Helmut Kerner, that I mentioned above, was tested in action filing and stock making for the two skills needed to get his Master's Cert. For the filing, he filed a plate 2cm thick, flat, 5x5cm, with parallel faces. He filed a 2cm square hole( I guess starting with drilled hole) in the plate and filed a 2cm cube that fit into the square hole at all orientations without light shining through the joints. It sounds simple. but it is not.
Mike