RE: [ATM] Turned Down Edge

["Jerry" <wa4guu@verizon.net>]

	All reliable sources would say that you need the channels in the
pitch while figuring an 8". Maybe on a small piece of glass, around 2 or 3
inches or less you might have no trouble without channels. You should never
work when they have closed. The channels do not necessarily have to go all
the way to the tool. You can use the depth as a variable to control the flow
rate somewhat. Deeper flows faster and shallow flows slower. If you think
your lap is soft use shallower channels. You will have to trim more
frequently but it will slow the flow.
	As you polish and the channels narrow you will usually see small
local spots where the facets on either side of the channel start to pinch
together. It might only be one spot and after you have recognized it, it
should only take one to tell you that the channels need trimming right now!
When this happens look at the overall channel depth and then set that as
just a little less than the minimum channel depth for that lap. Trim before
getting to the point of the facets pinching together. Those small spots will
not flow as well as the rest of the lap and you should feel some grabbing or
pivoting at that point. If you are working MOT and the glass is clear you
can see a color change of the rouge/pitch at that location also. That is
because the flow is limited there and therefore the pressure is increased at
that point on the lap squeezing out the slurry. The increased pressure
causes more heat at that location and since the pitch can't flow into the
channel directly because it is closed it has to move parallel to the channel
first to find open space to flow into. Even though the pitch at that point
is warmer and would flow easier it has farther to go and there is other
pitch in the way. It not only makes figuring harder, those localized spots
of more pressure heat and wear will cause dog biscuit.
	I would not worry about the micro-facets (from the netting) closing
more in the center. In fact it sounds like maybe this would be a good time
for you to try letting them close all together. Depending on the pitch
hardness and maybe some other factors, the micro facets are not always
helpful particularly if the pitch is soft. I have had laps that it helped
and laps that it hurt to use micro-facets. In fact I usually only use the
micro-facets while polishing and let them close for figuring. One of the
glorious properties of pitch is that they will go away if you find that they
aren't helping and can be put back easily if you find that they were
helping. Your micro-facets closing in the center first is probably because
that is the area of the lap that is working all the time on every stroke and
is warming more from the extra work and not being uncovered by the mirror
and exposed to cooling air.

	ATM is one of those endeavors where in some aspects the beginner
needs higher quality equipment/supplies than the skilled worker. In most
fields the beginner does not have the skill to use higher quality or more
sophisticated equipment. The Foucault tester is one of these things. The
beginner needs a stable precise tester to learn the process. The skilled
worker knows how in use to minimize the weaknesses of a lower quality
tester. And pitch is one of these things also. A skilled worker will be able
to adjust for difference in pitch hardness or know when the lap is so wrong
another is needed. A beginner doesn't know what to expect with the right
pitch let alone how to adjust if it isn't right. He needs the right pitch to
come to the proper understanding of figuring. The thumbnail test has never
shown me much about the pitch hardness. Seems I can put a very similar dent
in just about any but the hardest or softest pitch. The answer on pitch
hardness is not easy except to say a standard would be helpful. The best way
would be for everyone to use the same method to test the hardness. The
Twymann tester in Texereau's How to Make a Telescope (published by
Willmann-Bell) would be a good standard tester. But most people just aren't
going to build the tester. It would save them countless hours if in the
beginning they could see how important it is to have the right pitch at the
right temperature. So then the answer is to use a particular source of
consistent hardness pitch and use it at a particular temperature. Here I
will say as I have before that Willman-Bell and Edmund Scientific Co. sell
burgundy pitch that is pretty consistent and works well at 70F degrees. I
haven't used Gugolz so I don't have a clue about what hardness the various
numbers are and how they would compare to the burgundy. A few degrees
difference will make the lap behave differently. I will only figure when the
temperature is within 2F degrees of the temperature I have found the pitch
to work best at which is about 70F degrees.
	Here I will ask for some other opinions: In my experience I have
found that what ever the hardness of the pitch (within reasonable limits) it
will work best somewhere near 70f degrees. If using a pitch that would be
harder at 70 degrees because you are working at 80 degrees I have found that
it might work at 80 but it will still work best at 70. Anyone else out there
observed this behavior?

Well I have rambled again still haven't run out of babble. But I will save
it for the next edition.

Jerry


-----Original Message-----
Rich Ball
I took a look at my lap today and noticed that the impression left from the
netting was more obliterated in the center than on the edge. That suggested
to me that edge contact was not as good as center contact.

My pitch took a thumbnail impression in about 15 seconds, depth less that
1/16th" by my eye.  Pitch harness ok?

And here's a question:  Are the classic grooves in the lap of critical
importance at the figuring stage of life?  I have read various opinions from
seemingly reliable sources.

Thanks again,

Rich Ball
Oak Park, Illinois
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