Sole Flattening Report to the Teachers

[John Burrows]

Patrick, You are correct. I was in the glass tempering industry for over 30 years.
During the tempering process, the glass is heated to 1200 to 1400 degrees, which is visually red hot. In this state it is quite flexible and bendable, but not yet liquid, so it can deform during the rapid cooling process, especially if the cooling air is not distributed on both sides of the glass equally. So it can deform during tempering. Untempered float glass is your best bet, since it is drawn out of the tank onto a bath of molten tin. Tin is heavier than glass, so the glass literally floats on top of the tin creating a very even, flat and smooth sheet of glass.

Sheet glass however, is vertically drawn out of a tank and usually has visual inclusions and is not nearly as flat as float glass. I don't think sheet glass is made anymore, with the last plant, PPG, in Mt. Zion, Illinois, closing their sheet glass tanks a decade or so ago.

Probably more than you wanted or needed to know, but I thought I would chime in. Bob

Bob,

Float glass is not all that flat either. Is plate glass, which is ground and polished on both faces, even made today?

John

[Patrick McCarthy]

Bob Glenn, thank you for the insight. I am frequently amazed about the wealth of information available thru the creek. I have long heard the term "float glass" but had no idea what or how it came to be. Don't know if I will ever need it, but I do feel a little bit smarter today . . . and that has to be a good thing.

Best regards, Patrick

[Jim Koepke]

If finding flat glass is a problem, another source for a lapping surface might be a local monument maker. (often called tombstone carvers)

http://www.sawmillcreek.org/showthread.php?134511

My lapping station doesn't get as much use as it used to, but it is still a good thing to have.

Just like shopping for glass, take a straight edge.

jtk

[Chris Fournier]

An A Grade granite inspection plate is not expensive, it is durable and it is very flat. Buy one and it will pay for itself over the years.

The sharpie hash marks are just fine for this task, fallible at what increment I wonder? .001", .003"? Either way plenty reliable for the surface that you are trueing. The straight edge that you buy will have a similar tolerance...

As a woodworker and metalworker I can say that your efforts are just fine as long as your technique keeps the sole on the paper and not rocking. Personally I true soles on 320 grit but your piece may be pretty out of whack and be a slow process at that grit.

You could use your plane with a shooting board as you have it in the photograph and obtain good results.

[Pat Barry]

It sure looks like you have gotten it to the point that it's mostly flat with a bit of pitting. If you have scratch marks from the sanding spanning the area, even if not totally continuous you are done enough for the intended function. You can always go further if it makes you feel better