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

[Mike Lockwood <melockwo@uiuc.edu>]

Bill,

For my 20" F/3s and a 20" F/3.3 (1.25" thick), I used a 16" tile tool 
made from Hydrostone, about 1.25-1.5" thick.  I also used a 16" 
polishing tool.  I ended up with reasonably spherical mirrors with 
rotary polishing (fast turntable, slow eccentric) after getting the 
overhang and stroke set properly.

A 12" tool is not large enough for grinding or general polishing, IMO. 
  It will work more slowly and astigmatism can crop up more easily.
It's very useful for figuring, though.  You can start figuring your 
20" with the 16" polishing tool.

I have moved around a 22" plaster tool, and it's not fun.  I much 
prefer a 22"x7/8" Pyrex tool - it makes a wonderful pitch lap 
substrate, even though it is flat on both sides.

I very much like your article describing tile tool making, Bill.  Is 
it still posted somewhere?  I think the link I had is broken.

	Mike Lockwood

William Marriott wrote:
> Update on my 20” fused silica RC telescope project, short version: 
> 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.
> 
> 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.

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