I was doing some bike maintenance today and wanted to disassemble my rear hub. It turned out that I needed a 12mm Allen bit for that, which I don’t have. So I 3D printed one! And it worked! Torques safely to 5Nm and I only needed 4Nm for the job. Haven’t tested higher torques.
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I think you are looking for a caliper
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You could use a divider caliper to be able to translate the size of a fastener to a spot that is easier to measure, if you don’t have to swap between inches and metric it would probably be easy to be accurate enough with it without much practice.
I don’t think the tolerances would be too bad. A lot of prints that have tight tolerances have a test piece that you print and test against a known object, which let’s you adjust your print to get tighter tolerances. Once you correct for the expansion of the plastic, getting the right tolerances should be totally doable.