Hi Mark
Black sandpaper, is carbarundum and will work as an accessible alternative.
Body shops tend to be a cheap source. For health reasons its probably better to
work wet. A source of grit for grinding might also be a rock shop, who might
have it in stock for rock polishing.
All the best
Laurence Cuffe
Sent from QCloud
On Jul 13, 2015, at 09:00 PM, mark@xxxxxxxxxxxxxxxxxx wrote:
IF I understand this correctly, the frosted or front face of the ground glass
must be .197 from the wood on the back +/- .007 of an inch? Think I do have a
depth micrometer around here somewhere.
I also greatly appreciate the offer of a ground glass, and I very well may take
you up on it after I experiment a bit here.
I don't look at this as misery or a time waste, even if it fails. Though at
times it is frustrating, I am also learning a great deal and the education is
worth the time.
-------- Original Message --------
Subject: [pure-silver] Re: making ground glass
From: `Richard Knoppow <dickburk@xxxxxxxxxxxxx>
Date: Mon, July 13, 2015 12:51 pm
To: pure-silver@xxxxxxxxxxxxx
But, make sure the thickness is right. Most window glass is pretty thick, say 1/4 inch where camera glass is about half that. The thickness is critical only in that it must fit into the back.
In most camera backs the location of the ground surface, and hence the focal plane, is determined by the back, not the glass. The reference surface is the one the ground surface of the glass is held against. The construction of the back is such that the ground surface is automatically in the same location as the film. Here is a list of the ANSI film plane positions:
ANSI Standards for film plane placement in sheet film cameras.
Size Location Tolerance + or -
4x5 0.197 0.007
5x7 0.228 0.010
8x10 0.260 0.016
Dimensions in inches.
Film thickness is 0.007 The above does not include
film thickness.
Smaller formats than 4x5 are the same as for 4x5.
If the back is removable from the camera its easy to confirm the ground glass plane is the same as the film plane. You need a holder that you know is good. Load a sheet of film in the holder. Use a depth micrometer to measure the ground glass position in reference to any convenient surface. I lay a piece of thick sheet metal across the back, a piece of thick glass will do as well. Now put the holder in the back with the dark slide removed and measure again, the two should be the same. Note that in many holders the film is not actually pushed against the plenum. You should check your film holders the same way only lay the reference surface against the sides of the holder. I check near the corners and at the center. I've found that many older wooden holders are warped and some brands of plastic holders are even more warped. Generally the best are the Riteway holders up to 4x5 (they may have also made 5x7) and for larger holders the ones made by Graflex or Kodak (same holders). Check both sides since the plenums may have moved from the center.
On 7/13/2015 9:45 AM, Myron Gochnauer wrote:
By far the simplest thing is to go to a place that sells glass and ask them to
cut you a few pieces to the exact size you need.
There need be nothing special about the glass. Ordinary window glass will work
fine with a large format camera. You’re looking through the glass to see the
side that is ground (and illuminated by the image from the lens). Almost any
piece of commercially available glass will be clear enough and flat enough for
this purpose. The slight greenish tinge of inexpensive glass is unlikely to
matter at all since you use the GG only for composing and focussing.
My local glass supplier usually smooths off the edges and corners for me, too,
which makes handling very nice. (They use a large wet-belt sander.)
Myron
On Jul 13, 2015, at 1:23 PM, mark@xxxxxxxxxxxxxxxxxx wrote:
What I have seems to be working, but it is a mess. Locally I have no idea
where I could get the silicon carbide. It would have to be ordered, but that
is not total road block.
My biggest potential problem I think is going to be cutting the glass to fit.
The glass I am using is left over from another project, and truthfully I don't
remember what kind of glass I used. I have never been able to get any kind of
straight cut with the scoring method. It never breaks right for me. It looks
easy and I am sure its something to do with the way I apply pressure to break
it. Really doesn't matter. What ever the reason I stink at that.
If it was some form of laminated glass cutting with the right blade and a
jigsaw should be no problem if I take my time. Frame glass and I think it
might work. Might crack, but maybe not. Depends on how much temper was in the
glass. IF its tempered glass its going to be a mess. Gloves and eye
protection for sure and maybe the shop apron too. Will find out fairly
quickly. IF its going to shatter, it will shatter fast, and come to think of
it I may have some glass from an old 4x5 frame laying around.
