CNC CNC Maintenance and Troubleshooting 2 — Questions and Answers
Question 1: What is the most common cause of chatter (vibration) during CNC milling?
- Insufficient coolant flow to the cutting zone
- Excessive tool overhang, insufficient rigidity, or incorrect cutting parameters (Correct answer)
- Wrong spindle direction for the tool type
- Incorrect work coordinate system selection
Correct answer: Excessive tool overhang, insufficient rigidity, or incorrect cutting parameters
Chatter is self-excited vibration caused by insufficient system rigidity — long tool overhangs, light vise clamping, or cutting parameters that excite the natural frequency of the setup.
Question 2: What does a 'servo overload' or 'servo overcurrent' alarm typically indicate on a CNC machine?
- The servo drive's cooling fan has failed
- The axis is experiencing excessive mechanical resistance beyond the motor's rated torque (Correct answer)
- The control battery backup has discharged below safe voltage
- The axis encoder is sending incorrect position data
Correct answer: The axis is experiencing excessive mechanical resistance beyond the motor's rated torque
A servo overload alarm means the drive is drawing more current than rated, indicating the axis is working too hard due to binding, way lube failure, mechanical obstruction, or worn bearings.
Question 3: What is the purpose of 'backlash compensation' in a CNC controller?
- To automatically adjust feed rates for axis direction changes
- To add a small commanded overshoot to eliminate positioning error when an axis reverses (Correct answer)
- To compensate for thermal expansion of the ball screw
- To adjust spindle speed when the axis direction changes
Correct answer: To add a small commanded overshoot to eliminate positioning error when an axis reverses
Backlash compensation tells the controller to move the axis slightly extra when reversing direction to take up the mechanical play in the drive system.
Question 4: A CNC operator notices the surface finish deteriorating progressively worse as a production run continues. What is the most likely cause?
- The work coordinate system has shifted due to thermal expansion
- Tool wear is increasing cutting forces and reducing edge sharpness (Correct answer)
- The coolant concentration has become too high
- The machine table T-slots are dirty
Correct answer: Tool wear is increasing cutting forces and reducing edge sharpness
Progressive surface finish degradation almost always indicates tool wear — as the cutting edge dulls, cutting forces increase, temperature rises, and surface quality deteriorates.
Question 5: What is the purpose of CNC machine 'warm-up cycles' at the beginning of a production shift?
- To charge the backup batteries before machining begins
- To allow the spindle, axes, and structure to reach thermal equilibrium and stabilize dimensions (Correct answer)
- To verify that all tool lengths are correct before the first part
- To check that the coolant pump is operating at the correct pressure
Correct answer: To allow the spindle, axes, and structure to reach thermal equilibrium and stabilize dimensions
Thermal warm-up allows the machine's spindle, ball screws, and structural components to expand to their normal operating temperature, stabilizing the machine's geometric accuracy.
Question 6: What does a 'P/S alarm' (program/syntax alarm) on a CNC Fanuc controller indicate?
- A power supply failure has been detected in the control cabinet
- The CNC program contains a syntax, format, or parameter error the controller cannot execute (Correct answer)
- The program requires a password to execute
- The part counter has exceeded the programmed limit
Correct answer: The CNC program contains a syntax, format, or parameter error the controller cannot execute
A P/S alarm on Fanuc controllers signals that the control has detected a programming error — such as an illegal G-code, missing required address, or value out of range.
CNC CNC Maintenance and Troubleshooting 2
What is the most common cause of chatter (vibration) during CNC milling?