Re: [ATM] Stubborn TDE

[Mike Lockwood <melockwo@uiuc.edu>]

David,

David Steinhauer wrote:
> With the warmer summer temperatures, it takes about 2-3 hours for
> the channels to close.  I am amazed how much the softness can
> change with temperature, though.  In the spring (when I was doing
> hour-long polishing sessions), I polished for more than 10 hours
> before I needed to trim the channels.

2-3 hours for channel closing should be OK.  10 hours might have been 
to hard for polishing (or possibly good for making a sphere), but 
definitely too hard for figuring an F/5 mirror.

> I would say I am using moderate pressure, something like 10 lbs.
> The stroke is COC or a narrow W (either one seemed to have the same
> effect) with a stroke length of 1 inch.
> One thing I'm thinking now is maybe my 1 inch stroke is too short -
> so short it isn't doing much.  I might try increasing the stroke
> length to 1.5" (1/4 diameter).

Yes, the stroke length should be around 2" on average, that is, the 
mirror/tool should overhang approximately 1" at the ends of the 
stroke, with 2" of movement between.  Ideally this will vary in a 
random way, but not too much.

> The mirror is Pyrex, 1 inch thick.  The tool substrate is dental
> stone, also 1 inch thick.  The tool is full-size (6" diameter).

Sounds OK.

> Jerry suggested I try doing accented pressure closer to the edge,
> since the TDE is narrow. But I think I got about as close to the
> edge as I can - about 1 mm.  I applied pressure with my thumb at
> the very edge of the tool.  I tried various narrow and wide W
> strokes with this type of accented pressure, to no avail.  I
> wonder, should I try a straight forward and backward stroke (rather
> than a W) when I apply accented pressure, with this narrow of a
> TDE?

Jerry is trying to help you remove glass near the outer part of the 
mirror.  While this will work as a fix, I recommend that you first 
learn how not to get a TDE while doing normal polishing and learn how 
to fix it second.

You could try any combination of a straight stroke with W-strokes for 
polishing.  I try to vary it.  You might also try switching MOT and 
TOT.  Polish for 15 mins. minimum in each position, and at least half 
an hour overall.

How much weight are you using to press?  I assume the pitch is trimmed 
so the lap is a bit smaller than the mirror?

	Mike Lockwood

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