Holes come out smaller than they should
FDM printing has reasonable accuracy, but a hole never comes out exactly to size: the nozzle is round, the plastic shrinks and curves are approximated by segments. You compensate in the design, not by relying on luck.
What the failure looks like
You design a 5 mm hole for a 5 mm screw and the screw won't go in. An 8 mm shaft is loose or doesn't fit at all.
Causes, most likely first
- 01
Polygonisation effect
The hole is approximated with straight segments inside the circle, so it comes out smaller.
- 02
Material shrinkage
PLA shrinks very little (0.3-0.5%), ABS quite a bit (0.8-1.5%).
- 03
Over-extrusion
Excess plastic intruding into the hole.
- 04
Elephant foot
It narrows the first few layers of the hole.
Step-by-step fix
1. Print a tolerance test
A test piece with holes from 4.8 to 5.4 mm in 0.1 mm steps. Note down which one fits your screw.
2. Apply clearance in your design
Sliding fit: +0.3-0.4 mm diameter. Press fit: +0.1-0.15 mm. Clearance hole for screw: +0.5 mm.
3. Correct horizontal expansion
In the slicer, values from -0.05 to -0.1 mm adjust outer and inner dimensions simultaneously.
4. Check the flow rate
High flow reduces the hole size. See over-extrusion.
5. Add a chamfer in the design
A 0.5 mm chamfer at the mouth of the hole makes entry easier and prevents first-layer squish.
6. If critical, drill it out
For high-precision holes, print 0.5 mm undersized and run a drill bit through.
| Fit type | Recommended clearance (diameter) |
|---|---|
| Press fit | +0.10 - 0.15 mm |
| Sliding fit | +0.30 - 0.40 mm |
| Clearance hole for screw | +0.50 mm |
| 608 bearing | -0.05 mm (interference) |
| Embedded M3 hex nut | +0.20 mm flat-to-flat |
So it doesn't happen again
- Keep your printed tolerance test and use it as a reference for every new material.
- Document the clearance that worked for each project.
Related questions
Does tolerance change depending on the material?
Yes. PETG usually needs 0.05-0.1 mm more clearance than PLA because it is more viscous; ABS varies depending on shrinkage.
What about horizontal holes?
They turn out oval at the top. Design them with a teardrop or diamond shape if you cannot reorient the part.
Keep reading
Want to go deeper? We have a full article about this on the blog.
What if it isn't worth your time?
Sometimes the part is more urgent than the learning curve. We're a workshop in Viladecavalls: send us the file and we'll print it with the right material and settings.
Still not working?
Tell Marco which printer and material you use and he'll tell you what to change.
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