3D-printed plastic threads rarely fit a metal fastener if you model the nominal diameter directly, because FDM printers shrink internal holes through over-extrusion and the elephant-foot effect. This chart gives you the correct modelled hole size for each standard thread so the printed part actually accepts the bolt or screw.
Why printed threads need a different diameter
An FDM printer traces an extrusion path inward from the modelled wall edge, and the slight over-extrusion that keeps layer adhesion strong also intrudes into any internal opening. The result is a hole that prints 0.1 to 0.3 mm narrower than the CAD dimension. For a thread that must mate with a steel bolt, that shortfall is enough to make the fit impossible.
How it works
For a metric coarse thread the tap-drill diameter is the nominal major diameter minus the thread pitch:
tap drill = major diameter − pitch
modelled hole = tap drill + horizontal expansionFor M5 x 0.8 the tap-drill is 5.0 − 0.8 = 4.2 mm. On a printer with a
measured horizontal expansion of 0.15 mm, the CAD hole should be drawn at
4.2 + 0.15 = 4.35 mm so the printed result lands on the 4.2 mm target.
This leaves roughly 75 percent thread engagement — the standard that balances holding strength against the torque needed to drive the fastener in.
Common thread reference values
| Thread | Tap-drill (std) | Typical FDM model diameter |
|---|---|---|
| M3 x 0.5 | 2.5 mm | ~2.65 mm |
| M4 x 0.7 | 3.3 mm | ~3.45 mm |
| M5 x 0.8 | 4.2 mm | ~4.35 mm |
| M6 x 1.0 | 5.0 mm | ~5.15 mm |
| 8-32 UNC | 3.51 mm | ~3.66 mm |
| 1/4-20 UNC | 5.11 mm | ~5.26 mm |
The “typical FDM model diameter” column assumes 0.15 mm horizontal expansion; measure your own printer and use the tool to get the exact figure.
Imperial UNC and UNF
Imperial threads do not follow a simple major-minus-pitch rule, so the tool looks up each size in the published tap-drill table and applies the same horizontal-expansion correction on top.
Practical tips
- For small threads (M4 and below), print a plain hole at the corrected diameter and cut with a real steel tap. Plastic cuts cleanly when warm from the print bed and the result is far stronger than a printed thread form.
- Use 0.12 to 0.16 mm layer height when modelling thread profiles directly; coarse layers blur the helix geometry and reduce engagement.
- If a test print is too tight, increase horizontal expansion by 0.05 mm and re-print. If it is too loose, reduce by 0.05 mm.
- Print a calibration coupon — a 20 mm cube with a modelled 5 mm hole — to measure your actual expansion before committing to a functional part.
All figures are computed locally in your browser from the standard pitch and tap-drill tables.