Printed Holes That Fit
Designing Holes For The Printer You Can't Dalibrate
If you print the same part on three printers and measure the same hole, you'll get three different diameters. Holes generally print undersized, but not because the plastic shrinks - shrink is a percentage, so it would miss a larger hole more than a small one, and every machine I measured missed the small holes by more. The amount follows the machine and its tuning, and that is what makes it hard to predict how off each machine will be.
On your own printer, you can measure that error and correct for it. Unfortunately, once you publish or share a design, you lose the ability to do that — and the hole still has to fit, whether it takes a screw, a heat-set insert, or a bearing. So how do you design a hole that can be printed to spec on a machine you can't measure?
The fix is geometry, not tuning
- Entry chamfers that absorb first-layer flare
- Teardrop profiles that keep horizontal holes from sagging
- Polygon pockets with flat walls (which print more predictably than curved ones)
- Crush ribs that move interference into small features designed to yield
- Grip fins for fits that have to cover a wider spread of hole error
I wrote some design guidelines, with when to use each and the failure modes to watch: Printed holes that fit — design patterns that don't need calibration
If you want the exact number to model (drilled clearance, printed holes, insert pockets, and more), that's what FitCheck is for: it catalogs every standard size, and is free on the web. If you want those numbers corrected to your own machine's measured error, download the app!
Happy printing!
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