Accurate positioning of the CNC machine axes is typically achieved through a position servo system. These systems often operate using either closed-loop or semi-closed-loop control mechanisms. One key characteristic of (semi) closed-loop control is that any failure within the system can lead to inaccurate, unstable, or even completely invalid positioning. As a result, identifying and diagnosing faults becomes a critical part of maintenance. By understanding the control principles of the servo system and analyzing its interface characteristics, it's possible to effectively break down and assess the problem.
When a spindle servo system fails, there are usually three main symptoms: first, an alarm message appears on the CRT or the operation panel; second, the spindle drive device itself displays an error via an LED or digital display; and third, the spindle does not function properly without any visible alarm. Common issues in the spindle servo system include:
1. External Interference: Electromagnetic interference, poor shielding, or inadequate grounding can cause disturbances in the spindle speed command or feedback signal, leading to erratic speed fluctuations. To determine if interference is present, check if the spindle continues to move slightly when the speed command is zero, and whether adjusting the zero-speed balance and drift compensation fails to resolve the issue.
2. Overload Conditions: Excessive cutting force or frequent directional changes can trigger an overload alarm. This may manifest as an overheated motor, an overcurrent warning, or a displayed fault on the drive unit.
3. Spindle Positioning Jitter: During tool changes, fine retraction, or gear shifting, spindle quasi-stop can be achieved through mechanical stops, magnetic sensor-based electrical controls, or encoder-based methods.
4. Mismatch Between Spindle Speed and Feed: When performing thread cutting or using a feed-per-revolution command, if the feed stops while the spindle continues to rotate, this could indicate a problem with the spindle encoder. You can verify this by checking for alarm messages on the CRT screen, examining the encoder signal through machine data or I/O status, or testing with a feed-per-minute command instead of feed-per-revolution.
5. Deviation from Command Value: If the spindle speed exceeds the specified technical limits, consider the following possibilities: motor overload, incorrect analog output from the CNC system (usually 0–±10V), a faulty speed measuring device, or a malfunction in the spindle drive unit.
6. Abnormal Noise and Vibration: Distinguishing between mechanical and electrical causes is essential. Noise during deceleration is often due to drive issues, such as a faulty regenerative circuit. Constant noise at a steady speed may indicate a mechanical problem, especially if vibration persists during a free stop. Additionally, checking if the vibration frequency relates to the spindle speed can help identify whether the issue lies with the mechanical components or the drive unit.
7. Spindle Motor Does Not Rotate: In addition to the speed control signal, the CNC system sends an enable signal to the spindle drive, usually a DC +24V Relay voltage. Check if the system is outputting the correct signal, whether the enable signal is active, and whether all start conditions—such as lubrication and cooling—are met. If these are satisfied, the issue may lie with the drive unit or the motor itself.
Proper diagnosis requires a systematic approach, combining visual inspection, signal analysis, and component testing. Understanding the root cause of the problem ensures efficient and effective troubleshooting, reducing downtime and improving machine performance.
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