Re: ATM concentric hills

["Bill T." <twentiethwave@hotmail.com>]

Ryan Hess wrote:
>
> >  Probably your strokes were too regular.  Does do the hill spacing
> correspond
> >  to the channel spacing on your lap?  Did you rotate the tool?
>
> Yes to all of the above <g>.
>
> >
> >  One cure might be to spend some time on classic W polishing strokes,
> >  alternating mirror and tool on top.  This stroke has the advantage of
> >  averaging out lap induced artifacts over a wide area.
>
> Actually, I used this stroke to polish out the mirror, and it gave a
> "Gigantic hill" out to the 70% zone... which is probably the average of
> these two hills on top of each other.
>
> >
> >  1/7" is getting pretty thin.  Unless you're close to figured, it's time
> for
> >  retread.  As it thins down it will start to act like an overly hard,
> badly
> >  pressed lap.  You can very temporarily gain some extra thickness by
> >  channeling, if you're now un-channeled.
>
> Yes, it is thin, but it is a soft lap.  (I am starting to get worried
about
> the tiles showing up from beneath!)  Yes, that's right.  I used my tile
> tool...  Any ideas on how I can modify my lap to correct the problem?
> Thanks,

Ryan,

I see three possible solutions to the problem:

THE RESPONSIBLE SOLUTION: the only way out of a weird or irregular figure is
to go back to a sphere.  Use W strokes with about 7 to 10 vectors to go from
on side of the mirror to the other.  Alternate mirror and tool on top.
Overhang maybe 1/6 diameter.  When you're at the sphere, start parabolizing,
but this time do it right! ;^)

THE "GO FOR IT" SOLUTION: well, a big bump at 70% is kinda-sorta like
overcorrection, too much parabola.  One way to take down a parabola is: Use
W strokes with about 7 to 10 vectors to go from on side of the mirror to the
other.  Alternate mirror and tool on top.  Overhang maybe 1/6 diameter.
Keep checking until you've backed up to about the right parabola, sometime
before you hit the sphere.  Deal with any residual zonal problems.  Pray you
don't have a serious turned edge.

THE "NOT-RECOMMENDED" SOLUTION:  Pre-emptively knock down the 70% Hill with
Texereau's "method off accented pressure," pressing down one edge or the
other of the lap over the 70% zone.  A little bit of this goes a long way.
Then go on to one of the above.  This will wear down your lap fast.  Your
lap is dangerously thin and soft for this one.

Take your pick. I assume the "bump" is a big upward pointing hill at 70%,
not a trough, and that you determined that with a Foucault reduction program
like Tex.  Either way, that thin lap probably won't last long enough for any
of these solutions.  The sound of a tile breaking through the pitch is
something like "GRRRRR---SCREEEEETCH!!!!!!!!!"

Bill T.