Input shaping cancels the vibrations that cause ringing and ghosting on 3D prints, but it only works when the shaper is tuned to the printer's actual resonant frequencies. An accelerometer mounted on the toolhead measures those frequencies directly, replacing guesswork with data. The tuning process is straightforward, and the print quality payoff is dramatic.
What Input Shaping Does
Every printer frame rings at certain frequencies when the toolhead changes direction; that ringing shows up on prints as ripples near sharp corners, the familiar ghosting artifact. Input shaping works by splitting each motion command into a sequence of smaller impulses timed so their vibrations cancel each other. The timing depends entirely on the resonant frequency: shape for the wrong frequency and the cancellation misses, leaving the ringing untouched or even amplified. The shaper also needs the damping ratio, which describes how fast the vibration decays. Measure both from the real machine, because every printer, even two of the same model, rings slightly differently once assembled, loaded with filament, and bolted to its particular table.
Mounting the Accelerometer
The accelerometer, commonly a small ADXL345 board, mounts rigidly to the toolhead, aligned with the printer's X and Y axes. Rigid mounting is critical: a loosely mounted sensor measures its own wobble instead of the toolhead's vibration. Align the sensor's axes with the machine's axes as closely as possible; misalignment mixes the X and Y signals and muddies the frequency peaks. Route the wiring so it does not tug on the toolhead or snag during the test moves; a cable dragging on the frame injects noise into the measurement. The sensor only needs to survive the calibration routine, so a firm temporary mount with screws or strong tape is fine, but it must not shift mid-test.
Running the Resonance Test
The firmware's resonance test routine sweeps the toolhead through a range of frequencies on each axis while the accelerometer records the vibration. The printer must be in its normal operating state: heated to typical temperatures, since frame stiffness changes with heat, and sitting on its usual surface, since the table is part of the vibrating system. Keep hands off the printer during the test; touching the frame damps the vibration and corrupts the measurement. The routine produces frequency response graphs for X and Y, showing peaks at the resonant frequencies. Run the test a couple of times to confirm the peaks are repeatable; a peak that moves between runs is noise, not resonance.
Reading the Graphs and Setting the Shaper
Each axis graph typically shows one dominant peak, sometimes with smaller secondary peaks. The shaper frequency is set to the dominant peak's frequency, and the shaper type is chosen based on how clean the peak is: a sharp single peak suits a simple shaper, while a broad or multi-peak response wants a more robust shaper type that tolerates frequency spread. Set X and Y independently, since the axes usually resonate at different frequencies. Enter the measured values into the firmware configuration and restart. Then print a ringing test model, the classic tower with sharp direction changes, and compare against an unshaped print; the ghosting should be visibly reduced. If a particular feature still rings, recheck the measurement; the usual culprit is a peak misread from a noisy graph.
When to Retune
Input shaper tuning is not permanent. Any change to the moving mass, a different hotend, a direct-drive conversion, even a significantly different spool weight, shifts the resonant frequencies. Changes to frame stiffness, like tightening or loosening bolts, moving the printer to a different table, or adding an enclosure, shift them too. Retune after any such change; the test takes minutes and the alternative is mysterious print degradation. Some builders retune seasonally, since temperature and humidity affect belt tension and frame behavior subtly. Keep a log of the measured frequencies with notes on the machine's configuration; the log turns retuning from a chore into a quick check, and it reveals when something mechanical has changed.
Accelerometer tuning makes input shaping exact: mount the sensor rigidly, test in the normal operating state, set each axis to its measured peak, and verify with a ringing tower. Retune whenever the machine changes. Data beats guessing, and the prints prove it.
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