Fault Phenomenon
In collaboration with an elevator manufacturing company, our school has produced a stepped shaft used in elevator systems. The part is shown in Figure 1. The machine used for this process is the CAK4085ni economical CNC lathe manufactured by Shenyang First Machine Tool Plant. The workpiece is clamped using a chuck and a tailstock, with support provided by a center frame, as illustrated in Figure 2.
Figure 1
After roughing and finishing, it was observed that the outer diameter of φ26mm was out of tolerance. Measurement revealed an inverted cone of approximately 0.2 mm at the end of the φ26mm section (narrower on both sides), while the right end of the φ26mm and the outer diameter of the φ21mm section remained relatively stable. This issue does not occur consistently; sometimes it appears continuously, other times only occasionally. Due to its intermittent nature, diagnosing the root cause was challenging and involved some uncertainty.
Figure 2
2. Machine Tool Usage Status
The machine tool has been in use for about one year, and its internal lubrication and daily maintenance are in good condition. There have been a few minor collisions, but nothing serious. The machining accuracy of the machine is generally reliable, and during normal operation, the parts produced meet the required dimensions. However, the current issue is causing some irregularity in the finished products.
3. Troubleshooting Approach
We started by focusing on the most direct factors that could affect dimensional changes, then moved on to check other potential fault sources related to the machine. The problem can be addressed by analyzing both external and internal factors. External factors involve checking the machine’s base feet, while internal factors are examined in the following order: top, bushing, and center frame – tool holder system – middle slide connection system – bed saddle connection system.
4. Fault Analysis and Resolution
(1) Base Feet: If any of the base feet are loose, it could cause the bed to vibrate or tilt during machining, leading to surface irregularities on the workpiece. During inspection, we checked the tightness of the base feet, and found no looseness, so this factor was ruled out.
(2) Center Point, Bushing, and Center Frame: Based on past experience, size variations near the center point might be caused by issues such as a misaligned or blocked center point. Similarly, the center frame or bushing could contribute to dimensional changes if the center is not properly aligned. We checked the center frame’s stability, replaced the bushing and bearings, and adjusted the centering height, but the issue persisted. Therefore, these components were excluded from the cause.
(3) Tool Holder System: Issues with the tool holder, such as improper locking or loosening of the mounting screws, could lead to size variations. We checked the four mounting screws under the tool holder, used a magnetic dial indicator to test for movement, and confirmed that there was no looseness. Thus, the tool holder system was also ruled out.
(4) Middle Slide Connection System: This includes the lead screw nut, coupling, round nut, and support bearing. Any looseness in these components could result in the observed fault. We inspected each component and adjusted the tightness accordingly. However, after adjustment, the symptoms remained unchanged, so this system was not the primary cause.
(5) Bed Saddle Connection System: Similar to the middle slide system, the bed saddle connection was inspected step by step. During the inspection, we found a gap between the pressure plate and the guide rail, especially on the left side near the cutting area, measuring about 0.3 mm. After re-polishing and reinstalling the pressure plate, the gap was reduced to 0.03–0.05 mm. A trial cut showed that the issue was resolved, confirming that the excessive gap between the pressure plate and guide rail was the main cause of the fault.
5. Conclusion
After resolving this issue, I discussed it with my colleagues and engineers from the enterprise. They mentioned they had never encountered similar problems before, especially since their machines have been in use for five to six years. It was puzzling why such an issue would arise now. Upon further comparison, I realized that the equipment used in their factory is a larger 50-series machine, with larger tool holders (25×25 mm), which results in a heavier saddle. In contrast, our school's machine is a smaller 40-series model (actually only 36 mm in bed size), with a lighter 20×20 mm tool holder.
During roughing, the cutting force presses the tool against the workpiece. When finishing, the light machine may cause the saddle to rebound slightly due to the lack of sufficient weight, creating a small gap that affects the final dimensions. If the saddle is heavy enough, the rebound effect is negligible, and no such issue occurs. This explains why the problem appeared intermittently and why it didn’t happen on the heavier machines.
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