiency and ensured the verticality, position, coaxiality, and parallelism of both the groove and the bevel.

2) The introduction of the clamp block effectively prevented deformation of the pipe fittings during bevel processing. The production of this fixture made clamping more secure and straightforward, making it more suitable for large-scale manufacturing and significantly improving efficiency.

3) The machine tool’s versatility was expanded, fully resolving the production challenges associated with the bottleneck process.

4) The grinding of the keyway tool reduced tool change time and increased processing efficiency. We utilized an f6.5mm keyway tool to grind the cutting edge, transforming it into a drill bit. This keyway tool features a short cutting edge, high strength, and stable centering. These improvements significantly reduced tool change and installation time, lowered tool costs, and enhanced tool utilization and processing efficiency.

 

7 Comparison before and after tool improvement

By enhancing our tools, we provide precise positioning and orientation benchmarks for subsequent processes, ensuring that the processing and welding requirements are met effectively. Selecting the appropriate spindle speed and feed rate significantly improves the processing accuracy of each component. Using a custom-made clamp to secure the workpiece not only ensures reliable clamping and quick positioning but also reduces tooling production costs and shortens the production cycle. This approach guarantees product quality while increasing production efficiency. Additionally, by carefully selecting processing parameters, we minimize deformation during processing, which enhances both the quality and efficiency of the manufactured parts. The annual output of connecting pipes from 2019 to 2022 is illustrated in Figure 8.

8 Conclusion

By improving the processing method and tool of the pantograph connection tube, applying and promoting it in the processing of various pipe parts, the processing efficiency is improved, the production requirements of the process line are met, the stability and service life of the product are guaranteed, and value is created.

 

 

 

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With Anebon’s leading technology and our spirit of innovation, mutual cooperation, and development, we aim to build a prosperous future together with your esteemed enterprise as an OEM manufacturer of custom high-precision aluminum die casting parts, turned metal parts, and custom CNC milling steel components.