CNC Preventive Maintenance Schedules 3 — Questions and Answers
Question 1: Which parameter is verified during a CNC spindle PM to detect bearing degradation before catastrophic failure?
- Vibration signature analysis using an accelerometer (Correct answer)
- Spindle motor current draw at no load
- Tool holder taper contact percentage
- Coolant-through spindle flow rate
Correct answer: Vibration signature analysis using an accelerometer
Vibration analysis detects changes in bearing frequency signatures that indicate early-stage wear before failure occurs.
Question 2: A CNC PM schedule lists 'check hydraulic oil level and condition.' What visual sign indicates the oil needs to be replaced rather than topped off?
- Oil appears milky or foamy, indicating water contamination (Correct answer)
- Oil level is 10% below the full mark
- Oil has a slightly amber tint
- Oil temperature is 5°F above ambient
Correct answer: Oil appears milky or foamy, indicating water contamination
Milky or foamy hydraulic oil indicates water contamination, which reduces lubricity and promotes corrosion, requiring a full fluid change.
Question 3: What is the correct procedure when lubricating a CNC machine's automatic lubrication system reservoir during PM?
- Fill with the grade of oil specified on the machine's lubrication chart (Correct answer)
- Use general-purpose SAE 10W-30 motor oil as a universal substitute
- Mix two compatible oils to achieve the required viscosity
- Add oil until the pump cycles more than twice per minute
Correct answer: Fill with the grade of oil specified on the machine's lubrication chart
Only the oil grade specified by the manufacturer should be used to ensure proper viscosity and additive compatibility for all lubricated surfaces.
Question 4: During a quarterly PM, a technician performs a laser calibration check on the X-axis of a CNC machining center. What does this procedure measure?
- Positional accuracy and repeatability along the full axis travel (Correct answer)
- Axis rapid traverse speed
- Servo motor torque output at various positions
- Linear encoder scale cleanliness
Correct answer: Positional accuracy and repeatability along the full axis travel
Laser calibration measures actual axis positioning accuracy against commanded positions to detect leadscrew wear or thermal drift.
Question 5: A PM procedure requires the technician to 'clean and inspect the ATC (automatic tool changer) magazine pockets.' What defect is being looked for?
- Damaged retention springs or worn pocket liners that could drop tools (Correct answer)
- Incorrect tool number labels on the pockets
- Pocket diameter out of tolerance for standard tool holders
- Excessive magazine rotation speed
Correct answer: Damaged retention springs or worn pocket liners that could drop tools
Worn or broken retention features in ATC pockets can allow tools to fall during transport, causing machine damage or injury.
Question 6: What is the recommended action when a CNC machine's PM log shows the last lubrication service was performed 3 months ago on a machine rated for monthly lube intervals?
- Perform immediate lubrication and investigate why intervals were missed (Correct answer)
- Continue operating until the next scheduled annual maintenance
- Double the lube quantity to compensate for missed intervals
- Run the machine on reduced feed rates until lube is added
Correct answer: Perform immediate lubrication and investigate why intervals were missed
Overdue lubrication must be corrected immediately, and the PM scheduling process should be reviewed to prevent future lapses.
Question 7: Which of the following is a key inspection point for the CNC machine's electrical connections during an annual PM?
- Check for loose terminals, corroded contacts, and damaged insulation (Correct answer)
- Verify all wiring is the correct color code per NEMA standards
- Replace all terminal blocks regardless of condition
- Test each wire for continuity with a megohmmeter at 1000V
Correct answer: Check for loose terminals, corroded contacts, and damaged insulation
Loose or corroded electrical connections cause intermittent faults and can create fire hazards, making them a priority during annual inspections.
CNC Preventive Maintenance Schedules 3
Which parameter is verified during a CNC spindle PM to detect bearing degradation before catastrophic failure?