what grit for ground glass

[pdh]

An excellent "How To ..." http://www.apug.org/forum/index.php?...cus-screen.54/

[BetterSense]

I decided to do the following test:

I ground 2 glasses with 400 grit last night. I will grind one of them with 800 grit then use my D70 on manual mode to compare the brightness of the two screens.

[chris_4622]

I've ground different sizes of glass, from 5x7 to 16 x 20. I start with 25 micron, then 12, then 5. Sometimes I go down to 3 microns but haven't done any testing to see the difference between the 5 and 3.

Using this method is faster than when I tried to use the 5 as a starting point with a much more consistent grind over the entire surface.

[Jim Jones]

Most glass isn't flat, and so the first step is to flatten it. It's flat when there's an even scratch pattern across the screen. Rinse all the grit off of the grinding surfaces, and continue with finer grits until you get the level of diffusion/granularity that you want. You can start with the finest grit right away, but it'll take much longer to flatten the glass.

For most of us, a ground glass doesn't have to be perfectly flat. Any departure from perfect flatness in most glass blanks is a small fraction of the depth of field. A small tool may grind a slightly wavy blank with finer grits from the beginning.

[Peter De Smidt]

Hi Jim, I'm not worried about depth of field but evenness of granularity/diffusion. Maybe my last batch of glass was wavier than most.

[Dan O'Farrell]

Quite a few years ago, I made an 8X10 replacement glass for my Cambo. I used a good-sized (12"X12"?) piece of flat marble as my base, and spread the grit slurry over its surface.

The glass I used was regular window glass cut to size, with clipped corners. I don't recall which combination of grits were used.

I used suction cups (lever-action towel hangers) to grip the glass, and within an hour, had a beautiful (to me) groundglass, which served well until I received
a Yankee replacement.

It also did a nice job of polishing the marble.

All I missed were the gridmarks on the original ( and on the Yankee replacement); the focussing brightness and accuracy were excellent.

[Jim Jones]

One way to make neat unobtrusive grids on a ground side of a ground glass is to scribe them with a hard fairly sharp tool. A sewing needle, old style phonograph needle, or ice pick might work. The pattern for the lines is visible through the GG. Use a straight edge to guide the tool.

[BetterSense]

I ground one 400, one 600, and one with 400-then-800.

I conclude that 400 is too coarse. The two finished with finer grits are hard to distinguish. I further conclude that on small pieces of glass, starting with 400 is a waste of time vs. just jumping to 800 or 600.

Another aspect is my 400 I bought from The Rock Barrell in Dallas and it's black. The 600 and 800, although also supposedly SiC, is from a different vendor, green, and probably top quality.

[Peter De Smidt]

The source matters. I have some 1 micron aluminum oxide, which is finer than my usual grit, but it must be contaminated, as any screen made with it gets huge scratches.

[BetterSense]

Here is a picture I took with my cellphone through my loupe of the two glasses butted up against each other on the back of my camera. You can see the finer one on the right is both brighter and less grainy.