CNC machine tools often experience faults in several key areas, primarily in the spindle system and the feed servo system. In addition to these, issues such as problems with auxiliary control devices or open circuits in control loops are also common. This article outlines various troubleshooting methods and examples of typical faults found in CNC machines.
1. Detection Methods for Spindle Faults
The spindle drive system is crucial for the rotational motion of the machine tool. It should provide a wide range of constant power, fast acceleration and deceleration, high speed range, strong overload capability, low motor temperature, and minimal noise. Common issues include failures in the spindle drive system, hydraulic pressure issues, and problems with flow detection.
2. Examples of Spindle Faults
[Example 1] A German 13-meter CNC gantry milling machine experienced an issue where the spindle could not be loosened. After checking the PLC, it was found that the 24V power supply was active, but the solenoid valve did not operate. Further inspection revealed damage to the valve body. Replacing the valve resolved the problem.
[Example 2] A domestic 6-meter vertical CNC machine had an alarm at one of the four pressure flow detection points. Testing showed that the oil pipeline was blocked, so cleaning it restored normal operation.
[Example 3] A domestic 10-meter vertical CNC machine’s right tool holder failed to display correctly. Investigation found that the 24V power line to the handheld unit was broken. Restoring this line fixed the issue.
[Example 4] A CNC machine’s spindle screen did not move. Checking the encoder cable revealed no physical damage, but the encoder itself was faulty. Replacing the encoder restored normal function.
3. Detection Methods for Feed Servo Faults
The feed servo system is essential for precise positioning and movement of the machine. It includes components like the servo control unit, position feedback system, and the servo motor. Common faults involve issues with the servo enable signal, axis movement, and command voltage. Troubleshooting typically starts by verifying if the enable condition is met, checking axis values, and examining the servo unit for command voltage.
4. Examples of Feed Servo Faults
[Example 1] A domestic 200-bed CNC machine had Y-axis flying during startup. Diagnosis revealed a faulty grating ruler head, which was replaced to restore full closed-loop feedback.
[Example 2] A 17-meter CNC gantry milling machine displayed a Y-axis overload alarm. No mechanical issues were found, but oil leakage into the motor caused damage. Replacing the motor fixed the problem.
[Example 3] A 1680 CNC sleeper Z-axis could not return to the reference point. Inspection showed that the system was defaulting to a negative direction. Replacing the reference point wiring corrected the issue.
5. Other Common Faults
Failures in AC three-phase asynchronous motors due to phase loss or grounding are frequent. Grounding can cause a sudden current spike, tripping the circuit breaker. Open circuits in auxiliary control systems are also common. Here are some examples:
[Example 1] A CNC 1680 horizontal lathe's main oil pump would not start. Testing revealed a grounded motor winding, so the motor was replaced.
[Example 2] A CNC 260 boring machine’s turntable would not retreat. While forward movement was normal, the back movement failed. Further checks showed a broken operation line, which was replaced to restore functionality.
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