How do you design holes for 3D printing?
You punch a hole in CAD, print it, and the screw won’t go in. The edge rippled, the top sagged, or the whole thing shrank. Good 3D printing hole design starts with one question: what kind of hole is this? A side hole, a hole over air, a blind hole, a press fit and a hole that grips a rod each want a different fix.
Applies to
- Process
- FDM / filament
- Hole types
- Side holes, holes over air, blind holes, press-fit holes, grip holes
- Orientation
- Axis horizontal (side hole) vs. axis vertical (top hole)
Why it matters
A hole with perfectly sharp edges doesn’t exist in real life. The nozzle has a diameter and the machine has momentum, so it can’t trace an infinitely sharp corner. Slant 3D shows what that does up close: a little rippling right around the edge of a plain punched hole.
Then the plastic shrinks as it cools. A hole modeled at the exact size of the pin prints small, and the pin won’t go.
And the direction of the hole changes the failure. A side hole sags at the top because its roof is a bridge. A hole that starts over open air prints a nest of strands. A tight press fit splits the wall if the wall has nowhere to go.
One fix doesn’t cover all of them.
What good looks like
- Hole edges carry a small fillet, never a raw sharp corner
- Side holes have a point on top (a teardrop) so the roof can’t sag
- Holes that start over open air are supported underneath, or capped with a one-layer sacrificial skin
- Blind holes have a chamfered mouth that guides the part in
- One-time press fits use relief slots or crush ribs; reusable fits use grip fins
- The modeled diameter includes a shrink allowance
How to apply it
- Fillet the edges of every hole so the tool path flows around them.
- Work out which way the hole prints: axis lying flat (a side hole) or axis pointing up (a top hole).
- Side hole: add a point to the top so it prints as a teardrop.
- Top hole that starts over air: support it from underneath, or skin it with one sacrificial layer if it’s small and carries no load.
- Blind hole: fillet and chamfer the mouth so the part finds its way in.
- Press fit: model the bore at true diameter and add relief slots or crush ribs. Reusable fit: add grip fins.
- Add ~0.1–0.25 mm to the diameter and test-fit the real mating part.
Test it on your setup
- Print one small block with a hole of each type you plan to use, at your real sizes.
- Push the real screw, pin or rod into each one. Note which ones bind, wobble or have strands inside.
- Adjust the diameter allowance in 0.05 mm steps until the fit is right, and keep that number for that material.
Exceptions & tradeoffs
Electrical enclosures and rough industrial brackets can live with a slightly saggy side hole. Slant 3D says so plainly: it’s not the end of the world. Save the fussy fixes for holes where the fit actually matters.
Mind you, if you’re going to drill or ream the hole anyway, the shrink allowance matters less. The drill sets the size.
Common mistakes
- Punching the hole in CAD and calling it doneFillet the edges, then apply the fix for that hole type
- Modeling the hole at the exact nominal diameterOversize for shrink, then fine-tune from a test fit
- Printing a horizontal hole with a plain round topAdd a point to the top so it prints as a teardrop
- Using the same press-fit hole for a one-time install and a part you take apart every weekCrush ribs for one-time installs; grip fins for reusable fits
Frequently asked questions
- Why are 3D printed holes smaller than designed?The plastic shrinks as it cools, and the nozzle can’t trace a perfect circle. Oversize the hole about 0.1–0.25 mm on the diameter and test-fit.
- How much bigger should I make holes for 3D printing?About 0.1–0.25 mm on the diameter is a good starting point for FDM. Print a test hole in your material and adjust from there.
- Should holes be printed vertically or horizontally?A hole with its axis pointing up prints roundest, as long as it doesn’t start over open air. A hole lying on its side sags at the top, so give it a teardrop point.
- What types of holes are there in 3D printing?Side holes, top holes that start over air, blind holes, press-fit holes and grip holes. Each has its own fix, from teardrops to crush ribs to grip fins.
Sharp hole edges ripple; small fillets give the tool path a smoother motion and a cleaner hole — Design Better Holes | Improve Tolerances | Reduce Sagging ↗ at 00:13.
3D printing shrinks, so hole size sometimes needs compensating; the video then walks through each hole type — Design Better Holes | Improve Tolerances | Reduce Sagging ↗ at 04:04.
The hole types, the fillet trick and the shrink warning all come from Slant 3D’s video. The 0.1–0.25 mm oversize is our own FDM starting point, not theirs. It lines up with the ~0.25 mm clearance Slant uses for pins. I split each hole type onto its own page so you can jump straight to the one you’re modeling.