DESIGN LESSON // TOLERANCES
The 20.00 mm Magnet That Wasn’t
A tiny square hole, a supposedly standard magnet, and the reason nominal dimensions are only the beginning.
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.
The first test pieces made the real problem obvious: a nominal dimension is not the same thing as a useful fit. The magnet has manufacturing tolerance. The printer has dimensional error. Corners behave differently from long straight walls. The first layer can spread. Then I still need enough room to actually install the thing without attacking it 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.
Fit is a system
There is no universal “magnet tolerance” because the answer depends on the printer, material, orientation, wall strategy 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 possible coupon, 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: validate the interface first. Earn the right to print the pretty bit.
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