Re: ATM pitch lap problem (?)

["Bill T." <twentiethwave@hotmail.com>]

Lawrence Sayre wrote:
> After channeling my pitch lap with a soldering iron today I now have
> raised ridges (some pronounced) along the sides of a number of the
> channels, and also about 3 or 4 drips of pitch on the tool from where I
> inadvertently lifted the soldering iron out of the grooves.  It looked
> so beautiful prior to channeling, but now...  :-(
>
> My tool is POP (actually Tuf Stone, which is a Hydrostone derivative).
> What is the best way to regain the press fit with my mirror which went
> so well prior to channeling?  Should I heat the tool and mirror in hot
> water and then hot press for ~5 or so minutes?  I'm not sure the POP
> tool will hold up well in hot water, and my mirror is plate glass to
> boot (can hot water immersion shatter it?).  Should I cold press with
> weights for a couple hours?  Etc........?  How much weight, for how
> long, with an 8" mirror and my problems?
>
> Can soldering iron channeling be done without creating problems such as
> mine?  If so, either my iron or my technique (or both) need a change.

Lawrence,

Don't worry about the raised ridges, they'll quickly blend back into the
pitch mass.

The pitch drips stuck on the surface would be something to worry about
during the last stages of polishing and during figuring -- they can lead to
optical "gouges" on the mirror surface, painfully visible under the Ronchi
and Foucault tests.  You can get the same problem from broken-off pieces of
pitch getting pushed into the contact surface.  If you ever notice broken
off pitch pieces on your mirror, be sure to stop and remove all the pieces
before they get embedded in the surface.  (This is one reason why I like
soft Gugolz 55, it doesn't crumble.)  But for the early polishing stages you
don't need to worry about the drips, there will be enough time for the lap
to blend itself out before polishing is complete.

I usually use Hydrostone lap bases, either bare or minimally painted with
pitch dissolved in Turpentine.  I never have problems with broken off gritty
particles, even after the base has been wet for long periods and then
handled roughly.  Hyrdrostone is specifically designed to be water
resistant, and my experience backs this up.  Of course the Hydrostone has to
be very well mixed to start with.

My favorite method of softening the lap is to use one or more 250 watt heat
lamps at a distance of about 15", with the lap rotating on a turntable.  It
takes roughly 5 minutes to warm up enough for a good press, or about 20 to
30 minutes to be rechanneled by press-grooving with a piece of steel 1/4" by
2" by 36".  I poke the pitch with my fingernail to test for softness.  This
is much faster than the soldering iron technique.  But a lap rechanneled
with a soldering iron does have the advantage of pressing a little faster
and easier by virtue of not having the lap surface rippled so much -- it's
all a matter of trade offs.

Bill T.