Appreciate all the help
-------- Original Message --------
Subject: [pure-silver] making ground glass
From: Myron Gochnauer <goch@xxxxxx>
Date: Mon, July 13, 2015 7:46 am
To: Pure Silver group <pure-silver@xxxxxxxxxxxxx>
. . . I went to auto parts store and bought a small tube of valve grinding
compound. That's slow, but it is working.
It is vastly easier if you buy some straight silicon carbide. I don’t know of
any local source for most people, but if you are going to make ground glass in
the future, you should order some from a telescope-making supply. Here’s one
that might do: http://www.willbell.com/ATMSupplies/ATM_Supplies.htm It’s
located in Richmond, Va.
As you can see at that website, silicon carbide is pretty cheap. And it never
goes bad! I’m still using left-overs from 1962.
I can’t recall what grade I use for ground-glass. I *think* 220 is okay. I’m
pretty sure 80 was too coarse for my tastes, and 400 or 500 was definitely too
fine. You don’t need much, either. 4 oz. will make a lot of ground glass.
The other thing you need is a “grinding tool”. For 4x5 to 8x10 ground glass, I
use a piece of glass that is roughly 4 inches square and a half inch thick. The
exact dimensions don’t matter. It just has to be big enough to grip, and rigid
enough to transmit even pressure. You can probably make something out of a
piece of plate glass glued to a block of wood.
To grind the glass:
1) Place several thicknesses of newspaper on a sturdy work table. (Telescope
makers often use an upside down barrel so they can walk around it.)
2) Place the glass-to-be-ground on top of the newspapers. The newspapers will
cushion and “grip” it slightly.
3) Sprinkle a little carbide onto the glass. Keep it in an area a bit smaller
than your grinding tool. You’ll quickly learn how much to use.
4) Sprinkle a little water onto the carbide and place the tool on top. Move
the tool around gently to wet all of the carbide (you’ll feel it), and then
grind back and forth at various angles using moderate pressure. There’s
nothing magic about these motions. Just keep them fairly random and short
enough that you don’t constantly scrape off the grinding compound with the edge
of the tool. With a 4 or 5 inch tool I think I use a back-and-forth stroke of
something like 3 inches total.
5) You may need to add a bit of water from time to time, and in a short space
of time you will feel and hear the grit disappearing (it breaks down). When it
seems like you’re not really *grinding* any longer, rinse off the glass and
tool under running water and start over again with a fresh charge of silicon
carbide and water.
6) Within two or three cycles you should start seeing the glass beginning to
look frosted. Check as you go along to see where it needs more work, and
continue until it is ground and frosted uniformly.
7) If you later decide the surface is too fine or too coarse, you can regrind
it with a different grade, although it will be a little more difficult seeing
how your work is progressing.
The main difference between this procedure and one using automobile value
grinding compound is that this procedure is completely water-based, so it is
much easier to assess how you are doing, make adjustments, and clean up. The
basic idea is the same, though.
Myron Gochnauer
goch@xxxxxx
Got about 15 minutes into it and its clear the glass is grinding down as it
should. I am guessing its going to take at least another half hour or so,
maybe longer to get it like I want, but it is working. Will it be factory, but
from the looks of it I think it will be quite serviceable. There is something
to be said for having done it yourself.
Got to pick up a blade tomorrow that will fit in the jigsaw that will cut
glass. No way I could ever get a clean cut with a typical glass cutter. Tried
a few times and never mastered the scoring, or just never could get a clean
break.
This project is giving me a great appreciation for the work of the early
pioneers. When you see the images they created with equipment that would be
considered primitive, after doing this I can appreciate the work even more. I
have one huge advantage they didn't. I have their experience and all those
that came after them.
A while back someone posted they had a method to shim the ground glass to make
sure that what you saw as in focus on the ground glass was in focus on the
film. Would love to hear the procedure and if they prefer they are welcome to
email me off list.
Slow but sure, and this past couple of weeks have been a pain to find any time
at all to do much of anything. One day when I push the shutter for the first
time, it will all be worth it, at least I hope.
Mark
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WB6KBL
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