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.