Re: [ATM] First Timer Polishing Questions

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Meeks Robert-PT1784 wrote:
> If so, then I wonder why after ~15 hours of polishing I am still 
> seeing such a strong scatter reflection?  I have read conflicting 
> reports regarding the amount of pressure required.  In general, I 
> have not been using much downward pressure, so I am wondering if 
> anyone could provide a description as to the amount of pressure 
> required to get a proper polish?

Richard's previous reply of about 0.5 PSI is the first time I have
seen it quantified (but I haven't looked for that).  That's about 25 
lbs for an 8" mirror, which is probably consistent with my practice 
when working by hand.

> I generally have to re-channel after 20-30 minutes of polishing
> even with only moderate pressure.  So, I've been pressing with
> ligher weights (or else the channels close) to get good contact and
> then putting the lap in the freezer for about 10-15 minutes to
> harden it up. Seems to be working well.  I can go for an hour or
> two this way without re-channeling.  Just curious if anyone has any
> concerns with doing this?

I am glad to see someone judging pitch hardness by the time it takes
the channels to close - I have been trying to promote this idea,
because I feel it is the best way to judge pitch through cyberspace!

Anyway, working a cold lap on a room temperature mirror will cause
figure problems.  It may even cause the lap to go out of contact while
polishing, and it may have a bit to do with your polish issues.

I recommend that you cook the pitch for a while and make a new lap
from it.  This will harden it.  Don't get it too hot (should still be
syrupy) but you might need to cook it for half an hour or more.

Jerry wrote:
> Do you see a difference at the edge or center.  One almost always 
> lags the other in polish. If it is uniform it is likely just some 
> residue on the surface.  The usual advice of others (and me too) is
> that if you can rub it off it is just a residue.

That's a good point.  The difference in polish is a good way to judge
progress and see how your technique is causing the mirror to polish.
I tend to have "spots" of residue on my mirrors, which rub off to
reveal the true state of polish of the mirror.  I have been using a 
10X loupe to look for leftover pits, and a bright light or laser 
illuminating the surface from the side.

> I rarely ever press with weight in figuring. I feel that the weight
> can only bend something and the bend will have to be relieved 
> before working. The entire idea of pressing is to get the lap the 
> same shape as the mirror. Bending them in pressing just doesn't 
> seem to be a good idea to me and I get good contact pressing with 
> only the weight of the mirror or lap. That is another THEORY of 
> mine. I know others do OK working with a different theory.

I've experimented with this a bit, so I can offer some evidence to 
back up your theory.

With the softer pitch and lighter pressing weights used during
figuring, any error in the pitch surface due to bending of the mirror
or tool will be gone in a stroke or so, so I don't worry about it.
Weight is sometimes not necessary for pressing, but it does speed up
the process of getting contact.

Of course, a thinner mirror will cause the bending problem to become 
worse, and other variables come into play - how to support the back of 
the mirror while pressing, how to distribute the weight evenly over 
the back of a thin mirror or tool, etc.

With harder pitch and heavier pressures necessary for pressing a lap
to work on a large flat (say 10"), then the deformation of the
mirror/lap by the pressing weight is NOT negligible.  There is no
substitute in this case for simply waiting for the pitch to flow, or
doing a warm press and waiting for the temps to equilibrate.

	Mike Lockwood

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