How Flat Glass?

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I bought a new 2'x3' x 3/8" piece of plate/float glass and its not flat. Over the 3' there is a gap of .02" checking with a good straightedge. My No 7 plane actually rocks on the glass. The underlying surface is flat. I've read that all plate glass is now the same as float so I don't think I bought the wrong type.
I previously had (and cracked) a 1/4" thick piece that didn't vary by more than .003".
What is a normal tolerance? Does thicker glass have greater tolerance for error?
-- Mark
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Mark Morin wrote:

That is a pretty large error. Normally glass thicker than 1/4" is made by cooling it on a pool of molten tin. The tin melts at a lower temperature than the glass and since a fluid finds it's own level if not vibrated or disturbed the reason float glass is so flat. Nominal is .001 over a 1'.
Obviously the stuff you got has to be rolled or allowed to sag once it comes out of some rollers. Are both sides thin like this? Got a micrometer to check the thickness around the parameter?
Second thought is to buy a B grade granite surface plate.
Alan
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My suggestion, look at Grizzly at the granite flats/plates, grade B flatness and very thick, works like a charm here for scary sharp, and I don't have to worry about how much pressure I apply while sharpening - I think I paid around $20 for a 9x13 one from Grizzly
John
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Got a sink cutout of granite from granite shop gratis, used for scary sharp. About 12" X 18".
On Sun, 02 Oct 2005 22:35:39 -0500, snipped-for-privacy@interoz.com wrote:

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snipped-for-privacy@interoz.com wrote:

Ditto this, but if I were doing it again I'd get one that's 12"x18" (three inches thick they are). This allows for a full sheet of 9x11" abrasive to sit *flat*. The 9x12"s come with an eased edge on top, and as such the surface tension of paper is lost along the edge when using fluid to keep it in place. Plus, bigger is always better....right?
JP ************************** Zero tolerance anyone?
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I guess I was spoiled with my old 2' x 3' glass. I could put 6 quarter sheets on it for sharpening chisels. Is was also big enough to put belt sander strips on it for plane soles. I considered MDF but I use 3M spray adhesive and it comes off nicely when changing papers (maybe Formica-type top would be better, which I haven't tried).
I checked the granite -- price goes up quickly based on size and really quickly based on shipping.
I'm going to try another piece of glass before going the granite route.
-- Mark
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wrote:

removed from my mom's house when an addition was built. It seems rather flat to me (not measurably off by my crude measurements and certainly not off enough to rock a plane or a blade). It is about 3 feet or so long and wide enough for a sanding belt. Can set up a whole range of grits from 80 to 2000 in small sheets at one time. When done, it stands in a corner somewhere. Probably can find one somewhere where a building is coming down or something.
Dave Hall
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I've got a monster 4' x 3' sheet mounted to 5/4 birch ply and stiffened w/ steel. I don't bother pulling it out unless I'm lapping the sole of a bench plane or a slick. I've found the outfeed table of my jointer works really well.
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You need to go to a *junk shop*. My glass was $10 for a piece the size of 18"x18"x3/4" thick, awesome and heavy and FLAT.
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Mark Morin wrote:

Glass is very flat, but it's also bendy. Try placing your glass on a layer of foam over a reasonably flat surface. If you take the forces off it and let it float, then it should be flat.
Laminated glass and poorer quality toughened glass loses flatness, but new float glass ought to be the flattest thing in the average workshop.
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On Sun, 2 Oct 2005 17:54:37 -0700, with neither quill nor qualm, "Mark

So use a piece of MDF, or your kitchen countertop, fer Crikey's sake.

Good question. Glass does move; some say it's a liquid. I scratched my new utility door window while removing the label. 2 months later I noticed that the scratch had healed; it shocked the hell out of me.
Leave the glass on the table for a month and see if it has settled. It may have been stored upright where it took a bow.
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Thats an old wive's tale. Glass is amorphous, but certainly not liquid.

