CNC machining equipment is a mechatronic product that integrates both technology-intensive and knowledge-intensive features. With advanced technology, high intelligence, and complex integrated circuit-based control systems, these machines are prone to diverse and complex faults during long-term operation, making maintenance challenging. Fault diagnosis and repair are therefore essential to ensuring optimal performance. This paper categorizes CNC faults into those with display indications and those without, analyzes typical cases from actual production, and presents effective troubleshooting methods.
Keywords — CNC machining equipment, fault diagnosis, servo drive, encoder, overtravel, spindle control, maintenance.
1. Introduction
CNC machining equipment combines mechanical, electrical, control, and computer technologies. Due to their high level of integration and complexity, faults can be caused by mechanical wear, electrical issues, or control system failures. This paper is based on real-world maintenance cases, focusing on how to identify, analyze, and resolve both display and non-display faults in CNC systems.
2. Working Principle of CNC Machining Equipment
A CNC machine typically consists of the following subsystems:
- Input/Output device
- CNC unit
- Servo drive unit
- Electrical logic control unit
- Position detection unit
During operation, machining dimensions and process data are programmed according to system codes and formats, and transferred to the CNC unit via the I/O device. The CNC unit processes the data and outputs commands to the electrical logic control unit and servo drive unit, which in turn control the machine’s motion to complete the cutting process.
The position detection unit monitors the motion of moving parts and sends real-time feedback to the CNC unit for precise semi-closed-loop control.
3. Faults with Display Indications
3.1 Servo Drive Fault
Phenomenon:
A CNC lathe (model CKA6136, FANUC OTC system) displayed alarms 401 “SERVO X AXIS VRDY OFF” and 403 “X AXIS CNV LOW VOLT DC LINK” at startup. No motion was possible.
Analysis:
Alarm 401 indicates no servo ready signal, and alarm 403 indicates a drop in DC link voltage. Inspection revealed that the servo drive’s DC 24V input was missing due to a broken wire in connector CXA19B, preventing contactor KM1 from engaging.
Solution:
Re-soldered the broken wire. After testing, alarms cleared and normal operation resumed.
3.2 Encoder Fault
Phenomenon:
A machining center (model R560, MITSUBISHI 60S system) stopped suddenly during operation with alarms Z70 0001Y and S01 PR 0018Y.
Analysis:
Both alarms related to the Y-axis servo motor absolute position encoder. Checks eliminated other causes, confirming encoder damage.
Solution:
Replaced the encoder and recalibrated the Y-axis absolute position.
3.3 Overtravel Fault
Phenomenon:
A CNC lathe (model CKE6136, FANUC OTC) triggered alarm 506 “X AXIS OVER TRAVEL” during reference point return.
Analysis:
The zero-point limit switch was contaminated with coolant, causing oxidation that delayed signal closure. The CNC could not detect the zero-point signal before hitting the hard limit.
Solution:
Cleaned oxidation from switch contacts.
4. Faults without Display Indications
4.1 Spindle Speed Control Fault
Phenomenon:
A vertical machining center (model XH716, Guangtai CNC) failed to change spindle speed despite varying S commands. Actual speed stayed around 400 rpm.
Analysis:
Oxidation on terminal connections caused voltage drop in the VFD control circuit, fixing the analog speed control voltage at ~0.8V.
Solution:
Cleaned and reconnected oxidized terminals.
4.2 Automatic Program Execution Fault
Phenomenon:
A CNC lathe (model CAK6136, FANUC OTD) could not execute automatic programs, with no alarms shown.
Analysis:
Feed move commands (G01, G02, G03) failed due to missing spindle speed feedback. The spindle encoder was found defective.
Solution:
Replaced spindle encoder, restoring normal operation.
5. Conclusion
Maximizing CNC machine efficiency requires more than optimizing machining programs—it also depends on minimizing fault rates and repair times. Effective maintenance demands both theoretical knowledge and practical experience.
A systematic troubleshooting approach—observe, inquire, analyze, then act—is essential. By classifying faults and understanding their mechanisms as shown in these case studies, maintenance engineers can significantly improve repair efficiency and keep CNC equipment performing at its best.
📌 Note for Layout:
- Insert Figure 1–3 at appropriate points (servo drive wiring, encoder connection, VFD control schematic).
- Use table format for “Fault–Cause–Solution” summary for quick reference.
- Ensure consistent terminology in captions and labels.
