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
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
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
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.
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.
On Sun, 2 Oct 2005 17:54:37 -0700, with neither quill nor qualm, "Mark Morin" quickly quoth:
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.
Modern float glass isn't designed with a "flatness" tolerance per se, the flatness is really just a result of the way it is manufactured. For window glass the .02 discrepancy would certainly be acceptable. :)
If you're on good terms with the glass shop maybe the will let you swap it for another piece. Bring you straight edge with you this time. Or, you could buy a granite surface plate, the ARE manufactured to a specified tolerance.
Thats an old wive's tale. Glass is amorphous, but certainly not liquid.
I think you just looked in the wrong place. I scratched a microscope lens 35 years ago. It is still scratched exactly as it was then.
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?
it doesn't heal.
it doesn't take a set. it's pretty bendy. i have some 3/4" that bends over a
4' span. when i fuse it together to make a 1.5" thick piece, it doesn't bend very much over a short span, but does over a much longer one.if you want it to be flat, you have to put it on something that is flat already.
regards, charlie
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.
Please read this:
and of course, all those pieces of cast glass that the egyptians carelesslly just left lying around are simply puddles by now.
i don't know where you got the above ideas, but you might want to recheck them.
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.
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
On Mon, 3 Oct 2005 10:27:16 -0700, with neither quill nor qualm, "Charles Spitzer" quickly quoth:
That's what I used to think, but the window in my door did. I finally got some Oops! and removed the remainder of the label. The scratch was plainly there after all the adhesive, etc. were gone. Months later the scratch wasn't there any more.
Ohhhh, maybe someone came up to my house, broke in, replaced the glass, and sneaked back out, locking the door behind them.
Yeah, I suppose it could happen. What are the odds, C? ;)
You'd think he'd have straightedged the table he layed it on, wouldn't you, before determining that only the glass was curved?
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
An incredibly stupid comment, for a great number of reasons. Do you know anything about Egyptian glass making?
I'm sure you don't because you are too lazy to read.
Since they could see the difference by eye, why on earth would they bother with a micrometer?
Mike
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