Fault Phenomenon
In collaboration with an elevator company, our school has produced a stepped shaft used in elevators. The parts are shown in Figure 1. The machine used is the CAK4085ni economical CNC lathe manufactured by Shenyang First Machine Tool Plant. The workpiece is clamped using a chuck and a center, with support from a center frame, as illustrated in Figure 2.
Figure 1
After roughing and finishing the part, the outer diameter of φ26mm was found to be out of tolerance. Measurement revealed an inverted cone of about 0.2 mm at the end of the φ26mm surface (smaller on both ends), while the right side of the φ26mm and the outer diameter of the φ21mm section remained relatively unchanged. This issue does not occur consistently; it sometimes appears in sequence, and other times only occasionally. Due to its intermittent nature, diagnosing this fault posed some challenges and introduced uncertainty into the process.
Figure 2
2. Status of Machine Tool Use
The machine has been in use for approximately one year, with proper internal lubrication and routine maintenance. There have been a few minor collisions, but none that significantly affected the machine's performance. The machining accuracy remains acceptable, and during normal production, the parts meet the required dimensions. However, this specific issue has emerged, causing inconsistency in the final product.
3. Troubleshooting Approach
The troubleshooting process started with the most direct factors that could affect dimensional changes. We then systematically checked the machine tool for other potential issues. By analyzing external and internal factors, we were able to identify and resolve the problem. External checks focused on the machine bed feet, while internal checks followed a logical sequence: top, bushing, and center frame — then the tool holder system, followed by the middle slide connection system, and finally the bed saddle connection system.
4. Fault Analysis and Resolution
(1) Bed Feet – If any of the bed feet were loose, the instability could cause vibration and tilting of the bed, potentially leading to the observed surface irregularities. During inspection, no looseness was found in the bed feet, so this factor was ruled out.
(2) Top, Bushing, and Center Frame – Based on experience, size variations near the top or center frame might be caused by misalignment or wear. We checked the center frame’s stability, replaced the bushings and bearings, and adjusted the center height. However, the issue persisted, indicating that the top and center frame were not the root cause.
(3) Tool Holder System – Issues such as improper locking of the tool holder or loosened connections between the tool holder and the middle slide plate could lead to dimensional inaccuracies. We checked the four screws under the tool holder, tested for movement using a dial indicator, and confirmed that the tool holder was secure. Therefore, the tool holder system was not responsible for the fault.
(4) Middle Slide Joint System – Components such as the screw nut, coupling, and support bearing can cause irregularities if they become loose. After checking and adjusting these components, the issue still occurred, suggesting that the middle slide system was not the primary cause.
(5) Bed Saddle Connection System – Similar to the middle slide system, we inspected the pressure plate and guide rail interface. A significant gap was found between the pressure plate and the guide rail, especially on the left side near the cutting area, measuring up to 0.3 mm. After re-polishing and reinstalling the pressure plate, the gap was reduced to 0.03–0.05 mm. Following a trial cut, the problem did not recur. This confirmed that the excessive gap in the saddle connection system was the main cause of the fault.
5. Conclusion
After resolving the issue, I discussed it with my colleagues and enterprise engineers. They mentioned that they had never encountered a similar situation, especially since their machines had been in use for five to six years. It made me wonder why this issue hadn’t occurred in their facilities. Upon further comparison, I realized that their equipment was a larger 50 series machine with heavier tool holders (25×25 mm), whereas our machine was a smaller 40 series model (actually 36 mm in bed size) with a lighter 20×20 mm tool holder.
During roughing, the cutting force can push the tool against the workpiece. When finishing, the machine's light weight may allow the saddle to rebound slightly due to the guide rail’s reaction. If this rebound isn’t fully eliminated, it can lead to the observed irregularities. With a heavier saddle, the rebound would be less noticeable, and such a failure would be unlikely. Thus, the problem stemmed from the machine's lightweight design combined with the saddle’s insufficient weight to counteract the rebound effect.
Wheel Bearing,Automobile Wheel Hub Bearing Assembly,Automobile Wheel Hub Bearing,Hub Bearing Assembly
Shanghai Yi Kai Cheng bearing Co., LTD , https://www.ykchbearing.com