45° flanks
A symmetrical trapezoid with 45° flanks. The overhang never exceeds 45°, so bolts print upright and threaded holes print in any orientation — without supports.
—3D printing
SI3DP — the fastener standard created for 3D printing.
The ISO metric thread was designed for cutting metal. Printed upright, its lower flank is a 60° overhang, small sizes are shallower than a nozzle track, and clearances are counted in hundredths of a millimetre. SI3DP (screwit3Dprint) is a thread and hardware series designed for the printer instead — and screwit3D models it for you.
One number: D
The thread is proportional. Pitch P = 0.3 · D, profile depth 0.1 · D, flats P / 6. Everything follows from the nominal diameter — also for sizes outside the table.
Clearance built in
0.10 mm on the bolt and 0.15 mm in the hole are part of the standard, identical for every size. Need more for your printer? Set your own — clearly marked as off-standard.
Reinforced where prints break
A 45° cone under the head fills the clearance hole, followed by a 45° thread run-out. Right under the head there is full cross-section and no sharp notch.
01Try it
The same preview you get in the panel.
Rotate it, switch to the section, pick another joint. The card below shows exactly what the extension writes into your model and parts list.
- Thread cut into the part. A true helical thread modelled into your printed part — through or blind, across one part or two.
- Threads that mesh. Bolt, nut and hole threads are phase-aligned, so the model shows crest in groove — even with recessed heads and parts at an angle.
- Printed end caps. Plugs for tubes and holes with ribs that hold by friction, a 45° lead-in and a body 0.4 mm smaller than the cavity.
screwit3D
drag to rotate · scroll to zoom
What happens
02In SketchUp
From a box to printable parts.
03New in 1.1
Your own shapes get a thread. Then straight to the bed.
Boxes with screw-on lids, jars, caps, plugs — drawn as plain cylinders, threaded with one click, exported laid out on your print bed. Both tools are beta.
Thread on a shapebeta
Draw a neck and a lid. Get a thread that fits.
Model the neck of a box and its lid as plain cylinders. Pick Thread for a neck, click the neck — it gets a 45° thread sized for your printer. Click the lid — it takes its thread from the neck, shifted by itself so the two seat against each other, and the marks line up when tightened. Pitch, clearance, number of starts and hand are yours to set.
- Fit the lid seats it on the box exactly as it ends up after tightening — check the fit before you print.
- Lay out for printing puts it back beside the box, bottoms level, ready for the bed.
- Edit the pair any time — pitch, turns, clearance, starts — and both parts change together.
- Free version: 3 trial pairs per computer. Full version: unlimited.
STL and DXF exportbeta
From the model to the printer — or the laser — in one window.
Select the parts and press Export in the panel’s top bar. Name them, choose STL for 3D printing or DXF for CNC and laser cutting, and set the size of your print bed or material sheet. The parts are laid out flat with a gap between them; what does not fit is split across several beds or sheets — you are asked first.
- Lay out in the model places copies beside your model, spread over the bed — the model itself stays untouched.
- A selected joint is split into bolt, nut and washers, each as a separate part.
- DXF: every part lying flat, outline and holes as true circles — ready for the cutter.
- Export each part to its own file or all parts into one. In the free version too.
04The series
S6 to S20 — sized like the metric series.
| Size | D | Pitch P | Depth h | Bolt Ø core–crest | Nut / part Ø | Clearance hole | Hex key / spanner |
|---|---|---|---|---|---|---|---|
| S6 | 6 | 1.8 | 0.6 | 4.6 – 5.8 | 5.1 – 6.3 | Ø 7 | 5 / 10 |
| S8 | 8 | 2.4 | 0.8 | 6.2 – 7.8 | 6.7 – 8.3 | Ø 10 | 6 / 13 |
| S10 | 10 | 3.0 | 1.0 | 7.8 – 9.8 | 8.3 – 10.3 | Ø 12 | 8 / 17 |
| S12 | 12 | 3.6 | 1.2 | 9.4 – 11.8 | 9.9 – 12.3 | Ø 14.5 | 10 / 19 |
| S16 | 16 | 4.8 | 1.6 | 12.6 – 15.8 | 13.1 – 16.3 | Ø 18.5 | 14 / 24 |
| S20 | 20 | 6.0 | 2.0 | 15.8 – 19.8 | 16.3 – 20.3 | Ø 24 | 17 / 30 |
SI3DP v0.1 · all dimensions in mm · smallest usable size ≈ 15 × nozzle diameter (0.4 mm nozzle → S6).
05In the extension
Switch to 3D PRINT and click.
- Printable hardware. “Fastener only” hangs a bolt, nut or washer on your cursor — hex, socket or countersunk — ready to export as STL.
- Heat-set inserts. Brass inserts M3–M8 with the right pocket for your print — when you want steel bolts in a printed part.
- Metric in plastic. Prefer bought bolts? Cut ISO or ASME threads with print tolerance instead of SI3DP.
- Calibration plate. A test plate in ⚙ Settings with the clearances printed on it — check the standard on your own printer and adjust if needed.
06Questions
About SI3DP
What exactly is SI3DP?
SI3DP (screwit3Dprint) is a thread and fastener series designed for FDM printing: a proportional trapezoidal thread with 45° flanks, pitch 0.3 · D, clearance defined by the standard and reinforced head transitions. It is currently at version 0.1 and lives inside screwit3D, which models the threads, bolts, nuts and washers for you.
Do SI3DP parts fit metric steel bolts?
No — SI3DP has its own, much coarser thread, so printed SI3DP bolts go with SI3DP nuts and threads. Diameters and spanner sizes match the metric series, so your tools fit. When you want a bought steel bolt in a printed part, cut an ISO or ASME thread with print tolerance or use a heat-set insert; screwit3D does both.
Is 3D printing included in the free version?
Yes, in sizes S6 and S12: printed parts, threads cut into the part and heat-set inserts, up to 3 of each kind at a time in one model. The full version unlocks S6 to S20 and removes the limit.
What if my printer needs more clearance?
Print the calibration plate from ⚙ Settings, see which clearance fits your printer and set it in the joint. A custom clearance is marked as off-standard, so you always know which parts differ.