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

[Tim Sproul]

The way most glass is made today, it comes out very flat. The molten glass is made flat by laying the molten glass onto molten tin. Because the tin and glass don't mix and the glass is less dense, the glass floats and gravity gives you a nice, near perfectly flat surface.

The problem with glass used as a flat surface in woodworking (especially sharpening), is that glass is quite flexible. A large 36x36x0.5 piece of glass is only going to be as flat as the surface it is on.

For flattening your flattening stone, get an inexpensive 9x12x2 or x3 granite reference plate. The "B" grade are sold for $20 - $30 each and are more than adequately flat for woodworking. The granite, combined with the thickness of the granite, won't need a flat workbench surface to remain flat. You can also get granite plates in larger sizes....but for flattening a waterstone (the flattening stone or the grits you regularly use to sharpen), 9x12 is plenty large enough.

The method I like the most for flattening stones is to use DMT's DiaSharp extra-extra course (120 microng) for 2000 grit or coarser stones and the extra course (60 micron) stone for finer grits. Use plenty of water and you can maintain your stones in a flat state very easily. Be careful using finer grit diamond stones for flattening and not using enough water. If you go too fine on the diamond grit and too little on the water, the grit from the stone wears the nickel away, allowing the diamonds to fall off.

[Tom Esh]

Very unlikely it isn't flat. Check it with a straightedge if you have any doubts. With 1/2" deflection normally wouldn't be a concern, but since your piece is so large, you'll want to make sure it's adequately supported in the middle.
I got a diamond plate to flatten mine, but be warned - if you try one on a tool edge you may be hooked. Once I pop for a XX fine I'll probably dispense with waterstones (and the mess and the flattening) altogether. Not cheap, but they don't wear (that I can tell) and you don't need a slew of 'em (not as many intermediate grits required).

[Loren Bengtson]

Kevin,

Lay your piece of glass on your tablesaw top for support, then proceed. If you have a quality tablesaw, the top should be very flat.

Good luck.
Loren

[Dave Lehnert]

I use a granite 12x12 floor tile. I get them often on the clearance section at Lowes for around $2.00. I use the lapping grit from Lee Valley. A jar will last forever. Works well for me. See no reason to use anything else.