Focusing ground glass.

so I bought a Toyo Field Special, the 60's/70's one primarily for half-plate japanese holders. The back lacks the focusing glas:

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on this one tolerance around the edges is tight, like 1,5mm. After the black metal strip holders left and right are removed, measurement:

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I cut a piece of thick paper as exactly as possible, to fit inside. This is my template: 12x16,5cm

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buy at a cheap store the cheapest picture frame, 15x20cm. (check it has a glass not acrylic). And I grab the glass cutter:

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strip the frame, keep the glass, position the paper template with tape. There will be two cuts.

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check the margin between the blade of the cutter and the outer edge of its holder. Btw i prefer this kind of cutters because the large flat shape of the blade holder. Easy to slide against a guide when cutting:

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I position whatever thick enough straight length along the edge of the paper template, as per the measured margin. Use tape to secure it. Check it's ok all the length:

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a dip of the cutter in white spirit/terebenthine/terpentine/etc for lubrication is not vital but good:

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first cut, and because this is thin glass sheet (~1,5mm) I sandwich it and hold to avoid irregular uneven pressures that could cause break. Put something under the glass to have it a bit lifted, and the cut line aligned by the edge. Grab a piece of wood or MDF or whatever to distribute the push lengthwise, and give a clean straight push with the hand.

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I have reached the 10 pictures by comment, so follow on next one.

when cutting glass a short pre-cut should be done by the edge of the sheet, before doing the cut, otherwise there can be a glitch. It's tricky. Here a small glitch only:

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fix irregularities with a fine metal file, some sand paper (I use 80) for finish and smoothing:

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check this dimension fits:

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next cut:

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this one I was too conservative on the dimension, and have to file like ~0,5mm all the lenght. When filing I also clamp a sheet of metal/wood/MDF, hold with tape, near the edge of the glass, and always use a fine file, otherwise risks of cracks/splits.

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now i have perfect fit inside the holder:

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next comment, the sanding

i tape the sheet of glass, like 1,5mm tape around the edges, and grab F400 and F600 silicone carbide grit. Sold where stuff for stone polishing are sold for instance, or here and there online. I start with F400, then follow with F600, yet apply F600 only will work.

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use a piece of glass. I have one with a pad that i use with 8x10 glass but here I cut a little rectangle and glue some rubber ball, but can be used only with fingers:

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water for the grinding:

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when water with carbide powder sneaks its way around the edge of the tape and under the glass, remove the whole, clean, tape again, etc.

starting with F400 grinding produces a noise, but not really after when applying F600. A short video, I started with the bigger glass, then used a small cut:

https://youtu.be/zrhM1oOD0cM


Done:

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and mounted on the camera back:

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now I will make another one as reserve. Can have it inside a protective foam padded metallic case when traveling.

I do my focusing glasses from 6x6 MF to 8x10 LF like this. It's really trivial, simple, cheap. Now that I had to make one for this recently acquired Toyo, I thought I could illustrate it, because it's a recurrent question from people who just don't realize how simple it is to do.

not related the process, but just a note about protection of focusing glasses. Some holders have a flap cover. These old Toyo have rails to slide a protective sheet:

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Dear Landstrykere,

Allow me a question:
What kind of 'medium' would you suggest to mark gridlines on a DIY GG, and on what side the grind- or the smooth side?

I have used a fine felt tip permanent pen, but it erases easily and isn't that dense enough to be easily seen under some conditions. When used on the grind side the ink tend to run out and the lines aren't sharp and fine then.

Thanks,

I use laser printable.transparency sheets, with grids drawn in software. So overlay on the glass.
if i had to draw on the grinded side I would try fine point hard graphite pencil like 5H, maybe will be straighter lines than ink but may not hold long if back is taken on/off many times.

I have an old HP laserjet 15, a little basic black white printer. Opened GIMP on the desktop, went in main menu to "Filter > Render > Patterns > Grid" and draw different patterns on transparency/presentation sheets. For instance:

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I opted for a 3x3 thick lines, with long dotted halves, put under the retaining clamps of the ground glass:

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printable transparency sheets are also available for colour laser printers and for inkjet printers.

Thank you for sharing your process!

Two Questions:

One - Why the progression of two different (400 to 600) grit sizes? I would almost think this would be counterproductive - that you'd be creating deeper cuts with the 400, which would be very difficult to remove with the finer, 600 grit, leaving some deeper (and darker) "valleys" between more finely polished "peaks." While starting with a course grit and moving to a finer one is standard procedure when sanding/polishing (removing texture from) wood or metal, I'd think using grit for the purpose of creating texture from a perfectly smooth surface should be a "one and done" process.

And two - Do you rinse off and re-apply your grit(s) at several intervals during the grinding process? I've typically done this...assuming that the grinding process would create small shards of glass which would interfere with grinding and even possibly create extra "artifacts" on the glass surface, and that refreshing the grit several times creates a "cleaner" grind. But then again...maybe there's an advantage (aside from using less grit) to just going with one batch of grit for the entire grind? Just curious!

Quote:

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Originally Posted by John Layton

One - Why the progression of two different (400 to 600) grit sizes? I would almost think this would be counterproductive - that you'd be creating deeper cuts with the 400, which would be very difficult to remove with the finer, 600 grit, leaving some deeper (and darker) "valleys" between more finely polished "peaks." While starting with a course grit and moving to a finer one is standard procedure when sanding/polishing (removing texture from) wood or metal, I'd think using grit for the purpose of creating texture from a perfectly smooth surface should be a "one and done" process.

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I always had the feeling that starting with a bit coarser grit speeds up the grinding. Maybe subjective. When the finer grit is applied the "peaks" formed by the coarser one will be abrased. But then I could just do the finer one, then compare. My concern for a while was rather what size of grit is optimal: 400? 600? The screens I make with 600 are luminous enough.

Quote:

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Originally Posted by John Layton

And two - Do you rinse off and re-apply your grit(s) at several intervals during the grinding process? I've typically done this...assuming that the grinding process would create small shards of glass which would interfere with grinding and even possibly create extra "artifacts" on the glass surface, and that refreshing the grit several times creates a "cleaner" grind. But then again...maybe there's an advantage (aside from using less grit) to just going with one batch of grit for the entire grind? Just curious!

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yes I rinse and reapply few times, in order to wash away fine glass particles added to the wet grit pasta along the grinding, because as I see it it will reduce efficiency.. Like sanding wood, when enough wood dust lays under the sanding pad. Otherwise I have to wash and restart just because water with grit will at some point get under the tape retaining the glass.

I have no technical expertise in this, but I have done it so often I found it could be interesting for novices to see if they come across, it's a typical concern for people who don't tinker and there's money to spare easily. The glass cutting is in fact the tricky part that may need bit of practice. I started doing my screens for my 6x6 cameras, and because the tiny size i broke glass very often until getting it right.