RE: Re: [ATM] First Lap Tips - THANKS!

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

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-----Original Message-----
vorblesnak@peak.org
 
Wait a minute, we aren't done.  Are we done??

David Davis
Toledo, OR

> Thank you all for the incredible advice.  I've shaved a few laps
> worth of trial and error from all of your wisdom.  It is greatly
> appreciated.
> Thanks again!!!
> Bill
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Heck no! We aren't done. Bill can't get off that easy!

	Those who have the knack for cutting the facets with a razor blade
swear by it as the best way. Just like other parts of TM, with practice the
razor blade method looses its mystery and becomes as easy as parabolizing.
There is a proper angle to hold the blade, proper in pitch yaw and roll, and
a proper blade speed. Keep the blade cold by dipping in ice water a few
seconds every now and then and don't let pitch build up on the blade because
that is frequently what grabs the lap and pulls a chip off. A little
chipping is no big deal and big chips can be put back in place and pushed
lightly to stick it back in place if you wish. Flow rate of the lap can be
increased by cutting the channels deeper or decreased by making them
shallow. Absolute precision in the size and shape of the facets, while
making a "pretty" lap doesn't make it work much better. In fact you can't
align a bunch of same size squares in a straight lines and rows on convex
surface, or concave for that matter, and have uniform channels. There will
have to be "channel creep" or "facet distortion"; a little bending of the
channels or slight change in shape of the facets from center to edge. Even
if you use a mold you will probably want to trim the channels by blade to
keep the channels open as you work.

	Then there are those who use a mold and swear by them. If it makes a
good lap and you find it easier or less messy or you just like doing it that
way that is all that matters if it works. The razor guys, "Stitch Finger"
Sam, "Double Edge Laceration" Larry and "Bloody Lap" Bill will say "Why
spend the money for a lap mold?" Well, razor blades cost money too. And you
can go through some blades trimming laps. They don't last forever and I
wouldn't shave with them when they are through their lap trimming days.
Someone who uses the molds will have more to add to this paragraph.

	As for pouring I prefer the "cool melt" that when poured is just hot
enough to flow slowly to the edge and builds up a dome in the center. Then
place the mirror painted with polishing slurry on top and pressing the warm
pitch to come to the edge. The domed pitch will contact the mirror in the
center first and there will be no significant air trapped in the center. If
you have that air in the center or the "bugled facets" as previously
described just cut a shallow channel to it so that the air can escape when
you press. If you don't do that you can loose significant lap thickness
before getting full contact. This method also allows a little adjustment in
where you press to get a more uniform thickness of the lap.


I have only used turpentine once to soften pitch and I had much the same
result as Richard. I used very very very little turpentine and the pitch was
very very very soft afterwards. 

	The real answer is to buy soft and hard pitch and mix to your
satisfaction. A pitch tester will help here. I had a pitch lap that worked
perfect for me so I put a 1/4" steel ball on that lap and made a jig that
held a 2 pound bag of coffee (almost a kilogram for metricites) putting all
the weight of the bag on the ball and in five minutes I saw that it went
just slightly more than 1/3 of its diameter into the pitch. So I melted some
soft pitch and some hard and mixed small amounts in different ratios and
poured into 3/8" thick X 1 inch disks (about 10 X 25mm) and tested for
similar results with the steel ball and coffee. The mix that gave the same
result as the good lap is "normal" hardness for me and "soft" and "hard" for
me is relative to that.  The key here is I had a lap that from experience
worked better than any in the past so I tested to have the same hardness. It
is important to give some time before measuring hardness because the lap or
test tiles will harden a little more in the first several hours after
pouring. I wait a day. I have found the burgundy pitch from Willmann-Bell to
be just about right without alteration and the same for Edmund Scientific's.
Of course there is a little variation in hardness in what they ship but not
so much and I have some variation mixing my own. When I say just about right
I mean for working at 70F degrees.

	As important as pitch hardness is working temperature and of course
hardness is related to temperature. I might let the temperature vary when
polishing but I control the temperature when figuring and won't work if the
temperature is more than 2 degrees off from 70F degrees and if it hasn't
been that way long enough for the mirror and lap to be stable at that
temperature. I feel sorry for those who can't do that because it is an
element of control of the action. Whatever cliamate you have it is still
important to figure at a uniform temperature. I press with only the weight
of the mirror when figuring and leave the glass and lap together when not
working or testing. If I won't be able to work on it for a few days I might
take them apart to keep the lap from getting thin while not working.  


	I don't intentionally center the facet/channel pattern on the tool
but once ended up that way and had no trouble. I understand the theory of
how that could put zones in the mirror but there is enough variability in
working by hand, both in cutting the channels freehand and in the polishing
and figuring strokes to make this a non-problem for me. And besides as you
trim the facets when they fill in you can move the channels by only trimming
one side of them if it really bothers you.

"Sutures" Jerry 



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