Making ground glass

[BetterSense]

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?

[3e8]

Check out this tutorial (http://www.dokasphotos.com/techniques/ground_glass/), 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.

[Christopher Walrath]

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.

[BetterSense]

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.

[Ian Grant]

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

[AgX]

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.

[steven_e007]

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...

[Colin Graham]

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 (http://gotgrit.com/index.php/cPath/2_7?osCsid=6420912c80aba0fcde85d74e4a740b68)

[unclemack]

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.

[BetterSense]

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.

[Jim Jones]

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

[BetterSense]

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.