Re: ATM 10" help - TDE?

["Ken Hunter"<kb7h@onemain.com>]

On Thu, 26 Sep 2002 10:29:22 -0600 Harold <adams@rmci.net> wrote:

     This means the overhang is on the forward stroke only?   Also, im not
clear what you mean by steping sideways. Can you elaborate?


Harold...

My description assumes HAND WORKING with a barrel or other work support as is
typically used and before anyone complains... 

I know that it does not really work the way I describe. The glass does NOT get
pushed and dragged around. It is really being worked, abraded, polished etc
during the strokes but I find that when teaching beginners, it helps to
visualize the action in this way and so far it has always worked for me. 

So to proceed...

The pressure is applied on the outward going portion of the stroke and it
helps to imagine that you are pushing glass uphill to the edge with the center
of the tool. 

Once you do 2 of these strokes, you will want to work the rest of the pie
shaped segments of the mirror by side stepping around the barrel or work
support and also use another axis of the tool so you will need to rotate it a
bit also...

During the return stroke I suggest using mirror weight ONLY due to the
apparent fact that hand pressure (placed at the edge of the TOOL by
overhanging fingers while dragging the tool towards the body) tends to put
undue localized pressure right at the edge of the mirror which "plows" into
the tool and REALLY does roll the edge.

This is a REAL TDE, it's difficult to remove and is best avoided. It is
usually a narrow rolled ring around the extreme edge of the mirror and is
usually less than 1/8 or at worst 1/4 inch wide. 

Having a REAL TDE is much worse than having an apparent TDE where the edge
receives little or no attention because with the REAL TDE, the WHOLE SURFACE
OF THE MIRROR must be brought down to correct the defect. An apparent TDE can
be a zone or more in width but can usually be easily repaired or avoided by
properly attending to the edge zones. 

To avoid confusion, maybe this should be referred to in the future as an ATDE
for Apparently Turned Down Edge?

The "W" stroke is a modification (if you will) of the stroke I've described
(or maybe it's vice-versa, doesn't matter) where the center of the tool spends
a lot more time in the outer zones *AND* (the center of the tool) avoids
working the central zones. Properly done, the TOT "W" stroke makes several
strokes near the edge zone then proceeds rapidly through the center of the
mirror while keeping the tool moving then making several strokes as before at
the edge zones. 

Properly used, the TOT "W" stroke IS BETTER than my described stroke due to
the combined action at the extreme LEFT AND RIGHT zones of the mirror but... I
find that most people (newbies in particular) do not avoid the center properly
when doing the "W" stroke and end up doing COC strokes instead throught the
center zone causing a constantly deepening center and the apparent TDE. 

Another possibility using the "W" stroke is a REAL TDE caused by the above
mentioned hand position used when pulling the tool towards the body. 

After making a few mirrors, these natural tendencies to mis-apply strokes
usually (hopefully) goes away with practice and proper strokes become second
nature but in the hands of NEWBIES working on their own, any stroke not well
understood can be hazardous to their mirrors figure.

I hope this clarifies some of what I've been trying to put into words.

Ken Hunter