Re: [ATM] pitch lap help

["Bob May" <rmay@nethere.com>]

People keep geting into a tizzy about pitch laps for no good
reason.  They don't have to be perfectly formed and they don't
have to be of a certain thicknes or they are bad.
I've done pitch laps by many different means, including using a
propane torch to melt the pitch to a watery consistency but keep
coming backk to the cooler methods.
The double boiler will do well with even the 73 pitch.  I've even
done the job by getting the pitch warm enou8gh to pour the pitch
as a blob on the lap base and then putting everything under water
and pressing that way.  My turkey roasting pan did the work for
that one and I used a "mold" made from some silicone rubber with
lots of holes punched in it to get the channels desired by many.
The fi9nal layering from top to bottom was glass, microfacteing
material, mold, pitch and finally the lap base.  The temp was
slowly raised  and lowered because thge glass was sensitive to
temp variations (one of Schwartz's built up mirrors from plate
glass) and the glass survived the process.
The thing here is that you don't have to get the pitch so hot
that it boils and smokes but just warm enough to whee it flows
under a little pressure.  Bubbles in the surface don't matter and
rough edges on the channels don't matter either.  The  only thing
that does matter is that most of the surface matches the surface
of the glass.  Note that I didn't even say all of the surface,
just most of the surface.  With pitch that soft, the pads wil be
pressed down and the rest of the lap will mate with the glass by
the time that you hae a polish done.  I do like to see about an
eighth to a quarter of an inch for the thickness of the pitch
when the lap is finished and ready for work.  Pitch is kind of
like butter in that at the working temp it is fairly hard and
only a little increase of temp will soften it up to where it
flows under pressure.  It doesn't have a phase change like ice
does.
Also, don't bother to chip off the pitch if it is thin but just
warm it up a bit and pour on some more pitch.
I generally don't use a dam as I don't get the pitch that hot and
control the edge by pressing the pitch over towards any bare
area.  You don't even need to bring the pitch out to the edge of
the tool or mirror. A slight subdiameter lap will work fine and
often helps prevent  TDE.
Lastly, pour the lap and use it.  For a Qand D lap, see the way
that John Dobson makes laps.  His are fquick to make and do work
perfectly well.
Bob May
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