How do you get a perfect press fit with crush ribs?
A press fit into a plain round hole is a gamble on a 3D printer. Too tight and the wall cracks. Too loose and the part falls out. Slant 3D’s answer is crush ribs for 3D printing: design the hole at true diameter, then let the walls give, first with relief slots and then with ribs made to deform.
Applies to
- Process
- FDM / filament
- Features
- Press-fit holes for pins, rods, dowels and inserts
- Use
- One-time installs and glued joints
Why it matters
A solid round wall has to stretch to take an oversize part. Plastic doesn’t like that. It strips, deforms for good, or splits.
Printed holes also vary side to side. Plunge a tight hole straight in, and one side grabs while the other barely touches.
Relief slots let the wall spread and split on purpose, so the hole stops fighting the part. Crush ribs take it further. They’re sacrificial: the part strips them back as it goes in, and you get a tight press fit without loading the whole wall.
And each rib can be as thick or thin as you want. Thicker ribs grip harder. Thinner ribs press in easier.
What good looks like
- The bore is modeled at true diameter, not undersized
- Relief slots or ribs give the wall somewhere to go
- Ribs are evenly spaced so the part centers itself
- The part presses in firmly and the wall around it stays whole
How to apply it
- Model the bore at the part’s true diameter.
- For a lighter fit, cut a few relief slots into the outer side of the hole wall so it can spread.
- For a firm fit, put ribs all the way around the inside of the bore. Three or more, evenly spaced.
- Tune the grip with rib thickness: thicker to hold harder, thinner to press easier.
- If the joint gets glued, let the gaps between the ribs act as glue channels.
- Press the part in once. Crush ribs are a one-use feature.
Test it on your setup
- Print three test bores with thin, medium and thick ribs.
- Press the real part into each one, by hand or in a vise.
- Keep the one that goes in without cracking the wall and won’t pull back out by hand.
Exceptions & tradeoffs
Crush ribs are one and done. Pull the part back out and the ribs are already crushed, so the next fit is looser. If it has to come apart again and again, use grip fins.
Mind you, crush ribs earn their keep on glued joints too. Slant 3D points out a plain hole gives the glue nowhere to go. The gaps between ribs do.
Common mistakes
- Undersizing a plain hole to get a press fitBore at true diameter with crush ribs
- Hammering a part into a solid wall until it splitsAdd relief slots so the wall can give
- Expecting crush ribs to survive repeat assemblyUse them for one-time installs; use grip fins for reusable fits
- Gluing into a tight plain hole with no room for glueLet the gaps between ribs hold the glue
Frequently asked questions
- What are crush ribs in 3D printing?Small ribs around the inside of a hole that are meant to deform. The inserted part crushes them as it goes in, so you get a tight press fit without stretching the whole wall.
- How do you design a press fit for 3D printing?Model the bore at the part’s true diameter, then add relief slots or crush ribs. Tune the hold with rib thickness instead of shrinking the hole.
- Can crush ribs be reused?Not really. They’re a one-use feature. For a part you take apart often, Slant 3D uses grip fins instead.
- How many crush ribs should a hole have?Three or more, evenly spaced, so the part centers in the hole.
For a strong press fit, design the hole at true diameter with relief features so the walls can spread — Design Better Holes | Improve Tolerances | Reduce Sagging ↗ at 04:37.
Crush ribs: sacrificial ribs tuned by thickness, a one-use fit, and room for glue — Design Better Holes | Improve Tolerances | Reduce Sagging ↗ at 05:18.
Relief slots, crush ribs and the glue tip are all Slant 3D’s. The three-or-more rib count is standard practice for centering a part. The video doesn’t give a rib size, so a quick three-bore test print is how you land yours.