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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.
Nice! This got me thinking, is there a telescoping Allen key or socket wrench size determination tool? You slide down the increments until you achieve fit and this tells you what size wrench to fit? I'm guessing the tolerances would be way too tight for 3d printing. I'd keep a tool like this in my garage, I always run and get three socket wrenches from the basement to use in garage and half the time I still misjudge the size.
I think you are looking for a caliper
A 3d printed caliper is not a bad idea, but the shape isn't always great for working on cars and bikes. A vertical telescope mimics the approach of the tool. Its something trivial enough I would probably never buy it, but might print it.
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.