No surface plate needed. Use Whitworth's self-generating gage
technique. Build three planes and use each to flatten the other
The *sharper* and finer you set the irons, the flatter you'll shave
Whitworth claimed his cast iron surface plates -- which
he called "planes" -- were within 1 millionth of an inch of true
flat. Three #5's should true every bit as flat just as easily using
machinist's blue and a scraper, valve lapping compound, whatever.
Alternatively any company that deals with shopfront glass should be able
to sell you a chunk with the edges ground smooth for not much money. Or,
in fact, almost any glazier should sell it - if you can't find thick
plate glass then get two thin sheets and superglue them together.
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