Piston pin lubricating oil holes require precise fillets and chamfers with high surface roughness where they meet the outer circle and inner hole. This design ensures smooth oil flow and prevents damage to the pin and connecting rod during high-speed operation. Machining the outer circle’s transition fillet is relatively simple. Workers can complete this on a machining center or drill using a forming milling cutter or countersink.
Technical Requirements and Processing Challenges
The engine’s piston pin is made of alloy steel. It undergoes carburizing and quenching, achieving a hardness of over 60 HRC. This high hardness makes processing difficult. Therefore, we fine-grind the outer circle before quenching. This serves as a precise reference for subsequently chamfering the lubricating oil hole and inner hole intersection.
Chamfering between the piston pin’s inner hole and the lubricating oil hole poses challenges. Conventional countersinks struggle with positioning and installation. This is due to the chamfer’s width, angle, and its distance from the end face (over 135mm). The lubricating oil hole also has a large aspect ratio (7.3:1), which further complicates the process. Using a forming milling cutter often leads to insufficient tool rigidity and vibration.
Developed Processing Plans
Based on these challenges, we considered two main processing plans:
Plan 1: Machining Center with Forming Milling Cutter
This plan involves fine-grinding the outer circle to (d
outer_circle
- 0.4)mm. We maintain a 0.03mm tolerance, using this as a positioning reference. Then, on a vertical machining center, we use a special chamfering milling cutter with a dividing head and a special center. The milling cutter’s diameter is D
lubricating_oil_hole
- 0.5mm. It cuts from the lubricating oil hole, performing interpolation machining.
This CNC-based method completes chamfering in one clamping. However, the tool shank has a large aspect ratio (14:1). This makes it prone to vibration and limits cutting depth. To counter this, we recommend a carbide tool shank. This enhances rigidity, leading to better quality and efficiency. This approach works best for small-batch trial production.
Plan 2: Ordinary Drilling Machine with Special Fixture and Bidirectional Countersink
This alternative uses a common drilling machine (Z35). Similar to Plan 1, we fine-grind the outer circle as a reference. A specialized fixture ensures accurate axial and radial positioning. The key innovation is a bidirectional countersink and guide rod. This setup uses the already processed radial hole for positioning. It allows for the simultaneous chamfering of two inner intersecting holes.
This method is easy to operate, highly efficient, and ideal for mass production. It also uses ordinary equipment.
Conclusion
Considering the existing production batch, the company ultimately chose Plan 2: the hole chamfering solution. This solution uses a special fixture and a bidirectional countersink. It successfully addressed the previous issues of low chamfering efficiency and difficult tool clamping/positioning for the engine piston pin. This method ensures high-quality part processing. Its simplicity, efficiency, and suitability for mass production using standard equipment make it a valuable reference for similar part processing.