From the functional-yet-impractical files, a planetary-geared caliper set that measures the outside dimensions of objects up to 20cm. More or less accurate to within 2mm, based on my tests. Curiously, owing to the geometry of translating the angle between the caliper arms into degrees on the dial, the caliper is more precise at larger measures than at smaller. No kidding, it is tricky to assemble so that the zero on the dial matches the tick mark in the viewing window. Ach, herringbone gears.
If what you want is an accurate caliper, for Crom's sake, they are cheap as dirt online. If what you want is a working mechanical curiosity that you printed yourself, come in, friend, come in.