To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 


Rick,
  I did 30 minutes 1/3 to 1/2 stroke MOT COC.  As expected, there's a very
short radius hole in the center, a nice smooth sphere outside it, and the
edge doesn't look much different, though I'm willing to give it more of a
chance.

Richard,
  I tried that, and all I got was a deep well in the center, as if the
outside edge wasn't getting any wear at all.  I'm stumped!

=Matt

>Delivered-To: mlbrown{at}0
>From: "Richard Schwartz" 
>To: "Matthew L. Brown" 
>Subject: Re: ATM Still Edgy
>Date: Sat, 8 Mar 2003 22:02:42 -0800
>X-Mailer: Microsoft Outlook Express 6.00.2800.1106
>
>If you are working mirror on top of a full size pitch lap. just shorten your
>strokes to about 1/2" maximum displacement, and you can bust that edge.  I
>been there and done that.
>
>
>----- Original Message -----
>From: "Matthew L. Brown" 
>To: 
>Sent: Saturday, March 08, 2003 7:47 PM
>Subject: ATM Still Edgy
>
>
> >
> > I still can't clean up that edge on my 4.25" f9, after 40+
hours of trying
> > to make a sphere.  This is the closest I can get:
> >   http://host67.everstrive.com/~mlbrown/ASTRONOMY/TurnedEdge8Mar03.jpg
> >
> > (note, no www in that URL)
> > Knife edge is on the left.  There's a slight hint of a diffraction ring on
> > the left that doesn't show in the photo.  But both Foucault and ronchi
>show
> > the same thing.  As you can see, a depression is forming in the middle,
>all
> > the way out to about 80% radius.   From there it just turns over
>gradually,
> > and doesn't look very far off: the outer edge always nulls within an inch
> > of the center zone, so I don't think it is a grinding fault.  There's the
> > remnant of a hole in the middle from taking out a hill in the center and
> > overdoing it a little. I'm not worried about that.
> >
> > Now I got here, after many wayward corrections, with a 1/4 stroke, tool on
> > top, W stroke about as wide as it is long.  Continuing this just gives me
>a
> > turned up edge at about 85% radius, and still turned down on the
> > outside.  That is, it just digs a wide well in the center.  I persisted,
> > and the well just got deeper, with little apparent change to the edge.
> >
> > Shortening up strokes even more just decreases the radius of the well in
> > the middle.  Increasing the stroke to, say, 1/3 stroke, just digs a hole
>in
> > the center.
> >
> > I've also tried accented pressure (Texereau, pg 86, #6) and that just gave
> > me a depressed ring centered at about 75% radius, with a hill in the
>center.
> >
> > Nothing I've done seems to wear the outside edge.  Heck, I've even tried
> > centering the lap on the edge and doing a few rounds of chordal
> > strokes.  That left something indescribable, but the edge was still
>turned.
> >
> > I've also rebuilt my lap.  It had gotten thin (1/16") and very hard,
>taking
> > 2+ hours to cold press.  Now it is 1/4" thick, and cold
presses in about
>45
> > minutes.  I've also hot pressed, to no apparent effect.  This lap has a 45
> > deg bevel on the outside, so it is about 1/4" smaller
diameter than the
> > mirror.
> >
> > This is my 2nd mirror.  My first was a 6" f8 which would
snap to a sphere
> > with ease on each of my half dozen or so attempts at paraboloidalization.
> >
> > One thing that seems fairly repeatable is that the area on the glass that
> > is always in contact with the lap gets worn down the most.  So a long
> > stroke wears a narrow hole in the middle.  A shorter stroke wears a wider
> > hole, still centered at the middle.  Since my lap is slightly smaller in
> > diameter than the glass, so my newbie theory goes, the very outer edge
>will
> > always get less wear than anywhere else, if I am using centered or W
>strokes.
> >
> > Not at the glass throwing stage yet -- it seems too close for that, but
> > I've already spent more hours on spherizing this 4.25" than
I did doing
>the
> > entire 6"
> >
> > Hellp!
> >
> > =Matt
> >
> >
> >
> >

--- BBBS/NT v4.00 MP