Diagnosing and Resolving Worktable Noise in Vertical Turning Centers
Due to the issue of abnormal noise during the rotation of the worktable on the CNC vertical turning center (VTC), an inspection and analysis of the worktable’s transmission mechanism were conducted after disassembly. It was discovered that wear on the worktable bearing was causing the loud, abnormal noise when the worktable rotated at high speed. After disassembling, reinstalling, and debugging the worktable, the equipment operated smoothly without any noise issues.
PART 1 Introduction
The imported vertical turning center (VTC) is primarily used for end face cutting and inner hole processing of workpieces. Recently, the machine’s worktable has been making abnormal noises during rotation and is prone to vibrations while processing workpieces. In more severe cases, this instability can lead to tool breakage. It’s essential to address this issue promptly.
PART 2 Fault Cause Analysis
The machine tool worktable produces abnormal noises when rotating, resembling the sound of a running tractor. Additionally, it tends to vibrate during the workpiece processing, leading to excessive surface roughness. The possible causes of the abnormal noise from the machine tool are analyzed as follows:
1) The transmission motor of the worktable may be damaged, which includes issues with the motor bearing.
2) The transmission gear or bearing of the worktable could be damaged.
3) The hydraulic cylinder used to lock the workbench may not be completely released, causing it to rub against the workbench surface.
To troubleshoot, the workbench locking hydraulic cylinder was removed, and the abnormal noise persisted during the rotation of the workbench. This ruled out any issues with the hydraulic cylinder not being sufficiently loosened.
Next, the workbench transmission gear was removed (see Figure 1). After running the workbench rotation motor, no abnormal noise was detected, which eliminated any faults associated with the workbench rotation motor itself.
The workbench positioning gear pair, workbench bearing, and encoder were disassembled, and inspection revealed that the inner and outer ring raceways of the workbench bearing were obviously worn (see Figure 2).
After conducting an analysis, we identified that the root cause of the failure was the workbench bearing. Due to its extended service life, the lubricating oil had deteriorated, leading to a dry grind of the bearing. As a result, the bearing became worn and damaged, which caused abnormal vibrations and tool breakage during operation.
PART 3 Fault handling
To address this issue, you need to disassemble and replace the rotating bearing of the equipment workbench, as well as change the lubricating oil. First, reposition the center of the machine tool holder close to the tool magazine door for easier access during disassembly. Next, rotate the workbench to the zero position and mark it to prevent errors during reinstallation, which could trigger an alarm in the machine tool. Finally, ensure the machine tool is turned off before proceeding.
3.1 Replace the machine tool workbench bearing
The specific steps are as follows.
1) Disassemble the workbench locking mechanism carefully. Take care not to damage the limit switch and oil pipe during the disassembly process. Clean the surrounding chips before beginning the work. Be sure to mark the corresponding mechanical parts as you disassemble them.
2) Disassemble the encoder. The encoder’s cable (refer to Figure 3) must be cut and re-welded, and it should be marked and handled with care. Special attention should be given to the two planes of the circular grating during installation. The lower plane is fixed, while the upper plane rotates with the machine tool. It is important to maintain a gap of more than 0.2 mm between these two surfaces to prevent them from coming into contact with each other.
3) Carefully lift the machine table and move it outside the machine. Remove the worktable positioning gear pair. The gear and the shaft are connected and secured by an expansion sleeve. When removing the expansion sleeve, grind a chamfer on the new bolt to prevent it from becoming difficult to remove after force is applied. When installing, pay attention to the position of the expansion sleeve and ensure that the fracture direction is aligned properly.
4) Remove the worktable gear. Choose suitable lifting rings, lifting devices, and a bridge crane. It is recommended to use a 10-ton bridge crane for better stability. Be aware that there is an angular contact tapered roller bearing inner ring located at the center of the worktable, which needs to fit tightly with the center axis of the worktable.
5) Remove the damaged bearing and replace it with a new one.
3.2 Installation and commissioning of the worktable transmission system
The installation and commissioning of the transmission system are as follows.
1) Installation of the Worktable Transmission System: Begin by cleaning the installation area of the worktable. Next, install the bearing, positioning the angular contact tapered roller bearing at the center of the worktable. After that, attach the worktable encoder.
2) Installation and Commissioning of the Worktable Positioning Gear Pair: Once the table drive system is installed, it is essential to restore the milling mode. Refer to the manual to locate the hydraulic control valve needed for switching the machine tool to milling mode (see Figure 4).
Find the location of the machine tool milling mode sensor switch. Open the solenoid valve core and turn the shaft of the helical gear (see Figure 5) in the workbench positioning gear pair until the switching signal light (see Figure 6) lights up.
3) On the operation panel, press “system” → “PMC” → “PMCDGN” → “STATUS” → enter X10 “Search” to get the I/O detection interface, find X10.0 (milling mode confirmation signal), which is displayed as “I” , as shown in Figure 7. At this time, release the solenoid valve core (Y5.0) and install the gear on the workbench (Y5.1 milling mode locking valve will automatically lock).
4) Clear the lubricating oil circuit of the machine tool and replace the lubricating oil. Install the machine tool work surface (see Figure 8) and other CNC auto parts and auxiliary components.
PART 4 Machine tool trial processing
After installing the machine tool, begin with trial processing. Check the lubrication system, the machine tool program, and the workbench’s zero return condition to ensure they are all functioning normally. Next, toggle between milling mode and turning mode to verify that both functions work properly. Then, rotate the workbench at both high and low speeds for at least one hour, ensuring that no unusual noises occur.
PART 5 Conclusion
After conducting a detailed analysis and thorough inspection, we identified the potential causes of the abnormal noise issue with the workbench. The workbench was disassembled, and the bearings at its base were replaced as a targeted solution. Additionally, we replaced the oil in the workbench’s lubrication system to ensure the smooth high-speed rotation of the CNC vertical turning center’s workbench.
Our practice has demonstrated that regularly changing the lubricating oil of the workbench significantly enhances the performance of the major mechanisms in the transmission system. This proactive maintenance leads to smoother and more reliable operation, helps prevent various lubrication-related failures, saves considerable repair time, and improves overall maintenance efficiency.
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