Figure 1: Pin Shaft Parts
The problem under consideration is illustrated in Figure 1. Linde-Xiamen Forklift Co., Ltd. utilizes a static pressure transmission forklift, with the steering knuckle supported by a critical pin component. Originally sourced from Germany, this part required localization following a joint venture in China. The machining of this component presents several challenges:
(1) The raw material used is 42CrMoS4 tempered bar stock, exhibiting a strength of 1.2 GPa and hardness of 55 HRC—classified as high-strength and high-hardness material.
(2) The machining accuracy requirement for the middle section of the pin shaft is f33.33-0.013mm, falling within grades 5 to 6.
(3) The process must be completed in one turning operation without grinding, which adds further complexity.
To meet these requirements, specific measures were taken in tooling, equipment, and technology. Through repeated CNC turning experiments, a feasible machining program was developed, enabling successful high-precision batch NC turning of the part.
Figure 2: Schematic Diagram of Pre-Turning Tooling Process
In the machining of f33.33-0.013mm parts, the outer surfaces require high precision, with grade 5 to 6 accuracy. Additionally, the concentricity on four rotating surfaces (f40n6, f33.33-0.013mm, and f32t6) must be maintained within 0.02mm. To ensure coaxiality and meet accuracy standards, all lathe operations must be performed in a single setup.
Before finishing, the left end's inner hole and a portion of the outer surface (f39mm) are machined using the device shown in Figure 2. Then, both ends of the workpiece are positioned with a tip and clamped using a hydraulic chuck for finish turning (Figure 3). Due to potential eccentricity in the center hole, a special hydraulic compensation chuck ensures reliable clamping. This chuck evenly distributes clamping force across all jaws, ensuring secure grip even when the center hole is off-center.
Figure 3: Fine Turning Positioning Clamp
Turning Speed (m/min) | Cutting Amount (mm/r) | Tool Quantity (mm)
---|---|---
Rough Turning | 120 | 0.35 | 3.0
Semi-Rough Turning | 160 | 0.20 | 0.3
Finishing Turning | 160 | 0.12 | 0.05
Figure 4: Error Distribution Map on the f33.33-0.013mm Outer Circle
To achieve the required machining accuracy for the two f33.33-0.013mm outer surfaces, the CNC lathe must have a positioning accuracy of ±0.01mm and repeatability of ±0.003mm. Due to the high strength and hardness of the workpiece, large cutting forces are generated, leading to machine vibration that can affect turning accuracy. Therefore, a large-sized CNC lathe with strong rigidity is necessary.
We selected the PU-MA12-LB CNC lathe from Daewoo Heavy Industries, Korea. Its main technical parameters include a maximum bed diameter of f570mm, a spindle motor power of 26kW, a speed range of 20–2500 r/min, and positioning accuracy of ±0.01mm (X-axis) and ±0.02mm (Z-axis), with repeatability of ±0.002mm (X-axis) and ±0.003mm (Z-axis).
The turning process includes roughing, semi-finishing, and finishing. The tools used are Sandvik’s DNMG150608-PM 4025 for roughing, VNMG160402-PF 4015 for semi-finishing, and TNMG160408-PF 5015 for finishing. The turning parameters are detailed in the table above.
Key Measures to Ensure Parts Processing Accuracy
During the turning process, we observed that the dimensional fluctuations in the two parts were significant, with more pronounced overshoots, making it difficult to guarantee high-precision batch processing. To address this, we implemented the following measures:
(1) Before each official turning operation, the machine tool is allowed to run for about 30 minutes to reach thermal equilibrium. Several scrapped workpieces are then turned to stabilize the tool temperature field, simulating conditions during actual cutting. An empty cycle program can also be programmed to achieve this.
(2) Once the turning process begins, it should be continuous, with a full batch of workpieces turned at once. Interruptions, even if the machine remains running, can cause changes in the tool tip temperature field, affecting accuracy due to thermal expansion or contraction. Our practical experience has shown that this effect significantly impacts the accuracy of critical areas like f33.33-0.013mm.
(3) During the turning process, the workpiece's accuracy should be continuously monitored. Based on the dimensional trends after each cut, the compensation values for the finishing tool in the CNC system's tool offset parameters are adjusted accordingly.
By implementing the above process, tooling, equipment, and measures, we successfully achieved high-precision batch NC turning of the part, meeting the drawing specifications. Only a few parts showed minor deviations due to accidental factors. Figure 4 illustrates the error distribution on a f33.33-0.013mm circle after a batch of parts were machined. The dashed lines represent the upper and lower limits of the diameter. As shown, the error distribution is well-controlled.
This approach not only ensures high-precision batch processing but also reduces the number of processing steps, improves efficiency, and saves investment in cylindrical grinders, thereby lowering overall production costs.
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