1. Problem Description and Scope
This guide addresses positioning deviations in CNC machines (milling machines, lathes and machining centers) caused by mechanical wear, failures in feedback systems and misadjustments in servo drives. Symptoms include loss of dimensional tolerance, vibration marks on the part surface (chatter), interpolation errors and 'following error' alarms. Rating: Critical.
2. Safety Precautions
ATTENTION: KEEP FOCUSING ON SAFETY. SERVO DRIVE VOLTAGE MAY REMAIN ON THE CAPACITORS EVEN AFTER SWITCHING OFF. ALWAYS USE BLOCKING AND TAGGING (LOTO) PROCEDURES. THE USE OF PPE IS MANDATORY (INSULATING GLOVES, PROTECTIVE GLASSES AND SAFETY FOOTWEAR). DO NOT ATTEMPT MECHANICAL ADJUSTMENTS WHILE THE MACHINE IS OPERATING.
3. Required Diagnostic Tools
| Tool | Specification | Measuring Range | Objective |
|---|
| Centesimal Indicator | Magnetic base, sensitivity 0.001 mm | 0 - 10mm | Measure backlash and position deviation |
| Digital Multimeter | True RMS, CAT III 600V | V/A/Ohm | Check servo voltages and continuity |
| Oscilloscope | Portable, 4 channels | Up to 100MHz | Analyze encoder signals and noise |
| Thermographic Camera | Resolution > 160x120 | -20°C to 400°C | Detect overheating in components |
| Test bar (Ballbar) | Wireless, sub-micron precision | Variable radius | Kinematic and servo diagnostics |
4. Initial Checklist
| What to check | Action |
|---|
| Alarm history | Consult the CNC log (last 30 days). |
| Environmental conditions | Check room temperature (ideal 20-22°C). |
| Lubrication | Check ball screw pressure and oil level. |
| Audible noise | Identify abnormal sounds in spindles or shafts. |
| Recent changes | Check tool changes, repairs or power outages. |
5. Systematic Diagnosis Flow
- Symptom: Positional error when reversing direction.
- Check mechanical backlash: Measure with centesimal indicator on the spindle. If > 0.01 mm, check nut tightness.
- Check clearance in the motor/spindle coupling: Check elastic couplings.
- Symptom: Dynamic Following Error.
- Check servo tuning: Analyze proportional (Kp) and integral (Ki) gain.
- Check load and friction: Check whether the linear guides are locking or lack lubrication.
- Symptom: Thermal drift (error increases over time).
- Check CNC thermal compensation: Check compensation parameters.
- Check the status of the internal temperature sensor (if present).
6. Failure-Cause Matrix
| Symptom | Probable Cause | Test | Expected Result |
|---|
| Backlash | Wear on spindle nut | Measurement with a watch | Clearance < 0.005 mm |
| Instability | Incorrect servo tuning | Step response analysis | Fast setting time |
| Constant deviation | Encoder/scale error | Comparison with interferometer | Deviation < machine tolerance |
| Heating | Lack of lubrication | Thermographic camera | Temperature < 50°C |
7. Root Cause Analysis
7.1 Ball Screw Wear
It occurs due to lack of lubrication or contamination. Wear increases the axial clearance, generating inversion marks on the part. Confirmation: Physical measurement of the backlash when reversing the movement. Damage: Total loss of dimensional accuracy.
7.2 Feedback Errors (Encoders)
Failure due to electrical noise, damaged cables or oil accumulation in the encoder. Confirmation: Oscilloscope on A/B signals. Damage: Emergency stop alarms or erratic positioning.
8. Step-by-Step Resolution Procedures
- Backlash Adjustment:
1. Position axis in neutral zone.
2. Measure clearance with indicator.
3. Adjust backlash compensation parameters on the CNC (only if mechanical is correct). - Servo Tuning:
1. Check engine load via driver software.
2. Adjust Kp to increase stiffness (be careful with oscillation).
3. Adjust Ki to eliminate stationary error.
9. Preventive Measures
| Cause | Strategy | Monitoring | Interval |
|---|
| Wear | Automatic lubrication | Reservoir level | Weekly |
| Contamination | Seal cleaning | Visual inspection | Monthly |
| Misfit | Tightening of couplings | Torque meter | Semiannual |
10. Spare Parts and Components
| Description | Specification | When to Replace | UNITEC category |
|---|
| Ball chestnut | According to spindle model | Clearance > 0.02 mm | Mechanics |
| Elastic coupling | Rated Torque | Signs of fatigue/cracking | Transmission |
| encoder | CNC Protocol | Signal failure | Electronics |
See our catalog at https://www.unitecd.com/e-catalog/
11. References
- ISO 230-2: Accuracy testing for machine tools.
- ABNT NBR NM ISO 9001.
- NR-12: Work safety on machines and equipment.
