Flat Reference Surface To Tuning Wooden Planes

No surface plate needed. Use Whitworth's self-generating gage technique. Build three planes and use each to flatten the other two. The *sharper* and finer you set the irons, the flatter you'll shave the soles.

Whitworth claimed his cast iron surface plates -- which coincidentally, 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.