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How To Test For Backlash

Testing for backlash on a CNC (Computer Numerical Control) router is an important maintenance step to ensure accurate and precise machining. Here's how you can test for backlash:

Tools and Materials:

  1. Dial indicator
  2. Magnetic base or other suitable mount for the dial indicator
  3. Gauge block or other known reference material
  4. Wrenches or tools necessary to move the machine manually
  5. Pen and paper for recording measurements

Steps:

  1. Secure the Dial Indicator: Mount the dial indicator securely on the machine. Use a magnetic base or any other suitable mount to ensure the dial indicator is stable and can measure movement accurately.

  2. Position the Dial Indicator: Place the dial indicator's probe against the part of the machine that will move (for example, the router's spindle). The dial indicator should be positioned so that it can measure any movement along the axis you want to test.

  3. Set the Dial Indicator to Zero: Gently move the machine in the direction where you expect backlash to occur. Carefully set the dial indicator to zero at this position. This is your starting point.

  4. Move the Machine in the Opposite Direction: Move the machine in the opposite direction, ensuring smooth and steady motion. Measure the distance the indicator needle moves before the machine starts to respond. This movement indicates the amount of backlash.

  5. Repeat the Test: Repeat the test in both directions on all the axes of the CNC router. This ensures you check for backlash in all possible movements.

  6. Record the Measurements: Write down the measurements for each axis and direction. Compare these measurements with the specifications provided by the manufacturer. If the measured backlash exceeds the allowed tolerance, it might be necessary to adjust or replace the affected parts.

  7. Inspect and Maintain: If you find excessive backlash, inspect the leadscrews, nuts, bearings, and any other components related to the movement on the affected axis. Lubrication issues, worn-out parts, or loose couplings could be causing the backlash.

  8. Adjust or Replace Parts: Depending on your findings, you might need to adjust the existing components or replace them to minimize backlash. Consult the machine's manual or contact the manufacturer for guidance on adjusting or replacing specific parts.

Remember, testing for backlash regularly and addressing any issues promptly is crucial for maintaining the CNC router's accuracy and ensuring the quality of your machined products.

Almost all mechanical systems have some backlash - including brand new systems. Strain wave gearboxes and cycloidal drives can provide zero backlash. Assuming your system does not have these mechanical systems, you need to determine the maximum allowable backlash for your production needs. Repair or replace the mechanical components that are causing the backlash first. An axis with more than .003" of backlash will display accuracy, motion, and surface finish issues. Cutting a circle inside of a square that has been rotated 45 degrees will show how backlash will affect your part quality. G68 can be used to rotate the part. Make adjustments and run the same G-code to check your adjustments.

Signs of backlash:

  • A cut that continues along an axis until the backlash is taken up
  • Vibration, chattering, and servo hunting (with linear scales)
  • Witness/dwell marks at 3, 6, 9, and 12 o'clock positions while backlash is being taken up
  • Inconsistent and inaccurate part dimensions

If your CNC system uses scales to provide direct position feedback, it will continue to move the axes to reach the commanded position. However, there may still be motion (servo hunting) and surface finish problems while the backlash is taken up.  

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