When your tolerances drift
You measured it once. A pin that slides home with a satisfying, no-slop fit; a lid that clicks on and stays; a bearing seat that presses in tight. You dialled that clearance in on a test coupon, wrote the number into your model, and moved on — the way it's supposed to work. Then, weeks later, the same file off the same printer comes out wrong. The pin rattles, or won't go in at all. Nothing in the model changed. So what did?
The uncomfortable truth of a calibrated fit is that the number is never really about the geometry alone. It's about the geometry as produced by one specific machine, printing one specific material, in one specific state. The clearance you measured is a fingerprint of that whole system. Change any part of the system and the fingerprint smears — and your dialled-in fit drifts with it. This section is about the maintenance that keeps the system still enough for your numbers to stay true.
The number carries hidden conditions
When you write clearance = 0.2 mm into a parametric fit, you're not writing a law of physics. You're writing shorthand for something much longer: 0.2 mm gives a sliding fit, as calibrated on this nozzle, in this filament, dry, on a machine with belts in tension and a bed that's flat. All of those qualifiers are invisible in the model, but they're doing real work. The moment one of them stops being true, the number stops meaning what you think it means.
That's why "my tolerances drifted" is almost never a modelling problem. The model is faithfully reproducing the geometry you drew. What moved is one of the physical conditions underneath the calibration — and there are only a handful of usual suspects.
The four things that move your numbers
A worn nozzle is the slow one. Brass erodes, the bore widens, and every bead it lays down gets a little fatter. Fatter walls mean holes close up and outside dimensions grow — so a fit that was snug creeps toward tight, then toward seized, over dozens of prints without a single dramatic moment. You don't notice it happening; you notice that fits you trusted all quietly went wrong at once.
A new material or spool is the abrupt one. A different plastic — even a different colour from the same brand — can shrink and flow differently enough to shift a tight fit by a tenth of a millimetre. PETG spreads more than PLA; one nylon isn't another nylon. The instant you switch, every calibrated clearance is provisional until you re-check it. See Tolerances and fits for why the material sets the fit as much as the model does.
Damp filament foams the bead from the inside. Moisture flashes to steam in the nozzle and blows the extrusion wider and rougher, so walls thicken unpredictably and clearances close. This one is sneaky because it can appear overnight with no change to your hardware at all — a spool that sat out in a humid room. If a fit went off for no reason you can name, suspect the spool first (Keeping filament dry).
Loose belts or a skewed frame blur the dimension itself. Slack in the motion system shows up as ringing, backlash and parts that measure off-size or out-of-square — the machine simply isn't landing the nozzle where the model says. The fit didn't change; the printer's aim did.
The honest response is to re-measure
There's no way to reason your way back to a drifted number. The fingerprint is gone; you take a fresh one. Print the same small tolerance coupon you calibrated with in the first place, measure the real clearance, and update the number in your model to match the machine as it is today. That's not a failure of the model — it's the model working exactly as intended, holding a value you can re-baseline whenever the hardware moves under it. Reach for a coupon before you touch a single setting, the way Validate before you print argues.
So the discipline is simple to state and easy to skip: name the conditions your fits were calibrated under, and when a fit drifts, find which condition moved before you blame the geometry. The rest of this section walks the maintenance that keeps those conditions stable — and it starts with the prints that hand you the numbers in the first place, in Calibration prints that matter.