Chris Lindquist wrote:
I've been trying to find out exactly what process was used for
originating a surface plate in the good old days. I expect it involved
careful hands and a large amount of scraping, but what provided a
reference for "flat"? And how flat was flat around the turn of the
century?

I'm not planning to try this at home, of course. My Chinese granite
plate is plenty flat enough, thanks. But I've been going through my
books looking for an answer to this chicken-and-egg question without
finding a solution.

Can anyone help?

Thanks!

Greetings,

You don't make one plate, you make 3 of the same size. Match plate A to
B, B to C, and A to C. It's an iterative process, bringing down the
high spots on each plate a bit at a time. The same method can also be
used to generate straight edges, and with some variation master
precision squares.

Cheers,
Stan