In the old days, it used to be said that a Fitter could cut to within 0.005" (0.127 mm ) with a hammer and chisel.
There is a Cornish beam engine carrying an insert in the Piston Rod that is hardly visible. It was inserted Many Many years ago, by a fitter, who probably had no access to a lathe, (and if if he had, would have had to use callipers to measure the sizes, by feel ) A masterpiece of work with a file and emery!
Howard