Re: ATM Best lap-making methods

[schmidt@coyote.csusm.edu (Michael Schmidt)]

>Hello.  Although I'm not yet done with fine grinding on my 4" mirror,
>I'm beginning to think of the best way to make a pitch lap for
>polishing.

My postdoctoral advisor always said that if there are 7 different
recommended methods for doing something, it is because none of them work
particularly well. I think this is probably true for pitch laps.

I am no expert on this field, having just entered it a month ago. I first
tried pouring and then channeling with a wood tool while still warm,
according to Allyn Thompsons directions. Yuk. That lap went into the
freezer and ended up in my used pitch pile. Then I tried the molded pitch
method detailed in Thompson. I used 1/8" thick rubber sheeting obtained
from a local industrial supply with round holes cut with a cork borer.
(Ouch--too much rubber, too many holes, sore blisters on hands). This
worked ok for a while, but the pitch was too soft and flowed into one big
mass pretty quickly. No matter, time for method #3: channelling with
soldering iron. Works great. Only problem is the sore throat from inhaling
the vapors from the hot pitch. Even outdoors with a breeze, the vapors were
too much. And since the lap was still the same one with the soft pitch, I
had to melt the channels pretty often. Eventually, most of the pitch had
flowed into the channels and melted or chipped away. Time for Lap #3,
Method #4. Cooked the pitch longer. Made a nice "curved square" mold from
the same 1/8" rubber, 3/16" wide channels around 3/4" curved squares (used
same radius jig as for Thompson plan) cut with a sharp knife. Fewer cuts
than the Thompson design, and no painful cork borer. Took maybe 30 minutes
max to make mold. Beautiful lap, works great, no need to hot-press for a
long time because there are no ridges (as can sometimes occur with
soldering iron) and the lap is cast to the shape of the mirror. I've found
my method. For now. (I don't know if I want to make a rubber mold for the
36" in my future....)