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Old December 13th, 2006, 07:15 AM   #1
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Glass grinding?

I've heard of people grinding down their beatie's, maybe not beatie's, but other types of focusing screens to fit their needs. What do they use to grind them to a certain size?

-Rh
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Old December 13th, 2006, 07:30 AM   #2
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Hi,
Most commen way and is what I used is using aluminium oxide abrassive powder with a few drops of water, and grind against another peice of glass.

Different grit sizes of the AO will give larger or smaller grain, and will effect lightloss and diffussion/bokeh.
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Wayne.
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Old December 13th, 2006, 09:10 AM   #3
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I have used glass on glass in a slurry of water and aluminim oxide abrasive powder. a drop of dishwash liquid detergent seems to help.

Don't heed this suggestion without asking somebody else first as it may be a deteriment I am suggesting. There is not the slightest bit of science in my methods. Some disks have gone from 1.3mm thick to 0.9mm thick by the time I have made mistakes and reworked them..

I have also used glass on machined brass with good results. I tried glass on machined aluminium with poor results and glass on machined cast-iron like the prescription spectacle makers do but mine was disastrous, scratches and gouges all over.

I later found that the cast-iron I was using has hard particle inclusons in it and that the iron used for prescription optical work is a different material.

The beauty about doing your own is that once you have got the technique down, you can start customising your groundglasses with finer or coarser grinds for variations, finer for better low-light performance v/s coarser for better bokeh.

You can even try a very - very light cerium oxide backpolish on a coarser disk for a bit of a bet each way, just enough to begin to round off the microscopic sharp peaks of the grind but not enough to make them wider or flatter or an incomplete finer grade grind over a coarser grade to work the finish similar to the backpolish but without any microscopic clear transparent areas on the peaks of the grind.
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Old December 13th, 2006, 04:09 PM   #4
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Always remember that glass dust created when grinding if VERY dangerous. Do NOT brethe it in, wear a specific glass dust mask and wet the glass as much as possible.
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Old December 13th, 2006, 06:24 PM   #5
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A slogan from one of our local Occ Health and Safety campaingns.

"Spot the hazard, assess the risks, make the changes".

A good call I.J. Walton. We get caught up in our enthusiasms to soak up and share collective knowledge yet one of the most basic fundimentals so often just slips right on by.

You shouldn't ever grind or should I say dress the glass dry, so an airbourne dust issue in its on right should not stick its head up - except, if you allow the residues to accummulate and dry around your workplace. Then, once disturbed dry they could become airbourne.

A recommended practice when dressing your groundglasses in a water mixture is to wear disposable gloves.

I forgot to mention that I cheat a little. I made a machine to do mine, similar inprinciple to a cement mixer, not the correct mechanism for dressing groundglasses but with care and patience, close enough.
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Old December 14th, 2006, 09:56 AM   #6
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I think Rich is talking about grinding the screen to a smaller size than 36X24 to work with the leaf spring vibrating design that fits in thorlab tubes
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Old December 14th, 2006, 10:13 AM   #7
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Sorry, But I think grinding down a bigger size screen to fit a screen holder isn't a good idea. I think best is to work the other way. Make the GG holder to fit the screen instead. Focusing screen of any type are so fragile and damaged very easily. If you are not sure how to get the GG holder done up, there are a few people here who can probably machine one out with a CNC machine.
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