Calibrating Steps per Unit

Steps per Unit (usually Steps per mm) is the setting that tells grblHAL how many motor steps are required to move an axis exactly 1 millimeter (or inch).

If this value is wrong, your parts will come out the wrong size. E.g., a designed 100mm square might cut as 98mm or 102mm.

The Formula

The calibration process is simple math:

New Steps = Current Steps * (Commanded Distance / Measured Distance)

Where:

  • Current Steps: The current $100, $101, or $102 setting value.
  • Commanded Distance: How far you told the machine to move (e.g., 100mm).
  • Measured Distance: How far the machine actually moved.

Step-by-Step Calibration

Material Needed

  • A reliable ruler, tape measure, or digital caliper.
  • A pointed tool (V-bit) or a fine marker attached to the spindle/tool holder.
  • A piece of scrap material (MDF, wood, or paper tape on the bed).

Procedure (Do for X, Y, and Z axes)

  1. Check Current Settings:

    • Send $$ in the console.
    • Note down the current values for:
      • $100 (X-axis steps/mm)
      • $101 (Y-axis steps/mm)
      • $102 (Z-axis steps/mm)

  2. Set a Starting Point:

    • Jog the machine to a starting position.
    • Mark this spot preciseley on your material (or zero your caliper).
    • Zero the axis in your sender software (e.g., click "Zero X").

  3. Command a Move:

    • Move the axis a specific distance. The longer, the better.
    • Example command: G0 X100 (Move X axis 100mm).

  4. Measure Actual Movement:

    • Using your ruler or caliper, measure exactly how far the machine moved from the start point.
    • Example: You commanded 100mm, but measured 98.5mm.

  5. Calculate New Steps:

    • Use the formula:
      New Steps = Current Steps * (100 / 98.5)
    • Example: If $100 = 40.000:
      New Steps = 40.000 * 1.0152 = 40.609

  6. Update Firmware:

    • Send the new value to the controller:
      $100=40.609

  7. Verify:

    • Re-zero the axis.
    • Command the move again (e.g., G0 X100).
    • Measure again. It should now be dead-on 100mm.

Tips for Accuracy

Use the Longest Distance Possible

Measuring error is constant (e.g., ±0.5mm with a ruler).

  • Over 10mm, a 0.5mm error is 5%.
  • Over 100mm, a 0.5mm error is 0.5%.
  • Over 500mm, a 0.5mm error is 0.1%.

Calibrate using the longest travel your machine and measuring tool allow.

Account for Backlash

If your axis has backlash (loose fitting screws/belts):

  • Always approach your start point and end point from the same direction.
  • Example:
    1. Jog X+ to position.
    2. Mark Start.
    3. Command Move X+ 100mm.
    4. Mark End.

This removes backlash from the measurement equation.

Don't Forget Z-Axis

Calibrating Z is critical for accurate pocket depths.

  • Use a dial indicator or touch off the top of a block, move Z up, place a precision 1-2-3 block, and touch off again.

Troubleshooting

My dimensions are consistently off by exactly half or double.

  • Cause: Incorrect microstepping setting on stepper drivers vs. firmware.
  • Fix: Check your DIP switches on the stepper driver and double check your expected steps/mm calculation.

My circles are ovals.

  • Cause: X and Y steps/mm are not calibrated equally, or severe backlash.
  • Fix: Calibrate X and Y independently. Check for loose belts.

I updated the steps, but it got worse.

  • Cause: You likely inverted the formula (Measured / Commanded instead of Commanded / Measured).
  • Fix: Flip the fraction and recalculate.
    • Remember: If it moved short, you need more steps (ratio > 1).
    • If it moved long, you need fewer steps (ratio < 1).

Next Step: Once your machine moves accurately, let's make it move smoothly: Tuning Motion Settings