What ±0.25 mm means for your assembly, where the error actually comes from, and where to spend tolerance when it's scarce.
We hold ±0.25 mm in general, or ±0.3% on dimensions over 100 mm — whichever is larger. A 50 mm boss lands between 49.75 and 50.25; a 300 mm rail can be off by close to a millimetre end to end. That's FDM physics, not carelessness: plastic shrinks as it cools, and big parts have more plastic doing the shrinking.
Those numbers are what we'll stand behind on every order. Most parts land much closer — but design to the promise, not the average.
any dimension under 100 mm
shrink scales with size
layer-quantised — steps of 0.12/0.20/0.28
Cooling shrink is the big one, and it varies by material: ABS and ASA shrink most, PETG less, PLA least — one reason PLA prototypes measure so well. Shrink also explains why error is proportional: it's a percentage of the plastic in the dimension.
XY and Z behave differently. In the XY plane, dimensions are set by nozzle motion and are quite repeatable. Z heights are quantised to the layer — a 10.1 mm step at 0.2 mm layers becomes 10.0 or 10.2. Put critical heights on layer multiples and they land exactly.
First layers are also squished slightly wide for adhesion, so a part's bottom face carries a small flare — a 0.3 mm chamfer on bottom edges hides it completely and makes parts sit flat.
Cheap accuracy comes from design, not from begging the machine. Reference everything from one datum face instead of chaining dimensions — chains stack error, datums don't. Use one locating hole and one slot rather than two holes: the slot eats the tolerance.
Compliant features forgive what rigid ones fight: a slight crush rib inside a bore gives a snug fit across the whole tolerance band; a finger-width slit beside a tight hole lets it flex over a pin. And wherever two printed faces must sit flush, add a 0.3 mm recess on one so dust and squish never hold them apart.
One datum, one hole, one slot. That trio absorbs more tolerance than any amount of pleading with a printer.
For one or two critical dimensions, post-machining is the honest path: model holes 0.3 mm under and drill or ream; leave a face 0.5 mm proud and sand or file to fit. Plastic works in seconds with hand tools — a part with one drilled bore is far cheaper than chasing ±0.05 from any printer.
If a dimension is critical enough to verify, add a dimensional QC report at checkout: we measure your nominated dimensions against the drawing before dispatch — and if FDM can't hold what you need, we say so before printing, not after.
Drop an STL, STEP or 3MF and the quote applies every rule in this guide — mesh check included, GST included.