Topic – Grinding a Ground Glass

As mentioned in an earlier post my large format camera also had a rear standard for 5″x7″ but the focus glass was unfortunately broken. This is how I went about to make a new ground glass for it.

Earlier this week I bought a cheap diamond cutter for glass which broke after 20 cm’s use:( Another investment, even less expensive, resulted in a cutter with hard metal wheels instead. It turns out that the diamond cutter was much easier to use, but if it loses the pointy diamond after a few centimeters it is not much use… The hard metal cutter is trickier to use but my first attempt was actually successful. My second was really not, and much training at other pieces of glass made me no better. I simply don’t have the knack for it.

When you cut glass you get one chance! Measure twice, cut once… or the glass will divide in the most unexpected places. With careful measuring and a drop of oil on the little cutter wheel it can be done. Make an exercise on some scrap glass first. When there is a slight, even and non-jumpy, crushing sound you know you are making a fine track on the glass. Afterwards it should be visible as a thin white line across the glass. Then hold the glass over a table edge and give the glass the slightest twist and it cracks apart along the cut in a surprisingly elegant way! It really is that easy. (Don’t add a few millimetres, they cannot be removed except by nibbling, resulting in a unpleasant looking edge.)

For grinding you need some carborundum. Mine is the leftover of #800 grinding powder that I used for a 12″ parabolic newtonian reflector many years ago. It was saved to be used just before the final polishing with jeweller’s rouge, so it is finer than #800, probably #1000 and some parts even smaller. I have kept it in water in a glass bottle ever since, just in case. And have actually used it to grind frequency determining FT-243 crystals a couple of tens of kHz to 3575 kHz (a common vintage and QRP frequency here).

The pictures below explain the procedure and result:

groundglass (4)

I started off with two 4″x5″ sheets of glass. The cutting tool is in-between the rear standard (with the better looking sheet fitted for… well, fitting) and the second sheet of glass, the tool. The tool is necessary. Always use another pice of glass, preferably of the same size, as the grinding tool.

groundglass (3)

The piece of glass to be ground with (too much, less than half would also do!) grinding powder added. Make sure it is more wet than dry, ample amounts of water is the key! 

groundglass (2)

Now put the tool on top, press and with circular motions cover the entire lower glass. For me it took less than one minute of actual grinding time to get a smooth suede-like surface. Don’t overdo the grinding or the result will be too course. Remember the water! Too little water can result in ugly grooves and possibly two plates jammed to each other. You don’t want that, they might never part! “Water, water everywhere…”

groundglass (1)

The result is the finest and smoothest of ground glasses. Here inserted into the rear standard for testing if it was finished. It was. This is actually a much better, finer, and brighter ground glass than my 4″x5″. I now should make a new 4″x5″ glass.

groundglassThe end result in place! This was a very rewarding exercise, the result better than anticipated. Less than ten minutes’s (?) of work. Writing this blog entry took longer…

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