A hole that should take a bolt does not. Measure it and it is smaller than the model says — usually by a couple of tenths of a millimetre, occasionally by more, and always in the same direction.
That last part is the clue. Errors that come from random causes scatter both ways; this one is systematic, so it has a systematic explanation.
Three errors, all pointing inward
Chord approximation. A circle in a sliced file is a series of short straight segments. Every one of those segments is a chord, and a chord sits inside the arc it replaces. The printed shape is therefore inscribed within the true circle, and the effect grows as the segments get longer relative to the radius.
Bead placement on a concave path. The extruder lays a bead of finite width along a curved path. On the inside of a curve, the material on the inner edge is being compressed into a shorter arc length than the material on the outer edge, so it thickens slightly and encroaches on the hole.
Contraction pulling inward. As the loop around the hole cools, it shrinks toward its own centre — which is the middle of the hole. On an external feature the same contraction makes the part smaller; on an internal one it makes the void smaller too.
All three act in the same direction, which is why holes are reliably undersized and almost never oversized.
Why small holes suffer most
The errors above are roughly proportional to bead width and to segment length, not to the size of the feature. So the same absolute error is a large fraction of a small hole and a negligible fraction of a large one.
An 8 mm hole that comes out 0.3 mm small still admits most things you would put in it. A 3 mm hole with the same absolute error has lost a tenth of its diameter and will not take a 3 mm pin at all. This is why the fault seems to come and go — it is size-dependent, not intermittent.
Measure your machine's real allowance
Rather than trusting a number from a forum, print a gauge once and keep it.
- Model a flat plate with a row of holes covering the sizes you actually use — 3, 4, 5, 6, 8 and 10 mm is a reasonable set.
- Print it in your usual material, at your usual layer height and line width, with your usual nozzle.
- Measure every hole with calipers or with pin gauges, and write the actual sizes on the plate with a marker.
- Keep it next to the machine.
You now know your machine's error at each size, in the material you print in, which is more useful than any general rule. Repeat it if you change nozzle diameter or material family; both move the numbers.
Correcting it
In CAD, for parts you design. Add the allowance to the hole in the model. This is the most honest place for it, because the compensation lives with the part rather than the profile, and a hole modelled to print correctly stays correct if you send the file elsewhere. A clearance hole for a bolt is a place to be generous anyway.
In the slicer, for downloaded models. Hole compensation — the exact name varies — expands internal perimeters by a fixed amount. Around 0.1 to 0.2 mm is typical. It applies the same absolute correction everywhere, which suits a part with holes of similar size and over-corrects large ones on a part with a wide range.
With more segments. Some slicers let you increase the resolution of curve approximation, which addresses the first of the three errors directly. The default is usually fine; a model exported from CAD with a coarse tessellation is not, and that is worth checking on any part whose holes are badly out.
Calibrate flow first. An over-extruding machine adds its own error on top of the three above, and compensating for the sum leaves you wrong the moment flow is corrected. See over-extrusion.
The workshop answer
For a functional part, a drill bit through a printed hole takes seconds and gives a result no compensation scheme matches. Print the hole undersized deliberately, then drill it to size — the plastic cuts easily and the printed hole acts as a pilot. This is standard practice for anything that has to fit a bearing, a shaft or a threaded insert, and it is not a defeat.
How much each polymer closes a hole
Anything that contracts more closes the hole further: the polyamides most of all, then ABS and ASA, then PETG. Fibre-filled grades are the exception in a useful direction — the fibre restrains contraction, so filled nylon and filled PETG hold internal dimensions considerably better than their unfilled versions. PLA sits at the easy end, which is why a hole that fits perfectly in PLA can be tight in the same file printed in ABS.