DESIGN LESSON // TOLERANCES

The 20.00 mm Magnet That Wasn’t

A tiny square hole, a supposedly standard magnet, and the reason the number on the packet wasn’t the whole answer.

SEPTEMBER 2026 · 5 MIN READ · LESSONS LEARNED

If you buy a 20 × 20 × 2 mm magnet, it is very tempting to design a 20.00 mm hole and call the job finished. CAD certainly encourages that kind of optimism.

The printer has other ideas.

A 20.00 mm hole still didn’t fit. The magnet and printed hole weren’t quite the perfect sizes I’d drawn. Corners and a slightly spread first layer added their own annoyances. I needed enough room to push the real magnet in without attacking the print with a file.

I tested tighter and looser cavities rather than guessing at the finished part. Around 20.10 mm was still too tight for the practical fit I wanted. 20.20 mm became the useful baseline.

Your setup gets a vote too

There is no universal “magnet tolerance” because the answer depends on the printer, material, orientation, how the walls print and whether the magnet should press in, slide in or get glued in.

For this project I also wanted the magnet flush and exposed, because covering it with extra plastic was just adding distance between two already fairly weak magnets. That meant the seat depth mattered just as much as the width.

The useful workflow was boring but effective: print the smallest useful test piece, change one dimension, label the result, and test the actual hardware.

Why coupons win

A full decorative print is a terrible calibration object. It consumes more material, takes longer and gives you the exact same answer about one tiny feature.

A ten-minute coupon can save an hour-long print. Better still, it gives you a dimension you can carry into the next design instead of repeating the same experiment.

That is now part of the way I approach hardware fit: test the little hole before printing the whole decorative piece.