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Calibration: Gear Rotation Distance

Every MMU has at least one gear stepper moving filament, and like any extruder, its accuracy depends on rotation_distance - how far the stepper actually turns the filament per commanded millimetre. Gate 0 always acts as the reference gate: it's the one you calibrate first, and (on designs with per-gate gears) the one every other gate is measured against.

Until you calibrate, Happy Hare uses the installed default from mmu_hardware.cfg - a reasonable starting point, but real gears vary build to build, so accuracy improves once you measure your own.

MMU_CALIBRATE_GEAR - one gate, by hand

  1. Select the gate: MMU_SELECT GATE=0.
  2. Get filament to where it emerges from the MMU - the bowden exit, or a combiner/splitter exit on designs like Box Turtle's Turtle Neck buffer. MMU_TEST_MOVE MOVE=50 a few times to advance it into view if needed.
  3. Cut the filament flush at that exit point.

  4. Run a measured test move, retaining grip so the cut end doesn't slip:

    MMU_TEST_MOVE MOVE=100 GRIP=1

  5. Measure the emitted length with a ruler or calipers, then feed it back:

    MMU_CALIBRATE_GEAR MEASURED=102.5

    Gear stepper 'rotation_distance' calculated to be 23.117387 (currently: 22.7316868)
Gear calibration for gate 0 has been saved

Happy Hare computes the new rotation distance as current_rd * measured / commanded and saves it by default. Cut flush again and re-run MMU_TEST_MOVE (no GRIP=) to confirm - it should now move exactly the commanded distance.

A longer test move improves precision on a design where 100mm is hard to measure accurately:

MMU_TEST_MOVE MOVE=200 GRIP=1

MMU_CALIBRATE_GEAR LENGTH=200 MEASURED=205.25

RESET=1 restores the installed default for the current gate.

MMU_CALIBRATE_GATE - remaining gates, automatically

If your MMU has an encoder, you don't need to repeat the manual procedure above on every gate. MMU_CALIBRATE_GATE uses the encoder plus gate 0's already-calibrated rotation distance as a reference to work out each other gate's rotation distance on its own - just feed loose filament into the target gate and run:

MMU_CALIBRATE_GATE GATE=1

Calibration move of 6x 400.0mm, average encoder measurement: 404.7mm - Ratio is 1.011872
Calculated gate 1 rotation_distance: 22.941324 (currently: 22.672165)
Calibration for gate 1 has been saved

ALL=1 sweeps every gate in sequence (feed loose filament from gate to gate as prompted) - this is also what the legacy alias MMU_CALIBRATE_GATES runs (MMU_CALIBRATE_GATES is kept only for backward compatibility; MMU_CALIBRATE_GATE ALL=1 is the same thing). RESET=1 ALL=1 clears every gate's calibration except the reference gate.

This requires gate 0's rotation distance and the encoder itself to already be calibrated - see Calibration: Encoder. If a gate's measurement comes back more than 20% different from gate 0's, Happy Hare rejects it rather than saving a bad value - usually a sign filament wasn't actually moving, or gate 0 itself isn't calibrated correctly.

Do you need to bother with every gate?

It depends on your MMU's design:

  • Tradrack explicitly disables variable rotation distance in its installer profile - every gate shares gate 0's value, since its single-gear design can't have per-gate variation in the first place. There's nothing to do beyond the one reference-gate calibration.
  • Most other multi-gear designs (ERCF, Box Turtle-family, and other Type-B MMUs) can have real gate-to-gate variation - different BMG gears sourced from different production runs are a common cause - so calibrating each gate individually, or via MMU_CALIBRATE_GATE ALL=1, genuinely improves accuracy.
  • If you'd rather not calibrate every gate up front, autotune_rotation_distance (off by default on every MMU type) lets Happy Hare quietly refine each gate's rotation distance during normal use instead, using sync-feedback or encoder telemetry as filament actually moves - see Calibration for where this fits alongside the other autotuning settings. Even with it enabled, running MMU_CALIBRATE_GATE once against a test length of filament gets you a good value immediately rather than waiting for it to converge.

See also