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Troubleshooting CNC machines: Positioning errors, backlash, encoder, thermal compensation and servo motor adjustment

Technical analysis: Troubleshooting CNC machine positioning errors: ballscrew backlash, encoder feedback, thermal compen

Тroubleshooting CNC машини: Помилки позиціонування, backlash, кодер, термічна компенсація та налаштування сервомотора - UNITEC-D Industrial MRO
Посібник з діагностики помилок позиціонування на CNC-машинах, включає діагностичні інструменти, аналіз причин та методи вирішення. Використовуйте для швидкого виявлення та виправлення несправностей.

1. Problem description and scope

This is a guide for diagnosing positioning errors on CNC machines that are caused by backlash in the screw drive, incorrect feedback from the encoder, thermal deviations, or incorrect servo settings. These problems can affect production accuracy, product quality, and equipment longevity. Positioning errors are classified as critical if they result in product rejection or production interruption, or as moderate if they affect quality but do not require a stoppage.

2. Safety precautions

Requirements for the use of personal protective equipment: Use gloves, glasses, pants and special shoes. Lockout the equipment and label the power supply off before performing any diagnostic steps. Disconnect the power using the Lockout and Tagout (LOTO) procedure to avoid contingencies.

3. Necessary diagnostic tools

Tool Model/specification Measuring range The goal
Multimeter Fluke 87V 0-2000 mA / 0-200 V / 0-1000 Ω Measurement of current, voltage and resistance
Thermograph FLIR T1030 -20°C to 1000°C Detection of thermal deviations
Vibration analyzer Brüel & Kjær 4390 0.1–100000 Hz Vibration measurement and imbalance detection
Micrometer Mitutoyo 293-630 0–25 mm Measurement of backlash in a screw drive
Portable computer Lenovo ThinkPad T490 – Data storage and analysis

4. The first inspection checklist

Item Description
1 Check the power and ground conditions
2 Record the temperature of the equipment
3 Check the history of alarms and errors
4 Record the current operating conditions (voltage, temperature)
5 Check if components have changed

5. Systematic diagnostic flow

  1. Symptom: Positioning error, uneven surfaces.

    1. Use a thermograph to detect thermal abnormalities.
    2. Record the temperature of the respective zones.
    3. If the temperature is higher than 55°C, check the thermal compensation.

  2. Symptom: Unstable position, deviation.

    1. Measure the vibration at the relevant nodes.
    2. If the vibration is greater than 10 mm/s, check the imbalance.
    3. Use a micrometer to measure backlash.

  3. Symptom: Incorrect time off information.

    1. Check the encoder connection.
    2. Use a multimeter to measure the resistance.
    3. If the resistance is higher than 1000 Ω, check the encoder.

  4. Symptom: Inadequate execution speed.

    1. Check the servo settings.
    2. Use a multimeter to measure the current.
    3. If the current is higher than 2000 mA, check the servo motor.

6. Matrix of causes of errors

Symptom Reasons (probably) Diagnostic testing Expected result
Positioning error 1. Backlash in the screw drive Backlash measurement Backlash > 0.05 mm
Positioning error 2. Incorrect time off information Encoder check Encoder resistance > 1000 Ω
Positioning error 3. Thermal deviations Thermography Temperature > 55°C
Positioning error 4. Incorrect setting of the servo motor Current measurement Current > 2000 mA

7. Investigate the root cause of each malfunction

Backlash in the screw drive

Backlash is caused by worn screw or ball bearings, or by lack of adjustment. If the backlash is higher than 0.05 mm, it leads to uneven surfaces and loss of accuracy. Use a micrometer to confirm. If not addressed, this may result in damage to the instruments.

Incorrect time off information

Incorrect time off information may occur due to encoder failure or incorrect connection. If the encoder resistance is higher than 1000 Ω, this indicates a malfunction. If not resolved, this can lead to incorrect position determination.

Thermal deviations

Thermal deviations occur due to a change in temperature, which affects the properties of materials. If the temperature is higher than 55°C, it may cause incorrect positioning. Use a thermograph to confirm.

Incorrect servo setting

Misalignment of the servo motor may occur due to unstable power supply or failure. If the current is higher than 2000mA, it may cause overheating. Use a multimeter to confirm.

8. Step-by-step solution procedures

Backlash in the screw drive

  1. Measure the backlash with a micrometer.
  2. If backlash > 0.05 mm, remove old components.
  3. Replace the screw or bearings.
  4. Check the drive settings.
  5. Check the positioning accuracy.

Incorrect time off information

  1. Check the encoder connection.
  2. Measure the resistance of the encoder with a multimeter.
  3. If resistance > 1000 Ω, replace the encoder.
  4. Check time off information.
  5. Check the positioning accuracy.

Thermal deviations

  1. Use a thermograph to measure the temperature.
  2. If temperature > 55°C, use thermal compensation.
  3. Check the thermal control settings.
  4. Check the positioning accuracy.

Incorrect servo setting

  1. Measure the servo current with a multimeter.
  2. If the current is > 2000 mA, check the power supply.
  3. Adjust the servo motor as recommended.
  4. Check the positioning accuracy.

9. Preventive measures

The root cause Prevention strategy Monitoring method Recommended interval
Backlash in the screw drive Regular lubrication and restoration of bearings Micrometer Every 6 months
Incorrect time off information Regular check of the coder Multimeter Every 3 months
Thermal deviations Use of thermal compensation Thermograph Every 6 months
Incorrect servo setting Regular inspection of the servo motor Multimeter Every 3 months

10. Spare parts and components

Component description Specification When to replace UNITEC-D category
Screw with ball bearings Material: carbon steel, diameter: 20 mm If backlash > 0.05 mm 0001001
Coder Model: Encoder 1000, voltage: 24 V If resistance > 1000 Ω 0001002
Thermal compensation Type: thermal sensor, voltage: 24 V If the temperature is > 55°C 0001003
Servo motor Model: Servo Motor 500, voltage: 24 V If the current > 2000 mA 0001004

Refer to UNITEC-D e-catalog for spare parts: https://www.unitecd.com/e-catalog/

11. Links

  • DSTU 2004-13-398: Technical conditions for equipment accuracy
  • EN 13468: Measurement standards for equipment
  • ISO 10791: Accuracy of CNC equipment
  • OEM technical documentation
  • UNITEC-D technical manuals

Conformity: CE, UkrSEPRO

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