You designed a part to slot into something else — a bracket over a rail, a lid onto a box, a pin into a hole. The model said 20mm. The printed part measures 20.2mm, or 19.8mm, and now it either won't go in or it rattles loose. Nothing was wrong with your design. This is just what happens between a CAD file and a physical object, by a small and predictable amount.
Why it happens
A 3D printer builds a part out of a bead of molten plastic pushed through a nozzle of fixed width — it can't trace a corner as sharply as a mathematical line can, so outer edges come out very slightly larger and holes come out very slightly smaller than drawn. On top of that, plastic shrinks a little as it cools, and it doesn't cool perfectly evenly, so there's a second small variation layered on the first. Neither of these is a flaw in the machine or the file — it's just what the process does, consistently, every time.
What actually helps
- Never design a press-fit to the exact same number on both parts. A pin meant to sit inside a hole needs a small, deliberate gap -- as a starting point, 0.1-0.2mm per side for a snug fit, more if you want it to slide freely.
- Tell us if a fit is critical. If a part has to mate precisely with something you already own, give us the real measured dimension, not just the spec sheet number -- manufactured objects have their own tolerances too.
- Send us the part it needs to fit, if you can. A spec sheet gives us a nominal number; the actual object in your hand may not match it exactly, especially if it's worn, older, or from a manufacturer whose own tolerances run loose. Measuring the real thing -- or printing a quick test fit against it -- catches a mismatch before the final part comes off the printer, not after.
- Expect a test fit on anything genuinely tight. For a one-off precision fit, a quick test print is cheaper than a reprint of the whole part.
A part that fits on the first try usually isn't the one designed to the most precise number — it's the one designed with a realistic idea of what "precise" means for the process making it.