[ATM] Jay's TDE - Tenacious

[Mike Lockwood <melockwo@uiuc.edu>]

Jay,

> The -2.513 got into my Sixtests input files before I really knew
> how to properly set them up, so it is meaningless

Okay, that's good.

> My lap is pretty hard - synthetic pitch in a room at about 68
> degrees F.  The channels really don't close up unless I do a hot
> press, which for me is a disincentive to doing hot presses.  When I
> reopen the channels, using even a new utility knife blade, I
> invariably get some chipping, which is always a little
> disconcerting.

For a "normal" lap for this focal ratio and size, I would expect the 
channels to close after 2-4 hours of normal polishing with good 
downward pressure on the work and good contact.  So, indeed your pitch 
is quite hard by my standard, and this is causing problems.  For me, 
hard pitch is great for making flats, but it makes keeping the lap in 
contact and spherizing curved mirrors much more difficult, at least 
for me.

> When doing the Bob May trademark short half-stroke to reduce TDE I
> have been pressing REALLY hard - producing a fairly roughed up
> surface, but it does take out the TDE quite steadily.

Good.  It's a good thing to have a technique for remedying this 
problem.  The roughness is a side effect of the hard pitch.

> Because of the roughness and zoniness produced by that, I have
> recently done a couple short sessions of the TOT, short W with the
> lap edge just over the middle of zone 4 (my most peripheral zone).
> This has worked to simultaneously smooth some of the roughness as
> well as reduce TDE.

TOT should help in general.

> I am currently thinking of continuing to reduce the TDE using this
> TOT W method.  According to Sixtests, when I use a sphere as my
> target conic ( b = 0.0 ) I am quite close to that shape except for
> that TDE.

Ideally, the lap would be softer.  It would make spherizing and 
figuring much easier, and would leave a smoother surface.  There are 
three ways to make the lap softer:

1)  Pour a new lap with softer pitch.  If you only have synthetic, 
this is not an option - you can't soften it.
2)  Warm the work environment.  This may be tough in the winter.
3)  Warm the pitch and mirror.  Quite doable.

I am assuming that 1 and 2 are impractical, and that leaves 3.  The 
nice thing is that it does nothing that you can't undo.  The lap is 
not damaged, and your heating bill is not increased.

Here's another technique for spherizing, using hard pitch:
----------------------------------------------------------
First, warm the lap and mirror to a temperature of about 75-80 
degrees.  Use a thermometer to check this - it's easy to get it to how 
if you just guess.  We want to get them to the temperature where the 
pitch is softer, but not too soft.

Under moderate weight, 10-15 lbs, press for about 5 minutes.  This may 
or may not be enough.  Remove the weight and take a few strokes with 
the tool and see how it feels.  Press longer if you haven't gotten 
good contact yet.  Try to keep the mirror/lap at around 75-80 degrees.

As soon as contact is good, commence normal polishing, using slow, 
deliberate strokes (standard 1/3D stroke, but only on average), and 
downward force through the center of the tool.  Getting one palm stuck 
to the back of the tool with a little moisture works well for this. 
Polish as long as contact seems good.  As the pitch cools and gets 
harder, you need to slow down and increase the downward pressure to 
maintain contact.  There should be LOTS of friction.  Keep the 
polishing slurry thin.  If you can polish for 1/2 hr with this 
technique with good contact, you should notice improvement in the 
figure.  This often turns UP the outer zones for me.

Observe the edge of the mirror - if the edge dries out between 
strokes, this can cause more wear there and a TDE.  A light nearby 
(within a few feet) can also have a heating/drying effect.  If you 
think this might be an issue, raise the humidity of the work area.
--------------------

Anyway, working with the softer pitch will generally help improve 
contact and give you a more spherical surface.  I also HIGHLY 
recommend warming the mirror/lap before figuring sessions such as 
parabolizing.  5-10 minutes of work will do a lot at that stage, and 
the mirror/lap won't cool much in that time.  And the surface will be 
MUCH smoother.

Of course, you can also do the "short half stroke" after you've warmed 
the lap.

Good luck.

	Mike Lockwood



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