A printed part is never exactly the dimensions in the file, and designing as though it will be is the most common source of parts that do not fit.
Sources of error
- Shrinkage. Materials contract as they cool or cure. The amount varies by material and by printer.
- Beam or nozzle width. The physical width of the deposited or cured material means edges land slightly off the nominal position.
- Layer height. Vertical dimensions quantise to the layer, so a 10.05mm feature at 0.2mm layers becomes 10.0 or 10.2.
- Elephant's foot. The first layers spread slightly under the weight above, widening the base.
- Warping. Uneven cooling pulls corners up, particularly on large flat parts.
Designing for it
- Add clearance for fits. Two parts meant to slide need a gap; the amount depends on printer and material and is found by testing.
- Print a tolerance test before committing to a design with fits. A small part with a range of clearances tells you your machine's actual numbers, which no general figure can.
- Avoid dimensions that must be exact. Where possible, design so the part is adjustable, or so the fit is on a surface you can sand.
- Chamfer the base to counter elephant's foot.
Orientation changes accuracy
Dimensions in the print plane are more accurate than dimensions in the layer axis. A hole printed vertically is rounder than one printed horizontally, which tends to come out slightly oval and undersized.
Holes come out undersized
Reliably, on most technologies. Designing them slightly oversized, or drilling them afterwards, is standard practice.
Generated models and tolerances
Reconstruction has no concept of a dimension. A generated part is a shape, and any specific measurement in it is coincidence. For anything that must fit, model the mating features in CAD and use generation only for the decorative geometry around them.