Plate Glass Flatness

[Kevin Blunt]

Good morning,
I am going to be heading down to my shop not long after posting this but I would like post this question/thought that I have wondering for a while.
I know alot of comments I have heard about the Norton Flattening stone have been negative but my experiences with it have been quite positive, but am now at the point where the flattening stone needs to be flattened. I have read alot of comments about just using a piece of plate glass with snadpaper adhered to it to flatten waterstones but it just seems to expensive in my experiences with it. In this case though I would like to use this method for flattening my flatenning stone.

I have only ever heard that plate glass is flat and can be used for this application. Currently, I am lucky enough to have obtained a large 1/2" thick piece of tempered arena board glass that probably measures around 36x36. Would this glass be flat enough for flattening stones? I am assuming it will be. Is there any clear plate glass that would be flat enough for this or is it basically all flat because of th eway it is manufactured.


Would really appreciate your thoughts on this,
Kevin

[Gary Herrmann]

I've used float glass for sharpening, not plate glass. Float glass is flat and named that because it cools on a molten bed of tin. Plate glass may be flat enough, but I don't know enough about it.

[Mike Henderson]

Most regular glass made today is "float glass", meaning it is continuous cast by floating molten glass on a bed of molten tin. The glass flows out due to gravity. I expect the glass is not flat to optical standards (say within a quarter wavelength of light) but is plenty flat for the purposes of woodworking.

The negative comments you've heard about the flattening stone are due to the fact that the flattening stone starts becoming convex the first time you use it, which means your working stones are becoming concave. Since you have to flatten the flattening stone, many people feel it's best to skip that step and go directly to some flat surface with their working stones. Common techniques for flattening working stones are sandpaper on glass or granite or a DMT diamond stone.

Mike