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How flat is glass?

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  • How flat is glass?

    Anybody know how flat typical float glass is? It sure looks flat, but the way it's manufactured, molten glass rolled out and floating on a bed of molten tin, doesn't sound like a good way to make something flat. Has anyone here with a granite slab ever checked? Thanks!

  • #2
    I think how flat it is depends a lot on how flat whatever it is resting on is.

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    • #3
      It isn't flat as machine precision flat, a.k.a surface plates. It can vary anywhere from 20 to 200 um in total.

      So no, it isn't workable as a surface plate or for lapping, if the goal is flatness.
      Amount of experience is in direct proportion to the value of broken equipment.

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      • #4
        Originally posted by Jaakko Fagerlund View Post
        It isn't flat as machine precision flat, a.k.a surface plates. It can vary anywhere from 20 to 200 um in total.
        microns, so high tech. Now I get my slide rule to find out what that means. 0.0008" to 0.008" (?!) over what distance? I don't believe it. That would cause horrible visual distortion, no?

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        • #5
          Originally posted by superUnknown View Post
          .......molten glass rolled out and floating on a bed of molten tin, doesn't sound like a good way to make something flat. ........!
          Can't get much flatter than the surface of a liquid!
          "Patriotism is the last refuge of a scoundrel"

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          • #6
            Originally posted by jep24601 View Post
            Can't get much flatter than the surface of a liquid!
            Yes you can, by scraping. Liquid has a curvature radius of roughly 6500 km, which isn't flat as we know it. For very short distances the flatness is quite good for that liquid, but over even a meter it starts to add up.

            Not to mention that the glass plate is kind of continuous cast on that liquid, so it will have distortions, warps and ripples on it. After all, they are meant as window panels et., not as a flatness reference.

            If you want something flat, scrape your own reference or buy a surface plate.
            Amount of experience is in direct proportion to the value of broken equipment.

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            • #7
              Originally posted by superUnknown View Post
              microns, so high tech. Now I get my slide rule to find out what that means. 0.0008" to 0.008" (?!) over what distance? I don't believe it. That would cause horrible visual distortion, no?
              Look at a car windshield. It isn't flat and doesn't give distortions.
              Amount of experience is in direct proportion to the value of broken equipment.

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              • #8
                Windshield might be curved but will be an even thickness throughout for the most part- that means no local thickness variations which would cause visual deviations. Most glass these days is pretty good, at least from what I've seen. I've used the visual check on glass and some tiles- usually granite as that is what I've been interested in using- and find that you can see distorions quite easily using the grazing incidence viewpoint. That's looking across it from a very shallow angle towards light sources and straight things. Doesn't matter how straight really, as the eye will pick up the differences between looking directly at it and seeing the reflections in the glass. This obviously isn't a lab quality check, but for anything I've looked at, then subsequently checked by laying it across a surface plate, it has given me a way to separate the good from the bad so to speak. If I eyeball it in the store and don't see distortions, then I'm also unable to find them using the surface plate.

                Glass is kind of interesting as you can get a bit of an indication when you use paper shims between the glass and the surface plate. With care you can see the degree of contact by looking through the glass at the shim. If you wrinkle the paper a bit, you can see the degree to which the wrinkles disappear as the contact pressure gets higher. I have found also that even the thinnest paper, which I find to be zig zag whites, is too thick to indicate a good flatness between the plate and the thing being tested- after all the plate is flat to what- .0001 corner to corner- and the paper shim is only going to be good for .001, and only for the size of the thing being tested.

                Glass is thin compared to a surface plate, and will flex measurably under its own weight. If you're using it as a surface plate- well let's just say that a cheap surface plate is going to be lots better than the piece of glass.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Originally posted by jep24601 View Post
                  Can't get much flatter than the surface of a liquid!
                  That may be true but as the work piece and surface gauge are sinking they do tend to cause ripping that upsets the flatness.

