Troubleshooting Common Defects in Piston Rod Surface Finishing
The challenges encountered in finishing the piston rod body have been analyzed, leading to the development of a combined processing plan. Existing tooling has been utilized to pair with an integrated forming milling cutter and a thread milling cutter. This setup enables the processing of equal-diameter holes and stepped holes on the circumferential surface of the surfacing layer. This approach not only provides theoretical support but also practical experience for finishing similar products in the future.
1 Preface
The piston rod body is a crucial component of underwater equipment, specifically designed with its operating environment and performance requirements in mind. To enhance its anti-corrosion properties, a method of local surfacing with nickel-based alloys is employed. The surface roughness of the sealing area must meet extremely high standards to ensure the reliability of the metal seal. This requirement increases the complexity of the manufacturing process and raises the risk of product defects. Therefore, the effectiveness of the finishing plan for the piston rod body directly impacts both the processing efficiency and the qualification rate of the finished product.
2 Analysis of processing difficulties
In recent years, the state has strongly supported the integration of schools and enterprises, encouraging educational institutions to partner with modern processing companies. This initiative aims to enhance teachers’ practical skills while using professional knowledge to help businesses address technical challenges. Students gain the opportunity to engage with real-world products, immerse themselves in corporate culture, and develop a better understanding of quality awareness.
By employing a brainstorming analysis strategy and examining the engineering drawings of the piston rod body used in Christmas trees, we can identify the causes of difficulties in the precise processing of this component. We have summarized the key factors affecting fine processing into five categories: equipment, personnel, measurement, materials, and processing methods. A fishbone diagram has been constructed to clearly outline these relevant factors. The analysis of the challenges related to the fine processing of the piston rod body is illustrated in Figure 1.
(1) Existing tooling cannot meet the requirements of workpiece processing.
To ensure compatibility between the workpiece processing position and the processing requirements with the existing equipment, we have decided to utilize a floor-standing boring and milling machine for this task. However, due to the small size of the workpiece structure, using conventional V-shaped iron tooling for clamping poses a risk of exceeding the processing range of the machine during specific operations.
Based on the external dimensions of the piston rod body and the available tooling, we conducted a CAD simulation for clamping. This simulation identifies areas where interference may occur. The clamping simulation for the Christmas tree piston rod body is illustrated in Figure 2. The end face machining interference area is shown in Figure 3, while Figure 4 displays the outer cylindrical electrical connector mounting hole machining interference area.
In Figure 2, the processing of the lateral holes on the outer circumference of the piston rod body is nearly complete. Figure 3 shows that the minimum processing position of the workpiece end face cladding layer is 89.8 mm from the worktable, which is very close to the machine tool’s low-position processing limit of 90 mm. This proximity raises concerns that this processing task may not be achievable. Additionally, Figure 4 highlights an interference issue between the machine tool spindle and the conventional V-shaped iron tooling. During processing, there is a risk of tool vibration due to the long suspension of the tool, as well as a risk of collision because of the minimal safety gap between the machine tool spindle and the tooling.
(2) The cladding layer in the workpiece finishing area exhibits poor machinability.
To enhance the corrosion resistance of the local area on the Christmas tree piston rod body, nickel-based alloy cladding is applied to this region. The processing tasks for the cladding layer primarily involve the creation of equal-diameter holes and stepped holes.
1) Processing equal-diameter holes in the cladding layer: According to the literature, the hardness of the corrosion-resistant nickel-based alloy is relatively high, and there may be localized hard spots after cladding. High-cobalt drill bits with surface coatings are often utilized to effectively process these equal-diameter holes.
2) Processing of stepped holes on the circumference.
The machining requirements for the stepped hole on the circumference are illustrated in Figure 5. This stepped hole consists of a main section with equal diameters and a transition section that features a tapered hole. It is essential that these two sections transition smoothly, with no visible cutter marks. The tapered hole section should be machined using a flat rotating disk on the machine tool, which can subtly increase the cumulative error, directly affecting the machining accuracy. Therefore, it is crucial to consider optimizing the machining process.
As the fundamental component of underwater equipment, this part must operate in an underwater environment for extended periods. Therefore, it requires a highly reliable sealing solution to ensure its proper functioning. To achieve this, the matching interface must possess excellent sealing characteristics. Designing multiple sets of metal sealing cones necessitates a specific surface roughness of Ra = 0.8 μm, making the development of an effective processing plan crucial.
Additionally, detecting the dimensions of the workpieces poses challenges. The small size of the processed parts renders conventional measuring tools ineffective. It is essential to control factors such as the size of the tool cutting edge and the machining accuracy of the equipment to enable equivalent measurement.
Furthermore, operators may not be familiar with the processing procedure. As the key component of a new product, this workpiece represents a first-time application of its processing technology. Variations in how operators interpret the CNC process documentation can introduce risks during the machining process.
3 Solutions
Design of Custom Tooling
The processing capacity range of the machine tool is fixed and cannot be changed. Therefore, we can only work within the processing limits of the machine by designing new tooling. To achieve this, we will focus on two key aspects based on the basic state of the workpiece to be processed:
1. Protection of the Processed Area: Since the end of the workpiece features an external thread structure that has already been machined, we will design a positioning protective sleeve. This sleeve will safeguard the thread and ensure that the clamping outer circle size remains consistent.
2. Modification of Existing V-Shaped Tooling: We will enhance the current V-shaped iron tooling by creating a new circular pad. The surface of this pad will be processed using a grinder to meet the required standards for both surface flatness and surface roughness, in accordance with the specifications of the reference pad.
The newly designed tooling is illustrated in Figure 6, while its application is shown in Figure 7.
(2) Use customized tools
Customized high-cobalt drill bits with coatings are used for drilling carbide cladding layers. For processing stepped holes, floor-standing boring and 5 axis CNC milling machines typically utilize flat disc cutter heads. Given the limited size of the processing area and the precision required for flat discs, after adjusting the process, an integrated composite reamer, an internally cooled adjustable fine boring cutter, and a thread milling cutter are employed for combined processing. A customized tool is illustrated in Figure 8.
(3) Solve the problem of processing inspection
Solve the inspection problem by reasonably selecting processing tools and formulating complete processing technology to achieve equivalent inspection.
(4) Effectively implement the process plan
In conjunction with the production schedule, the process engineer conducts technical briefings and participates in the processing throughout to ensure effective implementation of the process plan.
(5) Implementation of the plan and display of results
Promote the implementation of the solution throughout the process, and the processing results are shown in Figure 9.
4. Conclusion
This paper analyzes the causes of machining difficulties in the precision manufacturing of piston rod bodies and proposes improvements to the CNC machining process. It develops a comprehensive machining plan, modifies existing tooling, and introduces integrated forming milling cutters and thread milling cutters. These innovations address issues related to equal-diameter holes in the end face cladding layer and stepped holes on the circumferential surface, providing both theoretical and practical support for machining similar products.
As mass production approaches, challenges related to tool wear will significantly impact quality, efficiency, and cost. Therefore, ongoing process optimization for this product is essential, particularly in relation to upgrading CNC machine tools and utilizing integrated forming reamers.
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