Precision Chamfering Techniques for Piston Pin Oil Passages
The structural characteristics and processing challenges of the transition chamfer between the inner hole of the piston pin and the lubricating oil hole have been analyzed. A feasible process plan for chamfering the inner hole has been proposed. Issues such as low processing efficiency and difficulties with tool clamping and positioning are addressed through the design of specialized fixtures and bidirectional countersinks. Additionally, the advantages and disadvantages of two chamfering process schemes are compared to identify an efficient processing method suitable for mass production.
01 Introduction
The piston pin is a crucial component of the engine. It is installed in the pin hole of the piston and the small end hole of the connecting rod. Due to the high-speed sliding motion between the piston pin and the small end hole of the connecting rod during operation, a lubricating oil hole is designed in the piston pin. To ensure the smooth flow of lubricating oil, fillets and chamfers are integrated at the intersections of the lubricating oil hole with the outer surface and inner hole of the piston pin, and there are strict requirements for surface roughness.
The kinematic pair formed by the piston, connecting rod, and piston pin operates in a high-speed sliding state throughout the engine’s operation. If the fillet at the intersection of the lubricating oil hole and the outer surface, or the chamfer at the intersection with the inner hole, is not properly processed, sharp corners, burrs, or poor lubricating fluid flow may result. This can lead to excessive strain on the piston pin and connecting rod, interfering with the normal functioning of the entire engine.
The transition fillet between the outer surface of the piston pin and the lubricating oil hole is external to the part. It can be machined using a forming milling cutter or a countersink on a machining center or drilling machine. The area being processed is easy to observe, and the tool structure faces fewer restrictions from the part’s design. Chamfering can be accomplished using standard processing methods.
In contrast, the transition chamfering between the inner hole of the piston pin and the lubricating oil hole is located inside the part, which presents challenges due to limited operating space and difficulty in observation. Accurately positioning and installing the tool can be problematic. The use of a forming milling cutter in this scenario may face issues such as insufficient tool rigidity and the potential for tool vibration. Therefore, it is essential to investigate processing technologies that enhance production efficiency and processing quality to meet production demands.
02 Technical requirements
The structure of a specific type of engine piston pin is illustrated in Figure 1. This part is made from alloy steel and undergoes carburizing and quenching processes. After quenching, the hardness of the part exceeds 60HRC. Due to its high hardness, machining fillets and chamfers using standard methods is challenging. Therefore, the process involves finely grinding the outer circumference of the part before quenching, and chamfering the intersection of the lubricating oil hole and the inner hole, based on the outer circle.
03 Analysis of processing difficulties
The chamfer width (L) and the angle between the inner hole of the piston pin (D1) and the lubricating oil hole (D2) are both 80°. The distance (L1) from the middle lubricating oil hole to the end face must be greater than 135 mm. If the conventional countersinking method is employed and the processing area is far from the end face, while the inner hole of the part is small, this can lead to difficulties in tool positioning and installation.
To address these challenges, the oil hole down-cutting method is recommended. Given that the outer diameter (d1) of the part is large and the diameter of the lubricating oil hole is small, the aspect ratio of the lubricating oil hole reaches 7.3:1. Using a forming milling cutter for this type of processing may result in insufficient tool rigidity and increased tool vibration.
04 Formulate a processing plan
Based on the structural characteristics of the part and an analysis of the processing challenges, the following initial process plan has been developed:
1. Utilize a forming milling cutter on a vertical machining center. Before quenching the CNC auto parts, finely grind the outer circle designated as d1. The target size for the outer circle is (d1 + 0.4) mm, and the tolerance should be maintained within 0.03 mm; this will serve as the positioning reference for subsequent processing steps.
2. On the vertical machining center, a dividing head and a special center clamp will be employed to design a proprietary chamfering milling cutter for processing the 80° chamfer of the inner hole. Since the milling cutter must cut from the D2 lubricating oil hole to the chamfering area, its diameter is designed to be d2 = D2 – 0.5 mm. The milling cutter must meet the requirements for interpolation processing during chamfering.
3. The diameter of the tool bar is set to d3, and the tool bar length is determined to be L2. The internal chamfer of the piston pin created by the forming milling cutter is illustrated in Figure 2, where d3 = D2 – 0.5 – 2Ltan(40°) – 1.5 mm and L2 = (d1 – D1) / 2 + 30 mm.
This process utilizes CNC equipment to chamfer the intersection of the lubricating oil hole and the inner hole in a single clamping setup. However, due to the significant length-to-diameter ratio of the tool bar, which is 14:1, issues such as tool vibration and limited cutting depth can arise. To address these concerns, it is advisable to use a carbide tool bar to enhance tool rigidity and improve both processing quality and efficiency. This method is particularly suitable for small-batch trial production and requires a low investment.
2) Design specialized fixtures and countersinks to perform part chamfering on an ordinary Z35 drilling machine. First, the outer circle (d1) is finely ground before the part is quenched. The processing size of the outer circle is (d1 + 0.4) mm, with a tolerance controlled within 0.03 mm, which will serve as a reference for subsequent positioning processes.
Begin by drilling and expanding 12 lubricating oil holes (D2) on the ordinary drilling machine. Next, place the CNC machinery parts on the V-shaped positioning fixture (block 4) and install the positioning block (block 5) to achieve axial positioning. Insert the positioning pin (pin 6) into one of the lubricating oil holes to achieve radial positioning and secure the part with clamping nut (nut 3).
A tool delivery mechanism (mechanism 2) is designed at one end of the positioning fixture. This mechanism includes a tool positioning device that serves two functions:
1. It accurately positions and clamps the tool for processing the chamfer of the internal intersecting holes.
2. It accurately positions the tool to the mouth of the hole that needs to be chamfered.
Additionally, a bidirectional countersink and guide rod (guide rod 1) is designed to utilize the processed radial hole for positioning. This setup allows the bidirectional countersink and guide rod to simultaneously process two internal intersecting hole chamfers on the drilling machine. The bidirectional countersink ensures the correct chamfer angle and shape, while the scale on the drilling machine guarantees the correct chamfer size.
This solution utilizes standard processing equipment, making it easy to operate and highly efficient. It is well-suited for mass production. Figure 3 illustrates the use of special fixtures and tools to process the internal chamfer of the piston pin on a drilling machine.
05 Conclusion
By implementing the methods outlined above and taking into account the current production batch conditions, the company has selected Solution 2 for hole chamfering. This solution involves using special fixtures and bidirectional countersinks to address issues related to low chamfering efficiency and the challenges of clamping and positioning the tool at the intersection of the engine piston pin lubricating oil hole and the inner hole. This approach ensures high-quality processing of such parts. Additionally, the solution utilizes standard processing equipment, is easy to operate, and is well-suited for mass production. It also provides valuable insights and guidance for processing similar parts.
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