                  Lots of people in the "old" days, when you couldn't get a cheap surface plate, have used a thick piece of glass (bedded in a support materiel) as a surface plate substitute. Good enough for most home shop applications but hardly worth it anymore.
                  The shortest distance between two points is a circle of infinite diameter.

                  Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                  Southwestern Ontario. Canada

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                  • #10
                    It depends on how flat you want a surface to be , quite a few would be happy with the results from a piece of 10mm glass which is supported by a polished granite tile .

                    Getting a perfect surface is often not really required ,eg would 200um be something that can be easily measured with field instruments on a daily basis?

                    0001mm is hard to measure at times on machinery .

                    All I am saying is "Is the degree of precision necessary?"

                    Now where did I put the fire extinguisher?

                    Michael

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                    • #11
                      I've used a piece of mdf as a flat at times. Yes, it depends on the application. Currently I have a piece of mdf as a backup for a granite tile on my belt sander project. The mdf has been laminated both sides with formica- now how flat is the end result of that - The granite tile doesn't rock on my surface plate, and it is showing that my prepped mdf is close, but not flat. I'm prepared to shim it anyway, and I expected to need to. A piece of glass would have been at least good enough to suit in this instance.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                      • #12
                        Thanks for your help, Jaakko and Darryl. So concesus is, glass with steel backing ok for sharpening chisels, no good for lapping? Now to find a cheap surface plate.

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                        • #13
                          In talking about the flatness of glass, you need to look at both short term (distance) flatness and longer term (distance) flatness. Almost any glass, even 100 year old window glass, will have a short term flatness (over a mm or two) that is really very good. On the other hand, it could be off by over 1/10 mm difference in several hundred mm distance.

                          If you are talking about apparent distortion when viewing things through it at a normal (perpendicular) angle, glass does not have to be very flat for that. The most important characteristic for distortion free viewing through a window is that the glass be fairly uniform in thickness and not excessively un-flat. But the lack of distortion in such viewing does not directly translate to flatness.

                          If you are talking about comparing it to a surface plate, then you are going to have to take the lack of stiffness of normal thicknesses of glass into account. A normal window pane can easily be distorted by several thousandths by just placing it on an uneven surface or by resting a weight on it. So glass that is only fractions of an inch thick can not be expected to perform like a surface plate that is several inches thick or that has ribs that are of that order. As loose nut says, it would have to be embedded in a supporting medium, like plaster that is allowed to dry after the glass is impressed into it's wet surface. But even then, a lot of consideration would have to be given to how that is done. A fraction of an inch of plaster thickness under the glass would be less stiff than the glass itself so it would have to be several inches thick and then you may have stresses set in while it dries. Perhaps a thin layer of plaster between the glass and a thick concrete or granite block. But, surface plates are dirt cheap so this is a lot of work for little savings.

                          If you tried to "print" a piece of glass on a surface plate, the glass, being relatively thin, would confirm to the plate over longer distances (several inches) and the test would be worthless. Yes, you could see any short term variations, but they probably do not exist. In fact, short term (short distance), the glass is probably better than the surface plate.
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

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                          • #14
                            A 3/4 or 1" piece of plate glass would be better than nothing, but even the worst chinee granite surface plate with its certification/map of dubious merit would be better than a piece of plate glass.

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                            • #15
                              i don't care how much the yokels may gripe: glass is the absolute worst lapping surface for precision metalworking. Glass abrades many times faster than any metal surface you lap against it. Since a lapped surface is a negative of the the lap any dish abraded into the glass thanslates to a convex in the work. Have I made my objections clear and my arguements cogent, objective, and experimentally repeatable. OK then. (Huff! Puff! Mutter, mutter)

                              As was earlier stated, thin glass sags under load to conform to its supporting surface. Yes glass is among the stiffer common materials but it will elastically deform under load. Window glass more then plate glass, plate glass more than entry door glass etc. The thicker in proportion to width and lenght the better. A piece 18" x 24" 3" thick will challenge granite in the stiffness department.

                              More later, gotta go. Heavy date. With an actual girl.
                              Last edited by Forrest Addy; 03-17-2014, 10:59 AM.

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