Re: [ATM] polish time for 13 inch

[Dominic-Luc Webb <dlwebb@canit.se>]

On Mon, 23 Jul 2007, Mike Lockwood wrote:

> As Mel mentioned the magic 6:5 ratio has been written about before, in
> the ATM books, I believe.  I seem to recall it in a chapter about
> Hindle machines, though I could be wrong about the location.
>
> I would like to point out that many of us make mirrors with laps about
> the same size as the mirror, and I personally don't get turned edges
> if the pitch lap is in contact.  The laps I use run from 100% down to
> ~25% of the mirror diameter, depending on the mirror.  For large
> mirrors the oversize laps become more of a challenge.


For faster mirrors, they become more of a necessity (my experience).
Actually, I ended up with a few different sized laps. I have made
~F/3 mirrors and gotten no TDE, so experience was on my side. On
my really fast mirrors (F/1 to F/1.5), I got TDE that was introduced
during final stages of grinding and would not polish out. I have
since learned to scan the surface with a subdiameter spherometer
before attempting any polishing. At least this way, I know I do not
have deviations more then a micron or two. I have also tried using
fine grit on a lap to attack TDE that wouldn't come out with polishing.
In my case, I am after spheres. I gave up parabolic surfaces when I
decided to go with fast mirrors.

Testing, well, I was going to give Mel's idea of a plywood lap tool
a try, but couldn't find time. I got caught up in improving my
tests (caustic, using masks with lots of holes).

Dominic

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