CNC Preventive Maintenance Schedules 2 — Questions and Answers
Question 1: When performing a weekly PM on a CNC machining center, which coolant system check is most critical?
- Check coolant concentration using a refractometer (Correct answer)
- Inspect the chip conveyor belt tension
- Lubricate the coolant pump motor bearings
- Replace the coolant filter cartridge
Correct answer: Check coolant concentration using a refractometer
Coolant concentration directly affects tool life and corrosion protection, and should be verified weekly with a refractometer.
Question 2: A CNC lathe PM schedule calls for checking the chuck jaw repeatability. What instrument is used for this task?
- Dial test indicator (Correct answer)
- Vernier caliper
- Micrometer
- Feeler gauge
Correct answer: Dial test indicator
A dial test indicator measures runout and positional repeatability of chuck jaws during PM checks.
Question 3: Which lubricant type is typically specified for CNC ball screw assemblies during scheduled PM?
- Low-viscosity grease such as NLGI #2 (Correct answer)
- SAE 30 motor oil
- Cutting fluid with EP additives
- Dry PTFE spray
Correct answer: Low-viscosity grease such as NLGI #2
Ball screws require NLGI #2 grease applied in controlled amounts to prevent over-lubrication and contamination.
Question 4: During a monthly PM inspection, a technician finds the CNC machine's air pressure regulator is set at 85 PSI instead of the required 100 PSI. What is the most likely consequence if left uncorrected?
- Inadequate clamping force on pneumatic fixtures (Correct answer)
- Excessive spindle vibration
- Coolant pump cavitation
- Overheating of servo amplifiers
Correct answer: Inadequate clamping force on pneumatic fixtures
Insufficient air pressure reduces clamping force on pneumatic workholding, risking part movement during cutting.
Question 5: What is the purpose of checking the Z-axis counterbalance during a CNC vertical machining center PM?
- To ensure the spindle head is properly supported against gravity at rest (Correct answer)
- To calibrate the tool length offset measurement
- To verify the ATC arm travel distance
- To set the rapid traverse override percentage
Correct answer: To ensure the spindle head is properly supported against gravity at rest
The Z-axis counterbalance supports the spindle head weight so the Z-axis servo is not overloaded when the axis is stationary.
Question 6: A PM checklist requires inspecting the CNC control cabinet filters. How often are these filters typically cleaned or replaced?
- Monthly or per manufacturer specification based on environment (Correct answer)
- Only when the control overheats
- Every 5,000 spindle hours
- Annually during a full machine rebuild
Correct answer: Monthly or per manufacturer specification based on environment
Control cabinet air filters trap dust that can cause overheating; cleaning frequency depends on the shop environment but is generally monthly.
Question 7: During a PM procedure, the technician notices the machine tool way covers (telescoping steel covers) are dented and not sliding freely. What is the primary risk if this is ignored?
- Chips and coolant contaminating the linear guideways (Correct answer)
- Increased rapid traverse speed errors
- Loss of axis home position accuracy
- Overloading the servo motor current limits
Correct answer: Chips and coolant contaminating the linear guideways
Damaged way covers allow chips and coolant to reach the linear guides, causing accelerated wear and premature failure.
CNC Preventive Maintenance Schedules 2
When performing a weekly PM on a CNC machining center, which coolant system check is most critical?