High-precision knuckle pin CNC turning

In the context of high-precision machining, a critical component known as the pin shaft is used in Linde-Xiamen Forklift Co., Ltd.'s static pressure transmission forklift. This key part was originally sourced from Germany and required localization after a joint venture in China. The challenge lies in the material properties and tight tolerances that must be achieved during the machining process. The raw material used for this part is 42CrMoS4 tempered bar stock, which exhibits a strength of 1.2 GPa and a hardness of 55 HRC—both characteristics of high-strength and high-hardness materials. Additionally, the machining accuracy requirements are stringent, with the diameter of f33.33-0.013mm needing to meet grades 5 to 6 precision. Most notably, the entire machining process must be completed on a single turning operation without the need for subsequent grinding. These constraints necessitated a well-designed machining strategy, involving advanced tooling, precise equipment, and optimized technology. To meet these challenges, a pre-turning setup was implemented using a CNC lathe, as shown in Figure 2. The left end of the part, featuring an inner hole and curved surface, was machined first. Then, both ends were positioned using a tailstock tip and clamped with a hydraulic chuck for the final turning process (Figure 3). A special hydraulic compensator chuck was employed to ensure consistent clamping force, even when the center hole was slightly eccentric. This helped maintain stability and precision throughout the machining cycle. The turning process was divided into three stages: roughing, semi-finishing, and finishing. The cutting parameters varied accordingly, with different tools used at each stage. For example, the roughing stage used Sandvik’s DNMG150608-PM 4025 insert, while the finishing stage utilized TNMG160408-PF 5015. The machine tool selected was the PU-MA12-LB CNC lathe from Daewoo Heavy Industries, known for its high precision and power. It featured a maximum rotational diameter of 570 mm, a spindle motor power of 26 kW, and positioning accuracy of ±0.01 mm in the X direction and ±0.02 mm in the Z direction. To ensure consistent accuracy across multiple parts, several key measures were taken. First, the machine was allowed to run idle for about 30 minutes before each official machining session, allowing it to reach thermal equilibrium. Second, once the machining began, it was kept continuous, avoiding interruptions that could affect the temperature field of the tool and thus the accuracy. Third, real-time monitoring of the workpiece dimensions was conducted, with adjustments made to the tool compensation values in the CNC system based on dimensional trends. These efforts led to successful high-precision batch machining of the part, meeting all design specifications. While a small number of parts occasionally exceeded the tolerance limits due to unforeseen factors, the overall error distribution, as shown in Figure 4, was well within acceptable ranges. The implementation of this process not only improved efficiency but also eliminated the need for a cylindrical grinder, significantly reducing production costs. This approach demonstrates how careful planning, advanced equipment, and precise control can overcome the challenges of machining difficult materials, ensuring high-quality results in mass production.

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Meets SAE J2494-3

Plug-in configurations

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