Re: [ATM] 20" FS update and what size grinding tool

[William Marriott <bmarriott@pacbell.net>]

Hello everyone;

Update on my 20? fused silica RC telescope project,
short version:

1) Put together a variety of tools, so now besides the
9? barbell weight, we have a 7? barbell weight, a 5?
cast iron gear, with recessed faces, and a 4? CI hub
from a grinding tool. 

2) The 4? tool puts correction quickly where ever it
is needed.

3) The 5? gear set so it sweeps from one side to the
other, increases over all correction

4) 7? Barbell weight smoothes out zones, but does not
flatten out the curve at all. 

5) Spindle is at 90rmp, eccentric at 50rp, 60 PSI,
grinds quickly, even with fused silica.

6) I have now switched to 120 SiC, and have a f4.25
(more or less) curve from center to edge.  

7) Ordered 25 lbs of dental stone, will be here in a
couple of days, and made a trip to Home Depot for the
hard small tiles to make the  Richard Schwartz-ish
tile tool.

Thanks for the procedure Richard!

So, what size grinding tool to make?

I am in general an advocate of full size tools for
ginding mirrors, but, when you are dealing with a 20"
mirror, the thought of having to hump around a full
size tool, causes me some concerns. 

The question is, how hard will it be to keep an
uniformly spherical surface with a smaller tool.

12" would be nice, but effective?
16" I would hope I could keep spherical with it, and
not have to go to a 20" tool.

Then there is the question of how thick to make the
grinding tool.

SInce I have never had to deal with these issues and a
big mirror, any input about all this would be greatly
appreciated.

Bill




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