Plate glass as a surface plate?



You get a glass rod clamped horizontally and put jump leads from a car battery at opposite ends. You heat the middle of the bar with a couple of bunsen burners which make the glass conductive before it melts. The current then flows and heats the glass until it melts. Very spectacular when I saw it.

They have a bit of a thing for Billy Bass. They also show a pot of thermite on a table above a butane tank. Naturally there are lots of sparks, molten iron and kaboom.
It's worth looking out for.
Chris
--
Chris Eilbeck

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Sounds exciting, I may have a go at that. Would a gas ring do instead of a bunsen burner dyt ?

I'll keep an eye out but the 18 price I've seen quoted on the web is a bit dear for such a thing.
Cheers,
-- Boo
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Bugger, you mean you don't need a mains lamp for the demo?
Not seen the DVD though -
Steve
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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.
Steve R.
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On Sat, 24 Feb 2007 17:52:49 -0000, "Dave Croft"

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.
Jim
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Good tips Jim, thanks
Steve
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Chris Eilbeck wrote:

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.
Nick
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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.
Steve R.
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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!
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I wouldn't have trouble using it. Depending on your other interests, you might have a table saw or jointer with a flat surface that you can press into service.
Wes
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snipped-for-privacy@lycos.com writes:

Unfortunately I don't have anything else I can use - too many hobbies, too much stuff, too little space. As it stands, I have to move the bikes out of the shed before I can work at the bench.
Chris
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I recall reading the comments of John Ruskin 'When we build, let us think that we build forever'
I can't recall his deliberations on fishtanks but he did write by a far more extensive and expansive stretch of water- and I suspect- imagination!
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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
Les
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Excellent. Thanks. The fishtank may well go off to be new bottles ;o)
Chris
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Chris Eilbeck wrote:

Asda do them as well - I just bought myself one - 12 quid for about 24x18 inches.
I used to use a 12" square piece of marble fire surround, 'til I dropped it moving sheds.
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Avondale Audio wrote:

Ahem. At the risk of being monumentally off topic, can I ask a question about the Sugden Au51-P ?
BugBear
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be
a
No problem if the group will indulge......ask away
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Avondale Audio wrote:

It's horribly specific, I'm afraid.
My local shop has replaced the output relays for the 3rd time; they last around 5 years.
But now they say that Sugden have no more suitable spec relays, which means around 5 years from now, my (ahem) quite expensive (and much liked) amplifier will become scrap.
Unless (of course) a different relay can be used, or the amp modified to not have a relay.
Any advice?
BugBear
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I am no expert on such things, so please take this as a suggestion for further research! However, AIUI the output relays on power amplifiers are included to protect the loudspeakers from switching transients - they delay connecting the output until the amplifier has stabilised. you could probably achieve the same protection by turning the volume control to zero while switching on, then turning up to listening volume after a few seconds (having, of course, by-passed the relay when it has died).
A little observation with an oscilloscope would probably show you what was going on.
I'm sure a better solution would be to find a suitable replacement relay, but I can't help with a suggested alternative. In fact, it is probably not a simple relay, but must incorporate a delay switch (thermal?).
Best of luck!
David
--
David Littlewood

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wrote:

We use a lot of relays, both PCB and power stuff, and also small DC contactors.
If the original relay type can be determined, and thus the ratings, it should be possible to find either a relay or a small contactor do do the job.
A lot of small-volume relays went out of the market when Schrack and Tyco were buying up the industry, but there are a lot of options still available and it sounds as though the originals may possibly have been too small anyway.
Get some details and let's have a look at it.
Peter -- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk http://www.prepair.co.uk
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