Making ground glass

Discussion in 'Camera Building, Repairs & Modification' started by BetterSense, Mar 10, 2010.

  1. BetterSense

    BetterSense Member

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    My pacemaker speed graphic's ground glass is old and grungy, and at some point something got between the glass and the fresnel, leaving splotches. I'm thinking of making a new ground glass out of window glass, which should only cost a couple of dollars if I could find cheap 10 micron grinding powder. Another option would be etching with HF, which I work with at my job. Does anyone know how commercial ground glasses are made, whether by etching or grinding and what kind of grinding compounds or acid formulas are used?
     
  2. 3e8

    3e8 Member

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    Check out this tutorial, which reccomends using aluminum oxide, down to 3 microns, which gives a better grind. According to the author, with carbide powders, the chips of glass are visible with a 22X loupe. The aluminum oxide gave a finer, brighter screen.
     
  3. Christopher Walrath

    Christopher Walrath Member

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    You could even use valve lapping compound available in most auto parts stores. Or most other very mild abrasive sources. The finer, the better view.
     
  4. BetterSense

    BetterSense Member

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    I looked at valve lapping compound at Autozone, but it didn't say the grit anywhere, and the datasheet available online didn't either. I think tomorrow I will take some glass into the lab and put it in some HF.
     
  5. Ian Grant

    Ian Grant Subscriber

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    The fine valve grinding paste is OK quite quick to use, 10 minutes maximum to grind a 5x4 screen, but it's still a bit to coarse.

    Find a Lapidary suppliers and get the #400 and #600 Silicone Carbide grits, they aren't expensive as a small amount goes a long way. Search Google there appear to be some suppliers in Dallas.

    Ian
     
  6. AgX

    AgX Member

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    The coarser the grinding is, the brighter is the image when viewed axial to the entering ray, but the more you need a appropriate fresnel lens to counter light fall-off with static viewing.
     
  7. steven_e007

    steven_e007 Member

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    I've made a lot of focussing screens using valve lapping compound.

    You need two sheets of glass - one is the griding 'table' to work on - but this isn't wasted. I made about half a dozen 5 X 4" screens using a 10 X 8" sheet of glass as the grinding table - and ended up with a nice 10 X 8 screen as a by product. They only take about 10mins each- but the secret is to use just a little paste and dilute it with something - I used alcohol. Keep wiping it clean and replacing it - otherwise the particles of glass get a bit crunchy and the grit loses its sharpness.

    The screens were a little coarse, but plenty good enough for my needs - but I'm sure the methods using the fine powder would be much better, but obviously cost more and take longer...
     
  8. Colin Graham

    Colin Graham Member

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    You can also use a piece of marble for the work surface, an ordinary tile works well. I dismantled a bad zoom and used the biggest element with a flat surface as the grinding tool.

    I've only tried aluminum oxide, 25 through to 9 micron makes a nice compromise between detail and brightness. Any finer and you start to get hot spots. I did try lapping compound once, very coarse with lots of pits when viewed under a loupe.

    A great supplier here of all manner of ATM stuff> http://gotgrit.com/index.php/cPath/2_7?osCsid=6420912c80aba0fcde85d74e4a740b68
     
  9. unclemack

    unclemack Member

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    I bought small amounts of 25, 15, 9 & 5 micron aluminium oxide for a total of £9 a while back on ebay. Enough for maybe 50 GG's - you use a tiny amount for each.
    It would surprise me if valve grinding paste is graded to anything like the standard of lapidary supplies. A single oversized grit could ruin the surface of your new GG of course.
     
  10. BetterSense

    BetterSense Member

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    Thanks for that gotgrit website. I think I will buy some 15 micron aluminum oxide.

    Since my fresnel is ruined, I had figured I would set my camera up to work without the fresnel, and if I ever wanted to add a fresnel later, I would put it on the back, outside surface of the ground glass.
     
  11. Jim Jones

    Jim Jones Subscriber

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    Caution: when fresnels are mounted between the GG and the lens, removing them induces a focusing error.
     
  12. BetterSense

    BetterSense Member

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    I understand that, so i will have to shim my plain GG when I get it finished. I don't think putting the fresnel behind the ground glass will affect the focus, but I don't quit understand how a fresnel behind the ground glass is supposed to help anything, since the light should already be scattered at that point.