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I have been given the following grit sizes: 80, 120, 220, 302, 303, cerium oxide. Have some clarifications switching grits.

ATM Mirror Making
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#1 Adriantamil123

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Posted 02 December 2021 - 09:20 PM

I am grinding a 10 inch F/6 to a sagitta of 2.64 mm.
Have already grinded till 1.32 mm with grit 80.
Questions:
1) How much deeper should I grind with 80 and then switch to 120 and at which point to 220, 302, 303
2) Most of the websites do not have information about 302 and 303 aluminium oxide grits. Are these to be used with the tile tool or the pitch lap (sorry if that sounds like a dumb question)
3) If I have to buy grits of different size than the ones provided which ones should buy and why?

#2 KBHornblower

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Posted 02 December 2021 - 10:12 PM

My 1964 Edmund Scientific kit came with alundum, a very pure and hard aluminum oxide grit, in sizes 80, 120, 220, 320, 400 and 600, with each number being the reciprocal of the grit size in fractions of an inch.  These were followed by #305 emery, which is finer than the 600 alundum.  This emery grade number is not the same sort of numerical measurement as with the alundum.  My best guess is that emery #302, 303 and 304 would correspond to alundum 320, 400 and 600.  These grits are all for grinding with the glass tool, or the tile tool in your case.

 

I hogged out the mirror to the full sagitta with size 80, and then alternated between mirror on top and tool on top to keep the curve close with the finer grits.  Polishing with the pitch lap is done with rouge or cerium oxide.

 

Your questions are dumb only if you don't ask them.  We glass pushers are here to help.



#3 Adriantamil123

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Posted 02 December 2021 - 11:06 PM

@KBHornblower I could imagine your level of experience when you talk about a kit from 1964. Thanks for answering my questions. It is clear now.

#4 gregj888

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Posted 03 December 2021 - 02:12 AM

Looks like 302 is about 20 um and 303 about 12 um...

 

Stellafane to the rescue--  https://stellafane.o...-refs/grit.html



#5 dodo

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Posted 03 December 2021 - 07:50 AM

1) I prefer to grind up to 80% of the sagitta with #80 then switch to #120 with 1/3 strokes till the desired curve is reached.
2) All the grits you mentioned are for grinding with the tile tool.
3) I have ground only with glass on glass and go down to 1 micron before polishing. I think starting polishing after #303 will take a lot of polishing time for your 10". I would say to finish at least with #1200. Also of you can polish for the first hour or so with the finest grit to speed up polishing time.

#6 Adriantamil123

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Posted 03 December 2021 - 08:09 AM

@gregj888 thank you. How did I miss that at Stellafane!

@dodo Useful inputs, I will keep these in mind. Since you are also from India, I will ask you this too, why don't any glass dealers have anything thicker than 19 mm?

#7 dodo

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Posted 05 December 2021 - 12:18 PM

why don't any glass dealers have anything thicker than 19 mm?

Well I don't know for sure but I think it is the limited application for thicker glasses that could be the reason. So here manufacturers limit their production to 19mm only.




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