Design pins & holes that fit every time
A printed pin that snaps off, or a hole that won’t take the pin — both come down to two things: a fillet at the base and the right clearance around the shrink.
Applies to
- Process
- FDM / filament
- Features
- Vertical pins & locating holes, alignment features
- Materials
- Higher-shrink materials need more clearance
Why it matters
A bare cylinder with no fillet at its base is a stress riser — it wants to snap off the moment the pin gets tweaked sideways.
If the hole is the exact same size as the pin, shrinkage means they won’t fit, or they jam and split the part. A small clearance and a funnel at the top let the pin find and enter the hole every time.
What good looks like
- The pin has a fillet where it meets the body so it can take a side load
- The hole has a chamfer/funnel at the top to guide the pin in
- The hole is a touch larger than the pin — a repeatable slip fit
How to apply it
- Add a fillet at the base of the pin to kill the stress riser.
- Chamfer or funnel the mouth of the hole so the pin self-locates.
- Start with the hole ~0.25 mm larger than the pin on diameter.
- Print a fit coupon; open the clearance toward 0.5 mm for higher-shrink materials.
Test it on your setup
- Model a small block with the pin and a row of holes stepping the clearance 0.15 / 0.25 / 0.35 / 0.5 mm.
- Print in your real material and orientation.
- Pick the smallest clearance that still assembles by hand without forcing — that’s your number for that material.
Common mistakes
- A pin that’s just a plain cylinderFillet its base so it survives a side load
- Hole and pin modeled the same diameterOpen the hole ~0.25 mm and funnel the top
Base fillets, funneled holes, and a ~0.25 mm clearance allowance — Design Unbreakable Pins with Perfect Tolerances ↗.
The ~0.25 mm starting clearance is Slant 3D’s guidance for FDM; the exact value depends on your printer and material shrink, so treat it as a starting point and confirm with a fit coupon.