Quote Originally Posted by DoctorSaul View Post
I thought that float glass plate was reasonably flat. I was wrong. I picked up 4-¼ inch glass plates (3" x 11") from a local glass/mirror place and lapped them on my DMT lapping plate. All four were certainly not flat but flattened easily with some effort on the lapping plate. I did it dry. I got these plates in anticipation of receiving some lapping films to evaluate how the new lapping films compare to stones, cubic boron nitride slurries on various substrate, and strops.

All four plates needed flattening. Two plates has several high spots that required significant work to flatten them (about 30 minutes of lapping). The other two, while needing flattening as well, had minor high spots that leveled off within about 10 minutes of lapping. The DMT metal lapping plate works well at flattening glass.
Thanks for sharing. I was honestly not aware of this. I have never used such a thick piece of glass, though.

The great advantage of glass over sharpening stones, is that you can check it easily. Just hold it near your eyes, at a tangent angle, and it should reflect a straight image, not a wavy one. Very high-grit stones might do the same; mines don't (up to 8000, Norton brand). Most stones just don't reflect an image at all.