<<snippage about surface plates>>
I saved a few pieces of granite tile from a bathroom I did and they
were not very flat. Probably ok for putting edges on blades, but
definitely not flat enough for lapping plane bottoms, IMO. Some tiles
might be ok, but not the ones I had.
If you have 3 of said plates, get some loose silicon carbide abrasive powder
and lap them together and they will be flatter than any woodworkers needs
Mix the abrasive with light oil...I use cheap olive oil or vegetable
oil...until you have a thin paste. Label the plates A, B and C or 1, 2 and 3
or whatever it takes.
Smear a coating of paste on plate A, place plate B on top of it and move
around in a figure 8. The pattern shouldn't be big...no more than 1/4 of the
plate size and smaller than that is better.
Turn plate B 1/4 turn and continue until both plates have been cleaned up
some...about half the surface...you should be able to see where it's cutting
and where it hasn't. Oh...be sure to keep the plates wet with the paste.
Clean off the plates, saving as much of the paste as possible.
Now put plate B down, cover with paste and lap plate C on it, using the same
Now plate C goes down and plate A goes on top. Continue.
Yes, you are seeing a pattern develop here and it should continue until all
the plates have a uniform surface on them.
Store 2 of the plates somewhere safe...if you ever need to touch up your
working plate, it'll go LOTS faster having those two plate rather than