Why can't I just model the exact thread diameter?

FDM printers over-extrude slightly at internal walls, so a hole prints smaller than modelled. To get a usable internal thread you model the standard tap-drill size and then enlarge it by your printer's horizontal expansion so the printed result lands on target.

What is the tap-drill diameter?

It is the hole diameter that leaves about 75 percent thread engagement when cut with a tap. For metric coarse threads it equals the nominal diameter minus the pitch. For M5 x 0.8 that is 5.0 minus 0.8 = 4.2 mm.

Should I print the thread or tap it after printing?

For threads M4 and smaller, printing a plain hole and cutting it with a steel tap gives the strongest, most accurate result. Larger threads (M6 and up) can be modelled directly with a thread feature, but still benefit from tolerance compensation.

Does layer height affect thread fit?

Yes. Coarser layers (0.28 mm+) blur fine thread profiles and reduce engagement, especially on threads printed vertically. Use 0.12 to 0.16 mm layers for printed threads, and prefer cutting a tap for anything finer than M5.

What horizontal expansion value should I use?

Print a 20 mm calibration cube and measure a hole of known modelled size. The difference between modelled and measured diameter is your horizontal expansion. Typical PLA values fall between 0.1 and 0.25 mm; enter your measured figure for best accuracy.

3D Print Thread Sizing Chart

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.

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

For M5 x 0.8 that is 5.0 - 0.8 = 4.2 mm. This leaves roughly 75 percent thread engagement, the standard target that balances strength against the torque needed to cut the thread.

Because an FDM hole prints smaller than modelled, the tool then adds your printer’s horizontal expansion to the tap-drill figure:

modelled hole = tap drill + horizontal expansion

So an M5 hole on a printer with 0.15 mm expansion is modelled at 4.2 + 0.15 = 4.35 mm.

Imperial UNC and UNF

Imperial threads do not use a simple major-minus-pitch rule, so the tool uses the published tap-drill table (for example 8-32 UNC = 3.51 mm, 1/4-20 UNC = 5.11 mm) and applies the same horizontal-expansion correction.

Tips

For strong threads, print a plain hole and cut it with a real steel tap. Plastic taps cleanly when warm from printing.
Prefer 0.12 to 0.16 mm layer height for any printed thread profile.
If a printed thread is too tight, increase horizontal expansion by 0.05 mm and reprint. If it is too loose, decrease it.

All figures are computed locally in your browser from the standard pitch and tap-drill tables.