Surface Plate

[Toolmaker51]

Depends on size you wish to attain.
If you have a Habitat for Humanity ReStores | Habitat for Humanity Int'l or other means for recycled materials, granite counter top is a good start. It cuts with abrasive blade in a circular saw. Don't use your best Milwaukee etc. Harbor Fright!! It's too thin beyond 12" x 18" or so, so bed it to a laminated wood base.
1/2" plate glass isn't too bad either, sliding gauges around though, the surface needs to be very clean to reduce deep scratches. Lap glass with wet silicon carbide [wet or dry] paper. Shiny is not related to flatness, air pockets in the surface are what help gauges slide. I hone and lap on a glass plate, probably have it 30 years.


https://www.google.com/search?q=lapp...utf-8&oe=utf-8

I found a suggestion for scrap aluminum cylinder heads, cracked but flat as carriers for lapping compound. Paste was applied with a little plastic wallpaper roller. Lapping is dependent on the lap softer than surface being lapped, so abrasive embeds there and cuts the surface. Water based abrasives are easiest to remove, cleaning has to be 100+%. Also found tips on reducing artist oil paint for hi-spot blue, less expensive and auto paint rubbing compound for fine grit abrasive. I'd probably start around 320 and work down from there.

Thickness and ability to support weight are related, being supported evenly is substitute. My A grade 3' x 4' with a tee-slot is 7". A B grade 5' x 6' will be about 10". Thicker granite is found at plants that produce headstones. Black granite is softest, lighter colors to pink [higher % of quartz] at other end of scale.
The real issue will be from securing 3 good flats, sufficient abrasive and Prussian Blue. Historically, cast iron plates go back I don't know how far, but a long time. They could scrape and flake planer beds, ways and rails to half-tenths or less [.00005], using the 3 plate method in the 1700's. The ideal surface is said to average 18 points of contact per square inch. It will be difficult to measure overall flatness [corner to corner] without collimators and such.

Or wait for free shipping at MSC, Travers or Enco. 12 x 18's will be in the $40-$50 range.

I will monitor and edit this as information or responses occur.