Originally Posted by
invention13
I did check it at work on a granite layout table with a dial indicator in our machine shop. It was within a few 10,000ths of an inch. Just how good do you think you needs it to be?
I think the surface you grind on matters less than you think. Both surfaces wear (the tool and the stone you are lapping). After a few strokes, they will 'fit' perfectly. With glass, the initial flatness is not particularly relevant since it will not be perfectly flat after a few strokes, but will be slightly dished. What matters more is how you vary and randomize the strokes. I recommended the granite tile simply because I was getting $20-30 quotes for a piece of 3/8 float glass vs. $4 for a tile.