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


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