Common faults in CNC machine tools often occur in the spindle section and the feed servo system. Issues such as failures in auxiliary control devices or open circuits in control loops are also frequently encountered. This article outlines some typical troubleshooting methods for common CNC machine tool problems. 1. **Spindle Fault Detection Methods** The spindle drive system in a CNC machine is responsible for the rotational movement of the machine's spindle. It should offer a wide constant power range, quick acceleration/deceleration, a broad speed range, strong overload capability, low motor temperature, and minimal noise. Common issues include failures in the spindle drive system, hydraulic pressure problems, and flow detection malfunctions. 2. **Examples of Spindle System Faults** [Example 1] A German 13-meter CNC gantry milling machine experienced an issue where the spindle could not loosen. After checking the PLC, it was found that input and output signals were present, indicating that the 24V power supply was active. However, the solenoid valve was not energized upon power-on, leading to the conclusion that the valve body was damaged. Even after replacing the solenoid valve, the problem persisted, requiring further investigation. [Example 2] A domestic 6-meter vertical machining center had an alarm at one of the four spindle flow pressure detection points. All multimeters showed normal readings, suggesting a blockage in the oil pipeline. Clearing the blockage restored normal operation. [Example 3] A domestically produced ten-meter vertical car spindle failed to rotate without any alarm. Upon inspection, it was found that the right tool holder was not displaying a normal axis value. The PLC program revealed that the handheld unit selection button was not functioning. Further inspection found an open circuit in the 24V power line of the handheld unit, which was causing the spindle to remain disabled. Reconnecting the line resolved the issue. [Example 4] A CNC ten-meter vertical car spindle display did not move. Inspection revealed that the encoder cable was intact but the encoder itself was damaged. Replacing the encoder restored the digital display. 3. **Feed Servo System Fault Detection** The feed servo system is crucial for the precise movement of machine tool components. It must provide high positioning accuracy, fast response, and stability. Common faults involve the servo control unit, position feedback, and the servo motor. The standard detection process includes verifying the presence of a servo enable signal, checking if the axis moves, and confirming the presence of a command voltage on the servo unit to determine whether the issue lies in the feedback system, motor, or mechanical components. 4. **Examples of Feed Servo System Faults** [Example 1] A domestic 200-bed Y-axis machine exhibited abnormal behavior during startup. The full-closed loop feedback from the grating ruler was not working, resulting in the system operating in a semi-closed mode. Replacing the scale ruler head restored normal operation. [Example 2] A 17-meter CNC gantry milling machine displayed an overload alarm on the Y-axis servo system. No mechanical issues were found, but further inspection revealed oil leakage into the motor, causing damage. Replacing the Y-axis servo motor fixed the problem. [Example 3] A 1680 CNC sleeper Z-axis could not return to the reference point. Although the axis was set to the forward direction, it moved in the negative direction during the reference point return. This indicated that the system had defaulted to the reference point block. Further checks showed that the zero operating line was incorrectly connected. Replacing the line with a spare one resolved the issue. 5. **Other Common Faults** Faults in AC three-phase asynchronous motors due to phase loss or grounding are among the most frequent issues in CNC machines. Grounding can cause a sudden increase in current, tripping the circuit breaker. Open circuits in auxiliary circuits are also common. Here are a few examples: [Example 1] A CNC 1680 horizontal lathe’s main oil pump would not start. The circuit breaker was off, and testing the motor winding with a multimeter confirmed a ground fault. Replacing the motor restored normal function. [Example 2] A CNC 260 boring machine turntable did not retreat. While the forward motion was normal, the reverse motion failed, and the input indicator was off. Further checks showed no signal at the PLC input point. Testing the button station revealed that the 24V power supply was normal, and the button was undamaged. The issue was traced to a broken operation line, which was replaced with a spare line to restore functionality.
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