ATM Variation on Randy's Theme (Hydrocal tools)

[CZambuto@aol.com]

Randy Johnson wrote:

>We use it regularly in our mirror making workshop for the ATM group here in
>Seattle.  We actually hog out the glass on a roughing tool to get the
>initial radius of curvature, cut a disk of tile from a sheet of 1" square
>tile to match the mirror diameter, make a 1" high dam  using two inch
>masking around the mirror blanks circumference, smear a little on the face
>of the mirror blank for a mold release, put the tiles face down inside the
>dam on mirrors face and pour in your mix of Hydrocal.  Allow the Hydrocal to
>set up for about 20-30 minutes after casting, remove the tape and separate
>the mirror from your newly formed tool.  Use something like a nail head trim
>out between the tiles.  (you need not remove all of the excess plaster
>between tiles.)

>Voila, a grinding tool matched to the radius of your already hogged mirror,
>ready for smoothing your mirror's surface.  Allow the plaster to dry out for
>three days to a week, a couple of coats of shellac or other water proof
>sealant to seal up the plaster on the back side of the tool and (optionally)
>some brushed into the space between the tile.  

>Does require the patience to let things sit for a week or so while it is all
>dried and sealed but yields an excellent tool.


Not too far south of Randy's TM group is the Nisqually Valley Telescope
Maker's Workshop, near Olympia, where we teach telescope making to the
general public. We also use tile tools as we grind a lot of mirrors. I like
Randy's technique, it sounds excellent. We use the same material (Hydrocal)
but I have found that after setting up for 24 hrs following the pour I can
put the tool in the oven on the lowest setting (ours will set electronically
at 170 degrees F) for 8-24 hrs, depending on the tool size, which ranges from
6" to 12.5". When the tool is too hot to touch, that means the water has been
evaporated out, and it is ready to go. 

We have not tried Randy's technique to date of putting the tiles on for the
pour. Our method is to pour the hydrocal on the hogged-out glass (with saran
wrap covering it, and a plastic strip dam). After the initial cooling from
the chemical reaction of the Hydrocal, about an hour or so, we strip the dam,
and take the tool off the glass. 24 hrs later it goes into the oven. Once the
"pie is baked", it is ready for coating and tiles, but here's what we do
different:

While the tool is still warm, we bevel the front and back edges at 45 degrees
with a grinding disk, and prepare a mixture of finish resin from our local
marine supply. The tool is inverted on the work surface, and elevated on a
disk of something, such as an ABS drainage pipe coupling. The tool is leveled
with a torpedo level, and we pour resin over the back and allow it to run
down the sides. That sets up in a few hours, to a day, depending on the resin
mix and the temperature. Then the tool is inverted again, (face up) the drips
are ground off the front edge by the grinding disk, and the tiles are
prepared. The mat of tiles is pre-cut (nipped) to the diameter of the tool,
and another resin batch is mixed. The resin is poured over the face, smoothed
around and over the sides by a disposable paint brush, then the tiles laid on
top. The brush is used to stroke the excess resin from the grooves, and the
tool left to set up. The grinder is used once more to remove the drips from
the back edge. The thin resin layer left on the surface of the tiles grinds
off quickly while the tiles are matching up to the glass (this would
typically be 120 grit).

This process gives us a resin-sealed tool with a tile face and nice channels
in about 72 hrs. The actual labor time spent on each one is considerably less
than an hour; the cost in materials for each tool shouldn't exceed a couple
of dollars, and we have never lost a tile from a resin-coated tool during
regular grinding. We've not made new tools in awhile, as these are reusable.
The old tools are used to rough-grind new mirrors (right down to the hydrocal
sometimes) then we mix a batch of resin and apply new tiles for the finish
grind.

-Two years ago for our Christmas class we pigmented our resin red and green.
It made for very lovely tools.

Carl Zambuto
optician/educator/Director of the Nisqually Valley Telescope Maker's Workshop
Rainier, Wa