ATM [Fwd: BOUNCE atm@shore.net: Non-member submission from ["Bob Kenne" <bkenne@bigcity.net>]]

[Mel Bartels <mbartels@efn.org>]

owner-atm@shore.net wrote:

> From: "Bob Kenne" <bkenne@bigcity.net>
> To: <atm@shore.net>
> Subject: Mirror Question
> Date: Mon, 28 Sep 1998 21:00:39 -0500
>
> I am currently working on my first mirror, a 10", F7, 3/8" plate,
> stabilized with 3/4" ribs and a 1/4" back plate. When I finished hogging it
> out, I found the focal length to be about 64". So I ground a bit more 60
> grit with the tool on top to try and get back to @ 70". The focal length
> didn't change much , so I went on to 120 and ground @ 2 hours with the tool
> on top. As of now, I've coated the mirror surface with WD-40 oil, and when
> I reflect the sun onto a white surface I get a sharp 1" circle at a focal
> length of 65".
>
> Here's the problem. I drew grid lines on the mirror and tile tool to check
> overall uniform contact and found that although the lines disappeared
> uniformly over the surface of the mirror over a couple of 120 wets, there
> remains a 2 1/2"circle on the center of the blank where I'm getting little
> or no contact. I' not sure if I have a problem here or should just continue
> with finer grinding. So here are my questions:
>
> 1) Do I need to go back to 60 grit rought grind until I get total uniform
> contact between the mirror and the tool or is this expected if I'm grinding
> only with the tool on top?
>
> 2) Is this an indicator of a problem or incomplete rough grind?
>
> 3) Is the reflected sun test an accurate indication of anything (good or
> bad) at this point?
>
> 4) Should I continue grinding finer?
>
> 5) Is there some other test I can do at this point to check for a good
> sphere?
>
> 6) I noticed that a bit of detergent in my water makes the abrasive
> distribute a bit easier-any problem with this?
>
> Any advice would be appreciated.
>
> Sincerely,
>
> Bob Kenne



--
Clear skies, Mel Bartels
http://www.efn.org/~mbartels