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Old February 28th, 2004, 09:49 PM   #1
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grinding powder

According to research and past discussions (35mm adapter stuff), 3 micron Aluminum Oxide powder is the finest powder for making ground glass, before polishing. Like the "Making Your Own Ground Glass" article says, I'd like to use 5 and finally 3 micron to grind my glass. My only problem is that I can't seem to find any companies with both sizes available. Has anyone purchased both 5 and 3 micron alumina powder? If so, where?
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Old February 29th, 2004, 08:49 AM   #2
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Go to a hobby shop, particularly one that sells rock polishing equipment. I forgot the name of that hobby. Michael's might actually sell it also. 500 grit or 600 grit is another size name if it doesn't mention it by micron size.

Back in my homebrew electronic days, we used to adjust the frequency of the crystals in oscillators with Comet cleanser. I don't know if that would work also.
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Old February 29th, 2004, 08:59 AM   #3
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the final source seems to be rockshed.com that has a 1000 grit alum.oxide
it is cheap, but they will probably run out of stock since this forum exists.... :-)
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Old May 22nd, 2004, 07:37 PM   #4
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aluminum oxide

Dental Equipment Supply shops also sell 50 micron aluminum powder... check Froogle.com under aluminum oxide powder.
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Old May 23rd, 2004, 01:44 AM   #5
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Nicoli.

If groundglassing a clean glass surface, I would be more inclined to try the 3 micron first and not to get the surface too opaque.

I find the best seems to be just barely on the obscure side of being able to see a bright dome lamp through the groundglass.

For sharpess of image, I have possibly been unwittingly cheating a little and getting a hint of the sharper aerial image as well as the diffused projected image on the groundglass.

Optical glass filters should already be a plane surface and not need correcting.

If you are dressing the disk on a sheet of glass, you may find it helpful to chamfer the edge of your intended groundglass and the edge of your grinding sheet with a diamond lap used for sharpening knives and available from some hardware/camping stores. Do it under water with short light circular strokes and wear gloves for all your glass finishing work.

(It is a small piece of wood with a sharpening surface like and emery stone set onto it.)

Do this and there will be less inclination for small pieces to spall off sharp corners, roll under and cause gouges if the groundglass sticks or when you slip it off the edge of the glass sheet to clean and check.

The optical people I have spoken to so far hint that dressing the groundglass on a glass sheet might be less preferable than on a finely machined castiron surface. One suggetion was a pitch surface which has been treated with the grit to make a lap ( grinding surface ). Another was the use of thick tight woollen felt. With the Ohara disks I am having to polish them first because they are raw and unfinished stock whereas you won't have to go this far with your filter disks.

If using a glass sheet, you may find it helpful to roughen it first with a coarse grit and piece of scrap glass, then clean it incredibly thoroughly before using the fine material.

This will lessen the problem of sticking and scratch/gouge problems. A little bit of household detergent or fine soap in the water apparently helps as well.

Take notice of more qualified advice than what I am casting here. You may already be more well read on this subject than I.
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Old May 23rd, 2004, 02:54 PM   #6
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Hi Bob,

I think a lot of this is going to be based on experimenting and personal preference. Quite some time ago, many people read an article that told how to grind glass with the glass-on-glass method and just stuck with that. I've read about other procedures that included felt and even leather instead of glass for grinding. The castiron idea sounds neat too.

The original post on this thread was started a while ago and, since then, I've experimented a bit with both 5 and 3 micron ground glass. I went with the glass-on-glass method and received good results using 5 micron to knock down any glass imperfections, then 3 micron to grind the final surface. However, like you said with glass filters, if you have an originally flat surface, there would be no need to use 5 micron first.

