Re: ATM Subdiameter pitch lap

[Anthony Stillman <atmer@flash.net>]

Bob,

I'm using a 12.5" diameter lap to polish out my 16" Walter's blank.  I've
got about 5 hours on it and except for a few ego brusing scratches its
coming along well.  As per Mel's advice I've feathered my stroke, spending
fully 2/3 rds of my time on or over the edge with at most a 1/3 mirror
radius overhang.

As this is a thin mirror I'm supporting it with a green plastic door mat
from Home Despot.  I'de liken the support the green plastic reeds provide
to that of whisk boom bristles.  In a pinch I suspect a bundle of such
bristle might do nicely.  I haven't even a hint of stig, a common problem
with thin mirrors.   Another Chabot regular supports his 16" 1" thick
plate(1) glass mirror in its scope with this same matting and is pleased
with its performance.  He's currently polishing a 10".  His polishing board
is covered with astro-turf.

I've only this one experience polishing out sub-diameter.  My thinking
before this effort was that about 80% the mirror diameter should limit the
how sub-diameter the lap could be.  I may relent or zones may become an
issue, we'll see.  In any case, I reserve the right to cast a full size lap
to drive it spherical.


Anthony




1)  The piece of green glass in his scope was float, but having been ground
and polished, on one side anyway, it can, at the very least, be called
half-plate.