Why printed holes come out small (and by how much)
A 5 mm hole comes out at 4.7. The cause, the numbers to compensate, and when it is better to drill afterwards.
It is the most common surprise on a first part: you draw a 5 mm hole for an M5 screw and the screw will not go in. It is not a machine fault. Printed holes come out consistently undersized, and the reason is geometric.
The cause: the chord and the arc
The nozzle lays down straight segments. A printed circle is really a polygon, and its sides are chords that run inside the true arc. On top of that, hot plastic shrinks slightly as it cools and the inner wall tends to close in on the centre. Both effects push the same way: inwards.
| Hole drawn | Comes out roughly | Draw instead |
|---|---|---|
| 3 mm | 2.7 – 2.8 mm | 3.3 mm |
| 5 mm | 4.7 – 4.8 mm | 5.4 mm |
| 8 mm | 7.7 – 7.8 mm | 8.4 mm |
| 20 mm | 19.7 mm | 20.4 mm |
Compensate or drill
For a screw clearance hole, compensating in CAD is enough and is free. For a hole that has to be genuinely round — a bearing, a shaft, a bush — do not compensate: draw it 0.5 mm under final size and drill or ream afterwards. A drilled hole is round; a printed hole is a polygon with good intentions.
Vertical and horizontal are not the same
- Vertical (axis perpendicular to the bed): the hole comes out round and only slightly small. This is the good orientation.
- Horizontal (axis parallel to the bed): the top half is an overhang and the roof sags. The hole comes out oval, wider than it is tall.
- If a horizontal hole is critical, make it a teardrop or leave it small and drill. Compensating it round does not work: the error is not symmetrical.
And if you are unsure, say so in the order notes. When a dimension is critical we check it before printing and tell you whether to compensate, drill, or change the orientation.