[ATM] Shop Practices: yet another heresy

["Richard Schwartz" <richas@earthlink.net>]

This is an evolutionary tale...

When I started making ribbed mirror blanks, I needed a way to make the ribs
all exactly the same width, with the opposite edges parallel.   I read the
books from the days of Twyman, Hindle, and Hitler, and blocked the ribs into
groups of about a dozen with pitch.   I miked the four corners of each block
to see where the grinding needed to be done, and worked on a flat steel
plate.   This method worked, until the time came to separate the ribs and
clean the pitch off of them.   The details of that operation can be found in
the archives by searching for "PRESSURE COOKER".    This was messy and
inconvenient, and sometimes the blocks would come apart.

I wondered if some mechanical method might be better.   The first thing I
tried was claming the ribs together with a C clamp.   With enough pressure
to keep the ribs locked together, the glass would crack.   A rubber cushion
with a quarter (U.S. coin about 1" diameter) did not help, so it was back to
pitch blocking.

Then I wondered if a jig could hold the ribs in the perpendicular position.
I made a simple jig of oxymoronic aluminum angle iron and a piece of flat
glass.   The metal was attached to the glass with cyanoacrylate crazy glue.
This method worked well.   I could guarantee parallel and equal edges of all
the plates in a block by removing the plates in two groups, then rotating
each group 180 degrees.  (The middle blocks are now on the ends.)  Then lift
out the middle third of the blocks and rotate that group 180 degrees.
Apply sharpie marks, and grind on a flat plate until there is contact all
over.   The problem with this method is that grit gets between the glass
plates and they get jammed into the jig very tightly.  This makes it
difficult to remove them and sometimes the pressure is great enough to break
the aluminum angles loose from the glass.   (Bob May has witnessed this
procedure.)

My latest heretical innovation required a leap of thought, not faith in
Texereau, Twyman, or Hindle.   Why do it the hard inconvenient way?   Just
fasten the block together with a rubber band!   I tried this, and it was way
Toulouse.   The rib plates would slide all over the place.    So I added
another rubber band, and things were a little better.   That sent me down to
the rubber band store, where I got a whole bag of 'em.   With about 15
rubber bands holding the block together, nothing was moving.   It is no mess
or bother to remove the rubber bands and shuffle the block or mix several
blocks to equalize the rib widths.   Rib grinding for the ribbed mirror
blanks will go much faster now.

One other minor point for those who would follow in Ric Rokosz' footsteps:
early in the game, Bob Goff told me that the edges of the ribs must be
POLISHED to get the best possible bond.   This I did not do; I did, however,
take them down to 3 micron grit.   The result was large flat bubbles in some
of the rib joints.   Let everybody know that if you want to fuze two flat
surfaces together without large flat bubbles, you should leave the surfaces
as ground with #120 grit.   This leaves a porosity in the interface that
allows air to excape.  There may be many very tiny bubbles, but they will be
tiny and round, and will not be barriers to stress and heat transmission
(stress and heat follow similar PDE's).

. . . Richard

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Mike Lockwood
Sent: Friday, July 15, 2005 8:07 AM
To: atm@atmlist.net
Subject: [ATM] Fixing oblate sphere/TDE (was shop practices)

Hi,

Mark Holm wrote:
> I'm not so sure of the short stroking cure for TDE for the same reason. 
>  My experience with TDE is neither extensive, nor terribly successful, 
> but I can honestly say I haven't seen MOT short stroking do a whole lot. 
>  With a full sized, or nearly so, lap, I have had better luck going TOT 
> and using a tangential stroke or a fairly wide W to get action over the 
> outer zones and the edge.

Just thought I'd comment on how TDE usually manifests itself for me, 
since I'm seeing REPEATED examples of it in my shop right now, and in 
pictures posted by list members.

A friend just sent me a mirror that I'm going to refigure for him. 
Also, I'm helping another friend in town finish an 8" F/6 that he has 
ground from the start, and it now trying to fix the edge on before 
figuring.  Both mirrors have the same problem.  It is the problem 
pictured in the first "Foucault image" posted by "RML" near Montreal 
(see the thread "12.5 inches BVC mirror polishing and figuring 
difficulties" from about a week ago.  His pictures are at

   http://www.axeinformatique.com/temp/Mirror/BVC_Mirror.html

See the second row of pictures, first photo on the left.  It shows 
(from the point where the knife edge is positioned) a classic oblate 
spheroid (assuming the Ronchi line enters from the right, which I 
assume based on his comment about a central bump).  I have seen this 
many, many times.

The central bump is the least of the worries!  The edge is a mess, but 
the fix is not difficult.  This is NOT a bad case of TDE, since the 
outer zones are high also.  Knock down the outer zones, and the TDE 
will be eliminated in the process.  The TDE is DUE TO THE OBLATE 
SPHERE FORM OF THE MIRROR.

What I believe is happening in the polishing process in all these 
cases is that the front of the lap is digging in on the forward 
stroke, and the back of the lap on the rearwards stroke.  This digs 
out glass inside the outer zones, causing the TU outer zones (as are 
characteristic of an oblate sphere).  Once the lap goes over the edge, 
it is riding on the last bit of the raised outer zone (the edge), 
which is polished away with a vengeance due to the high local 
pressure.  That causes the TDE.

The cure for me is to work TOT with good contact and slow strokes.  A 
wider W and longer strokes will often help.  Most importantly, one 
must avoid edge plowing.  I do this by moving the point that I apply 
downward force to the mirror away from the leading edge of the lap. 
So, when I move the lap forward, I apply more pressure with the heel 
of my hands.  When I move it back, I generally reduce the pressure and 
try to apply it with my finger tips.  This stops the plowing.

Mixing in a little MOT work with longer strokes will help deepen the 
center, and can help match the shorter radius outer zones with the 
longer radius inner zones more quickly.

Texereau's cure for TUE can also be used here, but it requires some 
practice.

Once you learn to "feel" how the lap is working, you can often tell 
just how bad the oblate sphere is by how the lap friction changes 
during the stroke, so long as the lap is in contact.

	Mike Lockwood




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