Quote:
Another was the use of thick tight woollen felt
I also tried using a nice wool felt. Basically, I just got a sheet of thick, non-died wool felt (not the cheap stringy polyester kind) from the fabric store. I used spray glue to layer the felt up to a nice thickness. After each layer was adhered, I used heavy weights to evenly compress the felt. Eventually I had a nice 4 inch diameter felt wheel that attached to a drill and spun at a low speed for grinding glass. Unfortunately, that didn't seem to work because the wool absorbed all the water and "trapped" all the aluminum oxide. I am thinking about trying this again with a different lubricant, maybe soap or a thin oil like machine oil. Or, and this is very likely, I may have just goofed something up and it will turn out fine next time.

Quote:
If you are dressing the disk on a sheet of glass, you may find it helpful to chamfer the edge of your intended groundglass and the edge of your grinding sheet
This is agreat idea since glass shards from the edges could easily cause serious deep scratches in the ground glass. I will definately implement this into my next experiment.

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If using a glass sheet, you may find it helpful to roughen it first with a coarse grit and piece of scrap glass, then clean it incredibly thoroughly before using the fine material.
I can vouch for this. One thing I quickly noticed was that two "flat" pieces of glass really give troubled results until they have been grinded a bit. Sticking was a big problem at this point. However, as soon as they were ground a little, the process went much smoother (pardon the pun).

Quote:
Take notice of more qualified advice than what I am casting here. You may already be more well read on this subject than I.
New ideas are always welcome, so please keep them coming. I plan to continue experiments until the result is perfect. At this point, 3 micron ground glass provides a nice, bright finish that is acceptable. However, my goal is to do even better than that. I believe it is ultimately possible to apply static ground glass to a 35mm adapter for use with high resolution camcorders, without any appearance of grain and very little light loss. Of course that would require a lot of diffusion through more than one condenser lens. This may end up just being a pipe dream, but that's never stopped me before :).
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Old May 23rd, 2004, 09:39 PM   #7
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Nicholi

Back when I was in the antique restoration field I used to resilver mirrors. In order to polish the freshly stripped back for a new layer of silver I would use a felt chalkboard eraser and jewelers rouge with water.
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Old May 23rd, 2004, 10:26 PM   #8
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I must have done something wrong then. I'm going to go back and try it again with the felt pad. Hopefully this time I'll get a better result.
Thanks, Jim.
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Old May 23rd, 2004, 10:27 PM   #9
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I was recently reading a history of "optical munitions" in Australia with reference to manufacturing and processing optical glass. Due to short supply in WW2, the locals here had to learn very quickly how to make and finish optical glass. They did it and actually nearly 50% of their dial sight production went to the US war effort.

Reference was made to jewellers rouge and that cerium oxide was an alternative.

I have also been doing some inventiveness on automating the polishing/dressing of the Ohara disks. I've broken two when I have been hastening the process along. (That's where the chamfering idea was prompted from the optical people I have been enquiring with.) Cracks were originating on the outer cirumference where the cut marks from separating the disk intersected. Add chamfer = eliminate breakage. I observed it conferred a scratch/gouge limiting benefit and my optical friends confirmed that method.

The inventiveness involves using a CD-R cover as a sort of cement mixer, placing my glass disk inside, loose on top of a few failed CD-Rs with a mix of water and powder, placing a large heavy metal hollow centred disk on top of that and rotating the whole thing like a gemstone tumbler. The metal disk rolls along the front of the Ohara disk and dresses it. Record turntables spin at the right speed but are not robust enough for continuous duty or the loadings whilst inclined at about 50 to 70 degrees.

So far with plastic disks to see how the rubbing motion occurs it appears to run from the centre of the disk out to the circumference almost along radius lines. If anyone has any ideas on reduction transmissions from any source, ideas are welcome. So far the only alternative might be broken oscillating desk fans but they are also a bit light for this task.
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Old May 24th, 2004, 06:34 AM   #10
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Furthur to the above, a bronze donut shaped disk about 60mm diameter, 20mm hole in centre and about 10mm thick, yielded some quick polished tracks on one of the disks when I manually tested the idea to see which was the best angle for smooth rolling of the donut across the glass disk inside the CD-R case with a polish slurry in place.

I did a quick test by manually polishing the disk with the donut. The initial results seem encouraging. More about this is on the Aldu thread.
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