How To Troubleshoot Common Faults of CNC Milling Machines?

1. Classify and troubleshoot based on the fault phenomena

Overtravel alarm ‌

Phenomenon ‌ : When the machine tool exceeds the set soft limit in a certain axial direction, the CRT displays an overtravel alarm.

Troubleshooting steps ‌ :

Check whether the program has set coordinate values that exceed the working range;

Check whether the limit switch has been physically triggered or damaged;

Confirm whether the soft limit parameter Settings in the numerical control system are correct.

Manually reverse the movement of this axis to clear the alarm and then return to the reference point.

Servo Overload alarm ‌

Phenomenon ‌ : The CRT prompts information such as "servo motor overheating" and "overcurrent", and the indicator light of the drive unit is abnormal.

Troubleshooting steps ‌ :

Check whether the lubrication of the feed drive chain is good and whether there are any foreign objects blocking the guide rails.

Observe whether there is an excessive load caused by frequent forward and reverse rotation operations.

Measure the insulation resistance of the motor to determine whether it is short-circuited or aged.

Check whether the power supply voltage of the driver is stable.

Coordinate axis movement or Crawling (Jittering/Crawling) ‌

Phenomenon ‌ : Uneven motion occurs at low-speed feed, and the machined surface is rough.

Troubleshooting steps ‌ :

Check whether the speed measurement feedback signal is stable and whether the encoder connection is loose.

Check whether the gain parameters of the servo system are set too high or too low;

To check if the mechanical backlash is too large, it can be adjusted by pre-tightening the nuts.

Check whether the coupling has cracks that cause poor synchronization.

The main shaft does not rotate or the rotational speed is abnormal ‌

Phenomenon ‌ : The main shaft shows no response or the rotational speed fluctuates greatly, accompanied by abnormal noise.

Troubleshooting steps ‌ :

Check whether the power supply line of the main shaft motor and the contactor are normal.

Check if the belt is slipping or broken;

Check whether the tool clamping mechanism is stuck and whether the hydraulic pressure meets the standard.

Check whether the main shaft bearings are worn and replace them if necessary.

Failed to return to the reference point ‌

Phenomenon ‌ : The machine tool cannot accurately find the reference point, or the position is inconsistent each time.

troubleshooting steps ‌ :

Check whether the deceleration switch is damaged or contaminated by coolant;

Confirm whether the Grid Offset parameter setting is correct;

Check whether the zero-point pulse signal of the encoder is lost;

Clear the debris between the limit stop block and the sensing element.

Ⅱ. Troubleshooting Methods Based on System Structure

Observation Method ‌

By observing abnormal phenomena such as ‌ sounds, odors, sparks, and smoke ‌ during the operation of the machine tool, obvious fault points can be quickly located. For example, smelling a burnt odor might indicate a short circuit in the circuit. Hearing the sound of metal friction might indicate that the guide rail is short of oil or the bearing is damaged.

Functional program test method ‌

Write a test program containing common G/M codes, run it and observe whether the machine tool operates normally. If the test program can be executed smoothly, it indicates that the basic functions of the system are intact. The problem may lie in the processing program itself (such as the misuse of the G61 single-segment stop command).

Swapping (Module Swapping) ‌

When a certain module (such as the driver board or CPU board) is suspected to be faulty, a spare part of the same model can be used for replacement to observe whether the fault has shifted. For example, if the X-axis drive is changed to the Y-axis and the fault shifts accordingly, it can be confirmed that the original drive is damaged.

principle analysis method ‌

Based on the working principle of the numerical control system, the analysis starts from the signal flow direction. For example: When the servo motor does not rotate, check in sequence:

Whether the numerical control system outputs an enable signal (+24V);

Whether the PLC logic meets the start-up conditions (lubrication, cooling, emergency stop release);

Whether the input signal of the drive unit is normal;

Whether the motor itself is disconnected or the brake has not been released.

Ⅲ. Handling of Common No-Alarm Faults

Some faults occur without system alarms and need to be judged based on experience:

Machine tool "freezing" or no response during operation ‌ : This is mostly caused by program dead loops, parameter errors or external interference. You can try restarting the system and check the PLC program logic (such as unstable signal when the tool change is in place).

Machining accuracy deviation ‌ : Check the levelness of the worktable, the concentricity of the tool installation, and whether the preload of the ball screw is loose. If necessary, perform pitch error compensation.

After ‌ power failure, the lifting platform slides down ‌ : Check whether the brake of the vertical servo motor has failed or whether a balance counterweight device is equipped.