Re: ATM Making Pitch Laps

[Bruce Swayze <swayze@europa.com>]

At 08:06 PM 6/13/97 -0400, you wrote:
>For a number of years now I have been making my pitch laps per Texereau,
>casting long strips of pitch and cutting them into squares, and sticking them
>to the previously turpentine mopped glass tool in the usual square pattern.
>This has always worked well though the process is time consuming.
>
>Someone on this list (I am not sure who) suggested casting the lap by placing
>a dam around the tool and pouring the pitch on if the mirror was slower than
>f 6 and up to 12" in diameter. Presumably these restrictions were necessary
>because of the much deeper sagitta of the larger faster mirrors.
>
>I tried this on an 8" f 12 .  Shortly after pouring the pitch I took a
>straight edge and lightly made the decentered checker board pattern on the
>lap. Then using these marks as a guide, when the lap had cooled over night, I
>used a cross cut saw to cut channels through the pitch to the glass. As long
>as I was careful and kept lubricating the cut with water, I had few problems
>with chipping facets. After finishing the saw cuts, I widened the channels
>with an exacto trim knife. This process was a laborious one, and occasionally
>a big chip of pitch would pop off a facet. Admittedly, my technique improved
>somewhat by the time I was finished...  There has to be a better way to cut
>channels. I'm sure if I had been on this mailing list over the past few
>months I would have been exposed to a better method. Unless I learn of an
>easier softer way of cutting channels, I think I'll return to strips of pitch
>cut to squares.

Bill,
We have been using the strips cut into squares for a long time, too.  My
brother even made a simple mold to pour the pitch strips into, and it makes
the process easier (line it with alum. foil). 

However, we have recently been experimenting with a very simple method.
It's very fast and clean.  All we do is melt the pitch, let it cool until
it's quite thick, and then pour small round blobs right onto the tool.  Pour
a bunch of these all over the tool, and then press with your cerium-covered
mirror and you're done.  You may have a few near the edge that run over a
bit, or a few that run into each other, but they can be quickly trimmed with
a razor blade under cold water.  Eventually, as you polish and the facets
run together, you will keep trimming and get smaller and smaller channels,
but these laps work great!  We have found from experience that large
channels and fewer facets on the lap are not detrimental in the least bit.

Bruce Swayze
swayze@europa.com
http://www.europa.com/~swayze    Swayze Optical home page
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