Re: [ATM] Current "favorite" for hogging?

[Michael Lindner <mikell@optonline.net>]

Jim McPherson wrote:
> I do have a question about the metal tool. Is there any advantage to the 
> wide surface area of the flange? I went to home depot today and bought a 
> 2" close nipple and a tee thinking that would work fine. Is there any 
> advantage to putting the flange on the end or will grinding with the 
> nipple ( or bottom of the HEAVY tee) be sufficient?
> Also, I'm assuming that 60-80 SiC is still the grit of choice with a 
> metal tool. Is that still true?

I would think the more surface area the better, but you do want the tool 
to be smaller than the blank (by at least 50%) to get a concave curve. 
I used 60, then 80 to hog out a 14" f/4.5. For a smaller than 12" mirror 
I'd probably start with 80. Once you're close to the curve (within about 
10% of the desired sagitta), I'd switch to 120 and make a tile tool - 
the curve at this point will be far from spherical, and the tiles and 
glass need the 120 to mate in a reasonable amount of time. Try to get a 
decent sphere before moving to 220 even if you have to spend extra time, 
as it's slow going to correct a bad curve at 220.

Clear skies and happy grinding.
-- 
Michael Lindner
http://www.starastronomy.org *** http://home.att.net/~mikel
http://www.atmsite.org *** http://www.atmlist.net
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