Is a piece of plate glass any use as a surface plate? It looks to be about 8mm thick. I know someone who's getting rid of a fish tank and realised I could do with something a bit flatter than my workbench surface but not necessarily the last word in accuracy.
Plate glass is fine if put into a decent wooden support to prevent damage and provide some restraint against accidents. I have two, a small one that is bench top friendly and is large enough to accommodate a height gauge and a small job, and a larger one also bench useable but it needs the whole bench .... which means a tidy up! Don't expect surface plate accuracy but for most work you will not notice a difference.
Hi Chris, I presume it is float glass that you are to use. I was working as a telephone engineer in St Helens in the late 1950's when plate glass was still being ground. Large piles of ground glass powder & abrasive were everywhere. At the same time I sometimes went to fix a phone in the "new department" I had to wait while canvas screens were put round the work equipment so I couldn't tip off the opposition about what they were trying. It was the start of making float glass.
I woudl assume so. Originally it came from a shop window and has already been recycled into a fishtank once. I'm tempted to have a go at making a telescope using another bit of it, laminating several pieces together and building a grinding rig.
Excellent! I've had the "hey, that looks cool, what you doing?", "don't ask and I won't have to lie to you!" conversation many times at work.
Before using that, I would support 3 corners of the glass and press down the forth. Not to forget the dial-indicator on that forth edge. I guess I would give up that idea, because it is not stable enough. My granite plate (350 x
350mm) is about 80mm thick. Granite plates aren't that expensive anymore. If you want to make serious and reliable work, it's the only way to go. IMHO.
If supported properly, true plate glass can come close to conventional surface plate accuracy. This is because ofthe tightly controlled grinding and polishing process.
Unfortunately it's pretty rare now having been almost completely replaced by float glass. Float glass is made by allowing a continuously flowing layer of glass to solidify on a bed of molten metal. As it solidifies it is drawn off while still semi solid and allowed to cool passing through a series of rollers. The float process ensures extremely uniform thickness but the flatness (i.e. curvature) is not as well controlled as it depends on the precise alignment of the cooling roller train.
With constant thickness, minor curvature doesn't matter for window glass because it doesn't distort the image. The same is not true for mirrors. Old mirrors are either true plate glass or the highest grade of float glass so large mirrors are a safer source when flatness is important.
A better bet is granite floor tiles. These are finished by a similar process to plate glass although not so tightly controlled. I have a slightly damaged 12"x12" x 10mm floor tile bought from Wickes(£2).
Supported on a piece of carpet and checked with a grade A straight edge the polished side is uniformly slightly concave.
0.0005" E300 video tape can be pulled through but an 0.001" feeler will not enter.
There's no guarantee that all tiles will be of this quality but the polished surface makes it easy to make an initial check. The eye is pretty sensitive to image distortion - view a distant sharp edged object by approx 45 deg reflection. There should be NO image change as the tile is tilted to move the reflection point over the area of the tile.
When I was in Technical school in the 1950s, the machine shop had a glass surface plate. It was in a shallow wooden box that provided support around the edges. The frame was filled with putty to provide further support. The plate glass was 1/2 inch thick, and had been ground against two others to produce a reasonable flat. It was left with a frosted finish, which was accurate enough for our purposes.
Well, the old 72 inch mirror at the Dominion Astrophysical Observatory in Canada has retained the accuracy of it's optical surface since 1918! It was replaced with a ceramic mirror about 25 years ago. The old glass mirror is currently on display, but is available as a spare. The new one is re-aluminised at least once a year, so there is always the risk of damage, as it is removed and lowered to the ground floor. The cleaning tray, and vacuum chamber are on that level.
Gentlemen, I am wise enough to have float glass covering a painting of the first steam loco going across the Border Counties Bridge at Berwick on Tweed. I am old enough to have old mirror glass dating back at least 150 years and also to have lived next door almost to a glassworks on the Tyne and whatever. With somewhat rheumy eyes I can still determine reasonably flat and the relationship between 8 mm fish tank and 12mm ground glass in a workshop's reasonably controlled environment. I made my somewhat ascerbic comments echoing the late Professor C E M Joad's quip of 'It all depends on what you mean by (?)'
Somewhat less seriously, there is a better solution!
Whether it is a myth or not seems to depend upon your definition of a liquid. If you define liquid by something that flows then glass basically doesn't flow appreciably at room temperature. If you define liquid as a non-crystalline condensed state of matter then glass conforms to this.
It doesn't really matter what you call it, there's no credible evidence (in the sense of scientific peer review) that it flows. I've never seen any despite looking quite extensively and no-one has presented any when I've talked about this before.
Tesco and I believe Argos, both offer ground granite slabs as chopping boards. They're some 300 X 200 X 25 and to a novice like me, appears to be well up to the task of behaving like a surface plate. I've sited one on a rubber mat on top of the bench and it suits my purposes well