Tolerances and fit — actual slot-together numbers
Why your hole was too tight, and the millimetre clearances that just work.
Why printed parts don't match the CAD model
FDM filament is hot when extruded and shrinks slightly as it cools. Holes come out a touch smaller than the CAD model; outer dimensions come out a touch larger. The difference is usually 0.1-0.4 mm — small, but absolutely the reason a 6 mm hole won't fit a 6 mm peg.
Add clearance to your design. Don't try to print to exact dimensions — print to dimensions + clearance, and you'll get parts that actually fit.
The clearance cheat sheet
Push-fit (snug, no slop): hole = peg + 0.15 mm. A 5 mm shaft slides into a 5.15 mm hole.
Sliding fit (lid on a box, drawer): gap = 0.25 mm per side, i.e. 0.5 mm total. A 30 mm cap on a 30 mm box → make the cap inner = 30.5 mm.
Loose / threaded screw (M3 screw through a hole): hole = nominal screw + 0.4 mm. M3 screw → 3.4 mm hole.
Snap-fit with a click: hole = peg + 0.3-0.5 mm and add a 0.3 mm chamfer on the peg's leading edge.
When in doubt, print a test piece
First time pairing two parts? Print just the joining area as a 10 mm test cube. Saves an hour-long print failing on the assembly step. Most slicers can render just a clipped Z range.
Holes shrink. Add 0.15 mm for push fit, 0.25 mm per side for sliding, 0.4 mm for screw clearance. Print a test piece before the full part.