During the later part of my Boy Entrant training, some of us fairies had to share a billet with some oily Riggers!
These guys all seemed to have kept their test piece and from memory they had been given a piece if 1 inch thick mild steel plate. They had to make it a 3" square, then put a 1" square hole in the centre.
A separate piece of mild steel had to be made into a 1" Cube.
The 1" cube had to fit into the 1" hole, any possible way, with not more than a 1 thou gap.
We fairies, played around with 'dominoes' of brass, ebonite and paxoline. All soft materials where it was dead easy to take too much off with one swipe of a file.