A regular knock or scrape, once per layer, always at the same point in the toolpath. Look at the print and there is a scarred, burnished patch where the nozzle has been dragging across something it should have cleared.
Treat the sound as a countdown
This fault escalates. The first contact is a light touch. Each subsequent layer prints onto a surface that has been disturbed by the last one, so the interference grows, and the sequence usually ends in one of two ways: the toolhead knocks the part loose, or the motors lose steps and the print shifts sideways.
Both outcomes waste the whole print, and both are preceded by minutes of audible warning. If you hear it, stop and look rather than hoping.
Something is higher than the machine believes
The printer positions the nozzle at a height calculated from the layer number. It has no idea what the surface actually looks like. So the question is what has risen above nominal.
A curled overhang. The free edge of an unsupported region lifted as it cooled. Common, and it has its own escalating dynamic — see curling overhangs.
A lifted corner. A part peeling off the plate raises everything above the peel. Look for warping, which is usually visible from across the room once you know to look.
Over-extrusion. Excess material has nowhere to go but up, so the whole surface sits proud of nominal. This is the version where the knocking is everywhere rather than at one feature, and it is fixed by calibration rather than geometry — see over-extrusion.
Stringing and blobs on a travel path. A deposit left on a surface the nozzle crosses can be tall enough to catch on the next pass.
A part that has already moved. If something knocked it earlier, it is no longer where the machine left it, and every subsequent pass hits it.
Triage while the print is running
If you catch it early, you sometimes save the print.
- Pause and look at the exact feature being struck. The scar shows you which one.
- Is the feature an overhang or a corner? Then it curled or lifted, and the print is unlikely to recover; stop and fix the cause.
- Is the surface uniformly rough and raised? Flow is over-set. Reduce the flow multiplier a few points on the machine's live adjustment and the print may finish.
- Is there a blob at one point only? Sometimes it can be trimmed away with a scalpel while the print is paused, and the job continues cleanly.
Never push the toolhead by hand to inspect. Move it with the controls, or you will introduce the layer shift you were trying to avoid.
Z hop is a mitigation, not a cure
Lifting the nozzle during travel moves stops it dragging across raised features on the way past. That genuinely helps, and it costs a little time and some extra stringing.
What it does not do is stop the nozzle contacting the feature while printing the layer above it. If a curled edge stands taller than one layer height, the toolhead still ploughs through it on the printing pass — so a machine that knocks with Z hop enabled has a geometry problem that has to be addressed at the source.
Enable it as insurance on tall models with small footprints. Do not treat it as the answer to this page.
Prevention that actually works
- Fix cooling on overhangs before adding hardware or settings, since curl is the leading cause.
- Calibrate flow so the surface height matches the model.
- Confirm the base is properly stuck down before walking away from a long job, because a part that lifts is a part the nozzle will eventually meet.
- Print several small parts sequentially rather than all at once only if your machine has the clearance for it — otherwise the gantry crosses finished parts, which is a different collision with the same result.
The materials that cause the most collisions
PETG curls readily and is sticky enough that a deposit stays where it lands. ABS and ASA lift corners. The polyamides do both, and add stringing on top. PLA is the least collision-prone material in normal use, so a machine that has started knocking on PLA usually has a flow calibration problem rather than a geometry one — which is a useful piece of triage on its own.