Marble vs Granite surface plate

The "3 pieces of equal size" requirement is to prevent dishing of your plates (or, one concave and one convex). A "dished" set will look good, because all areas are ground, but in reality are unusable. Grind Plate A and Plate B in a circular pattern for 100-200 circles, then grind Plate A and Plate C in the same pattern, then grind Plate B and Plate C the same way. Since you probably don't have a Repeat-O-Meter, so grind your surfaces until the scratches are all over the plates. Proceed to the next finer grit and repeat the same grinding. At the end, you'll have three usable "surface plate lite" that can have quite good flatness specs.

McMaster has the grinding powder (use silicon carbide or diamond powder).
 
@Brento I didn't realize that 'got grit' was still in business! When I was building telescopes (aeons ago) we got our supplies from Telescope Supply, now long defunct.
 
I went to got grit really quick but all they had was 5 pound containers. Does anyone make a kit for the material. I am also waiting as i need to still find 2 more plates of the same size. I was hoping i could use the big orange one with the big black on and get another to do the lapping. Its gonna be hard to fond the same sizes.
 
one of 2 [Edit] More like 5 of these will suffice for a 12 X 16" surface plate using the 3 plate method. [Edit] No you are better off with buying 5 packages of Got Grit abrasive...

Amazon.com: 7pcs 20 gram Diamond Lapping Paste Polishing Compound For Fine to Final Polishing Grits 0.25 to 5.0 microns : Industrial & Scientific

Buy 7pcs 20 gram Diamond Lapping Paste Polishing Compound For Fine to Final Polishing Grits 0.25 to 5.0 microns: Finishing Compounds - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com

I think I need to state again: by the time you've purchased the materials for lapping you could have bought 2 offshore surface plates:

www.busybeetools.com

SURFACE PLATE 12IN. X 18IN. X 3IN.

Precision Surface Plate - Black Granite. Harder than steel and does not nick or burr.
www.busybeetools.com www.busybeetools.com

P.S. Surface plates are way cheaper in the US than Canada.
 
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Im gonna have to check my black one to see the state it is in and then i guess break down and buy a smaller one i dont need a big one but it would just be nice.
 
Go to almost any auto parts store and get some valve grinding compound. That should suffice. You don't need a super polished surface, just a flat one.
 
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