I scratched a microscope lens 35 years ago. It is still scratched exactly as it was then.
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Toller wrote:

Super cooled liquid is the standard saying. And glass does flow. No fairy tale. Actual measurements show that glass standing vertically for a long period is thicker at the bottom. Of course it does take a long time.
Glass does heal. Anybody that works around glass or just uses his observation powers knows that freshly broken glass has much sharper edges than broken glass edges that have been around for a time. If that isn't enough for you, anyone that uses a microtome (you know one of those things that cuts very thin slices for microscope slides) and uses glass for the cutter, knows that you use a freshly broken surface. Let it sit around for a while and you won't be able to cut as thin slices as freshly broken, note we are talking about slices less than 10 microns thick. Of course, you could even more simply use a microscope to examine broken glass edges. The type of glass also makes a very big difference to how fast it heals.
Nonetheless, I don't believe the healed glass scratch in a window. Two months is too short a time to possibly observe that; 20-30 years for a very, very light nearly invisible scratch on window glass, maybe but more likely 100+ years. Probably was a scratch in the dirt on the glass or maybe just a thin cobweb.
I've been waiting for 5 years for a scratch in a new window to heal. Har. Har. Not really, the house will be torn down long before that glass heals.
As for the microscope lens, it might be very hard, heck it could be crystalline quartz, but not likely. In addition, the scratch would have to be on a vertically oriented plane to heal in any observable way.
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On Mon, 03 Oct 2005 18:57:36 GMT, George E. Cawthon

Please read this: http://tafkac.org/science/glass.flow / Just because your science teachers told you it does, doesn't mean they were right. The folks at alt.folklore.urban are damn good about tracking things down, and if you have something that isn't covered in their FAQs and is truly new information, they'd love to know about it.
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As I recall, FWW mentioned this myth long ago. The story goes is that old glass is thicker at the bottom because they made it using a technqiue that didn't make flat glass. Something about slab cooled, and one end being cooler than the other.
And when they placed it in windows, they put the thicker edge down just like they did with shingles. So the thicker end is due to habit.
But you are in good company, George. Marylin vos Savant (smartest person in the world) also got it wrong in her column.
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Dave Hinz wrote:

I read it and wasn't impressed. Just typical newsgroup bullshit and no scientific discussion or evidence. I did like the part that says that the greater thickness at the bottom of old panes of glass was due to original variation in thickness and that glaziers orient the thicker part to the bottom. I thought of this as a possibility for about 10 seconds before discarding it. First, most glaziers probably didn't give a damn how the glass was oriented and even if they did, they couldn't tell the minute difference in thickness without a micrometer on a consistent basis. That would howlingly be funny to see a guy in the 1800's using a micrometer on glass panes before glazing them.
I suggest you use a microtome with glass knives to find out for yourself and propose a different reason for why a freshly broken edge will cut a thinner slice.
BTW, I doubt that many of my professors considered or gave a damn about this burning issue. I'm sure there is plenty of data out
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Since they could see the difference by eye, why on earth would they bother with a micrometer?
Mike
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Michael Daly wrote:

Your statement assumes a fact not in evidence. The point is the glass worker couldn't see the difference by eye on each pane and they couldn't achieve the consistency of thicker at the bottom by chance. It depends on what era of glass making you are referring to. Some of the glass would require very good measurement to tell the difference in thickness, or are you saying that you can just look at a pane of glass and tell that one end is 0.001 inch thicker than the other? And some rolled glass has so many waves and bubbles, one couldn't decide which end was thicker.
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If you're talking about the oldest examples of glass then you most certainly can see the difference in thickness by eye. These are also the examples where studies have shown that a very significant majority are placed with the thick end down. Your objections are not based on fact but on your refusal to accept the facts.
Mike
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There is also no evidence that glass DOES flow. Can you point out one example of glass changing over time? Corning Museum of Glass has this to say:
http://www.cmog.org/index.asp?pageIdt5
Of course, if you have evidence, then send it to them. I'm sure they would appreciate it.
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