ATM Making Pitch Laps

[UC007@aol.com]

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) sugested 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.

Continuing on with the lap with chips, cold pressing for an hour with thirty
pounds of weight was followed by retrimming all the facets. This was followed
by again cold pressing with only the weight of the mirror for an hour. Then
polishing was begun in the usual manner, 1/3 diameter COC with slight W
stroke. I was happy to find the polish progressing evenly all the way out to
the edge. The chips out of the lap didn't appear to have caused any harm.

The casting in one piece is certainly attractive, if the channel cutting
procedure can be improved on.

Regards,
Bill Fries, EAS