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BetterSense
30-Apr-2016, 20:40
I have 400, 600, 800 grit SiC grinding powder. What is best?

Alan9940
30-Apr-2016, 20:54
I use the 400 and 600.

BetterSense
1-May-2016, 12:04
I ground one last night with the 400 and it looks beautiful. I think the only way to get my answer will be grind another with 800 and compare them on the camera. But I am debating whether to just use 800 for the next one, or start with 400 and finish with 800.

Peter De Smidt
1-May-2016, 12:51
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.

Alan9940
1-May-2016, 16:12
+1 what Peter said. Yes, you can start with a very fine grit, but ya better grab a pot of coffee and settle in because it's gonna take ya awhile! :)

goamules
1-May-2016, 16:51
I don't know how the grinding grit equates, but I always just use the medium Aluminum oxide grit size, .0002. I don't try to polish the glass flat, I don't care if it's not flat and don't note any patterns when I start from clear and go to .0002 directly. I do notice that the Alum Oxide makes very fine screens, where as a lot of home made ones with the carbide grinding compound seem to have huge micro-divits in the glass....that look like a bunch of clear craters, under a loupe. With the carbide, chips of glass flake out like what happens to flint when making an arrowhead.

Here is the reference I learned from, I believe: http://www.dokasphotos.com/techniques/ground_glass/

Peter De Smidt
1-May-2016, 17:41
I use 3 micron aluminum oxide for the final finish. If the glass is not flat, then the granularity of the screen will not be even. For instance, I recently started grinding a 4x5 screen using aluminum oxide. After 1/2 hour the edges were done, but there was a clear strip down the middle do to the glass not being flat. Grinding recommenced until the glass was evenly ground.

Tim Meisburger
1-May-2016, 17:41
Interesting article Garrett. The ground glasses I have made recently using the 300 and 600 are quite coarse under the loupe. Seems I need to switch to aluminum oxide. Where did you buy it?

Peter De Smidt
1-May-2016, 17:42
Lortone.

Steve Goldstein
2-May-2016, 03:28
This place sells a variety of grinding compounds:

http://www.willbell.com/atmsupplies/atm_supplies.htm

pdh
2-May-2016, 03:40
An excellent "How To ..." http://www.apug.org/forum/index.php?resources/making-a-ground-glass-focus-screen.54/

BetterSense
2-May-2016, 05:43
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
2-May-2016, 06:00
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
2-May-2016, 06:28
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
2-May-2016, 08:02
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
2-May-2016, 10:47
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
2-May-2016, 19:51
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
4-May-2016, 19:28
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
4-May-2016, 20:18
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
5-May-2016, 09:40
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.

barnacle
5-May-2016, 12:23
You are reading Neal Stephenson upside down!

Definitely a finer grind on the right, though.

Neil

goamules
5-May-2016, 16:50
And if you use Alum Oxide, it would look even better. I have a post somewhere that where I learned to just start with the .0002 Alum Oxide, and that's all you need. It makes a finer screen, with no divots.

Peter De Smidt
5-May-2016, 16:52
I agree with Garrett.

Tim Meisburger
5-May-2016, 18:09
Okay. I still have not found .0002 aluminum oxide. Is this what I am looking for? https://www.willbell.com/ShoppingCart/ShoppingCart/?id=4sat0smdgori4scbr0sbyoud&continueShoppingUrl=http://www.willbell.com/ATMSupplies/ATM_Supplies.htm

Peter De Smidt
5-May-2016, 18:20
It should be fine. I use this: #591-021 at http://www.lortone.com/abrasives_polish.html

BetterSense
5-May-2016, 19:12
I have heard that finer grinds reduce graininess, but also decrease brightness and increase apparent fall-off, but I haven't seen any disadvantage to the finer abrasives yet.

If I can find them I will try installing the cheap page magnifier fresnels I bought too.

Randy Moe
13-May-2016, 09:14
And if you use Alum Oxide, it would look even better. I have a post somewhere that where I learned to just start with the .0002 Alum Oxide, and that's all you need. It makes a finer screen, with no divots.

Ok, I eat Crow again! Tasty.

I made an 11x14 GG last night using Garrett's way and used 3 micron aluminum oxide that Peter gave me. One step on Home Depot framing glass. Then I held it up next to my 2 year old 11x14 DIY GG on the same glass, but made with 2 steps of 320 then 500. !!!! I need a way to test this. I don't 'see' a huge difference, but I was tired late last night with bad light.

Both methods took about the same time, One movie. So Garrett is correct that one step with the finest, which is darn fine, like soft mud, is all that is needed.

This chart is handy. http://stellafane.org/tm/atm/mirror-refs/grit.html#Grit%20Size%20Table


So I propose using 51 USAF target under good light in a dark room, interchange the GG, take a look see and take spot meter readings.

Any better suggestions for testing and comparing GG?



Scratching my fingernail across both felt very similar. Too similar!

Peter De Smidt
13-May-2016, 10:05
Did you use a loupe, and, if so, what power?

Randy Moe
13-May-2016, 10:22
Did you use a loupe, and, if so, what power?

Not yet, that is for tonight during the rain.

As soon as I finish breakfast, I am out into the Sun and shooting 35mm.

Randy Moe
14-May-2016, 13:46
First is 2 step 320-500 grit. Second is one step 3 Micron. Second is one stop brighter as measured by Sekonic spot meter.

Ipod shots of 11X14 GG of USAF '51 at 1 to 1 with 10X loupe focus of Nikkor 610mm at f22. Unfortunately the flash fired and made both exposures the same. I am not redoing it. Check exif and notice flash off glass.

I prefer the new finer 3 micron grind. :)

https://farm8.staticflickr.com/7760/26980293556_194b44b773_c.jpg (https://flic.kr/p/H7a4i7)GG (https://flic.kr/p/H7a4i7) by moe.randy (https://www.flickr.com/photos/tincancollege/), on Flickr https://farm8.staticflickr.com/7575/26945425581_884c2a29e6_c.jpg (https://flic.kr/p/H45mg8)Untitled (https://flic.kr/p/H45mg8) by moe.randy (https://www.flickr.com/photos/tincancollege/), on Flickr

Peter De Smidt
14-May-2016, 15:41
Good stuff, Randy! When I get some time, I'll make some comparisons of a 3 micron aluminum oxide glass, a Sinar glass, a Maxwell screen, and a Beattie Intenscreen +.

Fr. Mark
30-Aug-2016, 13:49
Reviewing some older DIY threads. You guys amaze me. I tend to think of float glass as a flatness standard and here y'all
Are talking about flattening it!

Two possibly dumb ideas:
Sandpaper will grind glass too. I used it because it was available. To make grid lines, draw them on with a pencil and a straight edge.

Scribing grid lines with a needle sounds to me like an invitation for the glass to snap on those lines.

A couple more dumb ideas: don't use glass, use acrylic (it scratches easily!), or waxed or oiled paper (durability issues) or a matte or flat varnish on glass etc.

Tim Meisburger
5-Oct-2016, 11:16
Okay. I finally got around t ordering this. I got the one Peter recommended (called pre-polishing), and after I made the order the invoice showed it as 3 micron powder (.0001) rather than 5. I hope it is not too fine!

Unfortunately, I am off on a work trip Friday, so will not have a chance to try it till I return in early November.