Quote Originally Posted by rossbotics View Post
After it's cooled down I just wipe it off and go to the grinder where I have a wire wheel and get the crud off, I then take some 220 grit by hand and clean it up, heat it back up to a straw color and quench her once more, I have two surface plates of different sizes and only use them for inspection Etc. go down to your local granite shop and get you one of there drops, they gave me mine, I got two pieces of that stuff there about 14" X 16"just for laying down full sheets of wet dry 600-800-1000 and do polishing or whatever on, this stuff is incredibly flat, you can use this plate like a lap with a piece of 600 wet dry, or you can grind it all over like I did, this is the best way, If you find your slot closed a little bit from the heat treating you can put it back in the mill with the same parallel setup and with a 1/4" carbide end mill and clean the slot up.

Hope this helps
Doug, thanks for your reply:
A stationary wire wheel I do not have. I've used them at friends homes, they just are too easy to have large chunks of skin removed (and my skin is getting thinner every day). Scary enough to use a buffing wheel, which is what I tried to use in the past to remove the oxide coating from heat treat. It was ineffective as I could not cut the oxide off without hours of effort.
Just a few weeks ago, a guy advertised polish black granite slabs, that at one time were the exterior facade of a bank gone long ago. I calculated the weight from the description, 296lbs. But for $20, how could I go wrong. And that was my intent was for a lapping plate. Way too big, so the next project is to cut it down with a diamond blade in a circular saw. It would make a good surface plate, as I assume it was polished flat via a lapping process. But at 4" thick, probably is concave if held near its edges (at it's current size). I also purchased sheets of wet dry sand paper (eBay), from 280 grit to 5000. I played a little on it with a piece of brass.