Design guide
Holes, bosses & threads
Anything you intend to screw, bolt, press, or thread needs a bit of extra thought in CAD, FDM is not a CNC mill, and the rules are slightly different.
Holes print smaller, plan for it
Printed holes come out smaller than the CAD model, the inside wall is laid down by an extrusion that sits half on, half off the nominal edge, eating into the opening. The hotter the material, the more it shrinks.
| Material | Add to diameter (mm) | Notes |
|---|---|---|
| PLA | +0.2 | , |
| PETG | +0.3 | , |
| ABS | +0.4 | ABS shrinks ~0.6% on cooling, bigger holes shrink more. |
| ASA | +0.4 | , |
| PA-CF | +0.3 | , |
| TPU | +0.4 | Holes deform under load; oversize and add a chamfer. |
For a clean fit on a bolt shank. For loose bolt clearance, add another +0.2 mm.
For holes larger than ~10 mm, drill rather than print, a hand drill cleans up the bore in seconds and gets you a much rounder, smoother surface.
Threads, print, tap, or insert?
For anything you'll assemble more than once, use heat-set brass inserts. For one-time assemblies, self-tapping screws into the right pilot hole work fine. Printed threads are a last resort, and only at M8 or larger.
| Method | Best for | Notes |
|---|---|---|
| Heat-set brass insert | All materials except TPU | Strongest, most repeatable threads. We can fit M2-M6 inserts during finishing, call it out in checkout notes. |
| Self-tapping screw | PLA, PETG, ASA, ABS | Fast and cheap. Pilot hole = screw outer diameter − 0.4 mm. Avoid for parts that get disassembled often. |
| Printed thread | M8 and larger only | Fine for one-off assemblies. Don't print threads smaller than M8, layer lines destroy them. |
| Captive nut pocket | All materials | Hex pocket sized 0.2 mm over the nut's flats. Bridge over with a thin ceiling so the nut drops in mid-print. |
Bosses & screw lugs
- Outer diameter should be at least 2× the screw nominal, so a boss for an M3 screw needs to be ≥ 6 mm OD.
- Lie bosses on the bed when possible. A vertical boss loaded in tension pulls layers apart.
- Add a small chamfer at the top of the hole. It guides the screw in and prevents the first layer from spalling.
- Reinforce with gussets to the surrounding wall, a free-standing boss is the most common breakage point on functional parts.
Captive nuts and pockets
For repeated assembly without inserts, captive hex-nut pockets are excellent. Size the pocket 0.2 mm over the flats-across dimension of the nut, depth equal to nut thickness + 0.4 mm. Bridge a thin ceiling above the pocket so the nut drops in during the print and gets trapped on the next layer.
We'll fit inserts for you
Tell us in checkout notes: “M3 heat-set inserts in the four corner bosses”, we'll fit them as part of finishing. We stock M2 through M6.
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Last reviewed May 2026 · Rigid Prints engineering team