How do you design a blind hole that fits in a 3D print?
A blind hole stops partway into the part, so there’s nowhere for a bad fit to hide. The pin goes in or it doesn’t. Plastic shrinks as it cools, so blind holes in 3D printing, plunged straight in at nominal size, often come out tight. Slant 3D’s fix is small: fillet the top and chamfer the mouth so the part finds its way in.
Applies to
- Process
- FDM / filament
- Features
- Non-through holes: pin sockets, dowel holes, locating holes
- Fit
- Slip fit to light press fit
Why it matters
FDM plastic shrinks as it cools. A blind hole modeled at the pin’s exact size comes out a touch small, and you find out when the pin stops halfway down.
A plain plunged hole also has a sharp mouth. The pin catches the edge, starts crooked, and either jams or shaves plastic off the wall.
A chamfer turns the mouth into a funnel. Slant 3D calls it a finding feature. The part gets started easily, then tightens as the hole narrows.
You get a good fit without sweating the exact tolerance.
What good looks like
- A fillet on the top edge of the hole
- A small chamfer at the mouth that guides the part in
- A bore oversized for shrink, so the part seats all the way down
- The floor of the hole prints on solid plastic, not over air
How to apply it
- Model the bore at the part’s diameter plus a shrink allowance, starting around 0.1–0.25 mm.
- Fillet the top edge so the tool path flows around it.
- Add a small 45° chamfer at the mouth as a lead-in.
- Orient the hole opening up, so its floor prints on solid plastic. If it has to open downward, its end prints over air: support it or skin it.
- Test-fit the real pin and adjust the allowance in small steps.
Test it on your setup
- Print a block with four blind holes stepping the oversize 0.1 / 0.15 / 0.2 / 0.25 mm, each with the same chamfer.
- Push the real pin into each one.
- Keep the size that starts by hand and seats fully at the force you want. That’s your number for that material.
Exceptions & tradeoffs
When you need a strong press fit, a chamfered bore isn’t enough. Slant 3D’s next step is a bore at true diameter with relief slots or crush ribs, so the walls give instead of the part splitting.
Mind you, if the pin gets glued in, a little extra room is your friend. The glue needs somewhere to go.
Common mistakes
- Plunging the hole straight in at nominal sizeOversize for shrink and add a lead-in chamfer
- Hammering a tight pin into a blind holeOpen the bore, or switch to crush ribs so the wall doesn’t crack
- A downward-facing blind hole whose end prints over airFlip it so the floor prints on solid plastic
Frequently asked questions
- What is a blind hole in 3D printing?A hole that doesn’t go all the way through the part. It stops at a floor, like a socket for a pin or a dowel.
- Why chamfer the top of a 3D printed hole?The chamfer is a lead-in. The pin starts easily and centers itself, then tightens as the hole narrows.
- What tolerance should a blind hole have in 3D printing?Start by oversizing the bore about 0.1–0.25 mm on the diameter, then test-fit the real part in your material.
- Can you press fit into a blind hole?Yes, but don’t just make the hole smaller. Slant 3D recommends a true-diameter bore with relief features or crush ribs so the walls can give.
Blind holes: watch the tolerance and shrink, fillet the top, and chamfer the mouth as a finding feature — Design Better Holes | Improve Tolerances | Reduce Sagging ↗ at 03:59.
The fillet-plus-chamfer finding feature is Slant 3D’s. The oversize range and the coupon steps are our own FDM starting points.