ATM [Fwd: BOUNCE atm@shore.net: Non-member submission from [et@ugcs.caltech.edu (Ernest S. Tomlinson)]]

[Mel Bartels <mbartels@efn.org>]

owner-atm@shore.net wrote:
> 
> >From atm-owner  Fri Oct  4 14:21:35 1996
> From: et@ugcs.caltech.edu (Ernest S. Tomlinson)
> Subject: Fine grinding
> Date: Fri, 4 Oct 1996 11:21:29 -0700 (PDT)
> 
> I'm pretty pissed at the moment because I wasted about 4 hours of work
> during fine grinding.  I was finishing up work with #400 carbo
> (alternating mirror-on-top and tool-on-top between wets, as per usual)
> and then managed to put a giant scratch in.  I had to return to #220
> and grind about six wets with that.  In any case, I doubt that I'll
> be able to polish for a while yet.
> 
> While grinding I tried to investigate where scratches originate.
> The huge, deep scratch that I mention above obviously came from
> some stray particle of dirt, but when I get scratches they're
> usually very shallow lines that are clearly visible only when the
> disk is wet and the light hits them just right.  They grind out
> quickly and tend to concentrate fairly near the edge of the mirror.
> _And_ they all seem to get there when I'm grinding tool-on-top.
> 
> These facts seem to indicate to me that the scratches aren't
> coming from contamination, but from the abrasive itself.  When
> I'm grinding with tool-on-top, the mirror tends to dry out much
> sooner.  I'm guessing that the abrasive layer isn't distributing
> itself well, and that's where the scratches are coming from.
> OTOH, mirror-on-top wets last longer and seem to give a smoother
> finish.  (I tried mixing liquid soap with the water to get better
> wetting of the disks; this seems to give better distribution of
> the abrasive.)
> 
> Is it safe to finish fine grinding entirely with mirror-on-top?
> When I ground my only-other-mirror two years ago, I ground
> exclusively mirror-on-top and got a hyperbolic surface and poor
> edge polishing.  But maybe if I use a very short stroke (< 1/4
> diameter) I can keep the surface spherical.
> 
> Cheers,
> -et