[ATM] Theories on what causes TDE

[wa4guu at verizon.net (Jerry)]

	There are more than one cause for TDE.

	There is the failure to carry the curve to the edge somewhat like
Ken is saying, using a stroke that more or less hogs a hole in the middle
area, shortening the radius and leaving the outer zone alone long.

	There is the opposite of the first and that is just plain too much
work on the edge making it longer without working the central area longer.

	And then there is the turned down edge caused by the fact that most
laps are a little smaller than the mirror. The "change in curve" occurs at
the edge of the lap. Sure the whole lap wears the glass but the difference
between the new curve caused by a stroke and the old curve before the stroke
occurs where the left and right edge of the lap passes across the disk on
forward and back strokes. If the lap is smaller than the mirror then there
are some strokes that where both sides of the lap cross inside the perimeter
of the mirror surface even when doing W strokes. And that is the case on
every stroke when doing center over center strokes. You will have a turned
edge or zone the size of the uncovered glass when the lap is centered on the
mirror. This cause is why some ATM's like the idea of a lap larger than the
mirror they are polishing. 

	There is the plowing of the edge into the lap when working MOT
caused by the force to move the glass necessarily being above the surface of
contact between lap and mirror. This levers the leading edge of the glass
into the lap. This would not be significant except for the next reason.

	Then there is the "real TDE" that is caused by the elastic lap
curling around the edge of the glass as you polish. The pitch is not
absolutely rigid and it yields under the pressure between the lap and mirror
and that results in the lap rounding the edge. That is why on a perforated
mirror such as for a Cass that the edge around the central hole turns also.
Or if you have a bubble on the surface or deep scratch you will see some
turn down along the edge. I call this the "real cause of TDE" because it is
the one that really can't be done away with. The only way around this one
that I can think of is that the mirror disk must be larger than the finished
mirror in order for the turn down to not be on the finished curve. This is
done either by masking the TDE, grinding it off or building an annulus of
some material around the perimeter of the mirror that will be where the TDE
is made as you work the mirror and when finished it is removed taking the
TDE with it.

	With all these things working to make TDE there is no wonder that it
is the most difficult aspect of figuring for most glass pushers.

	These are my theories. But then I think they are more than theories.
I have proven each to my satisfaction.


Jerry

PS   That is 5 of them. But I do not find them contradictory in any way.  




-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Guy Brandenburg
Sent: Saturday, February 28, 2004 1:14 PM
To: ATM
Subject: [ATM] Theories on what causes TDE

How many different theories can y'all (or youse, if
you prefer) come up with on what causes Turned Down
Edge during figuring or polishing? 

(I know of about 5 such theories, and they are mostly
mutually contradictory. I also know - by experience -
how to get rid of it. But having TDE is a real pain in
the a**, and it would really be nice if we could
actually figure out, for real, by experiment, what
causes it and thus how to prevent it.)

Guy Brandenburg