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Old 09-08-2021, 01:36 PM
legswilly (Werner)
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Location: Wauchope, NSW, Australia
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Where is grit for sale

I am looking for a supplier of grit for grinding a mirror. I particular, I am looking for aluminium oxide, #1000 and #1200 (9 and 5 microns).

The current supplier has no aluminium oxide in the sizes, only silicon carbide. From what I have read silicon carbide is more aggressive and sharper, but mirror grinders use aluminium oxide. Is there a reason for using this?
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Old 09-08-2021, 02:34 PM
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mura_gadi (Steve)
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Location: Canberra
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Hello,

The shatter caused by silicon carbide is greater than that caused by aluminium oxide at the same grit. The fractured surface material generated by aluminium oxide has smaller pits, fractures, smaller loosened bits of material etc to clean away. That makes progressing between grits a little friendlier and gives a smoother final finish than Sic at the same grit. Ideally if you had a lot of glass to hog, you'd go 80/120 silicon carbide for fast hogging, then 180-220 aluminium oxide to rough figure/figured focal length onwards.

"Keep in mind that silicon carbide leaves pits about three times deeper than aluminum oxide" from The Joy of Mirror Making - Mel Bartels.

Hobby Gem shops and lapidary clubs for polishes and grits etc... a quick google will give you a lot providers.

Kemet industries Sydney, or any high end polishing/lapping company really in your area will have them.



Luckily NSW is very good for options.
Steve
Ps. At 5 and 9 micron, see if you can find microgrits and I would not suggest spending extra on "optical grade" material. Get microgrits or check to see if your oxides need a wash prior to use.

Pps. Keep an eye out for zirconium oxide over rouge if you can find it for later.

Last edited by mura_gadi; 14-08-2021 at 03:49 PM. Reason: added quote from Mel Bartels webpage
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Old 10-08-2021, 04:39 PM
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mura_gadi (Steve)
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Smaller grits

Hello,

The smaller your grits the harder they will be to work with effectively, too dry, too wet, too muddy(old material), warmed the mirrors incorrectly etc. At 9micron your operating with a .009mm grit.

Grits generally smooth to 1/2 the grit size. Ie 12micron will give a 6micon smoothed surface.

For a diffraction spot you normally go 1.22x0.56xF/l. In the case of a F6, that would equal 4.1microns(1.22 x 0.56 x 6) and you would set that as your polish limit or close too. A very good reason why mirrors get harder as they get faster, not just spherical to paraboloidal curve to consider.


Steve
1.22 - don't remember why
.56 being the most sensitive light wave
Focal Length

Last edited by mura_gadi; 10-08-2021 at 05:03 PM.
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