I was watching this video which made the claim in the subject line above. Unfortunately they do not explain it (there is a brief graphic but it made no sense to me).
The video is about extreme precision in tools made in the 19th century. Remarkable precision for the day using mechanical means. One of the things they managed were very, very flat surfaces which were achieved (it seems) via the method in the subject line.
But unless I am missing something obvious I do not understand what they were doing to achieve these really, really flat surfaces.
Can anyone shed some light on this?