When we get new
people in the telescope making class
a common discussion gets underway about what they need to get
started. One of the items on the checklist is called a "tool".
In mirror grinding and polishing we use a tool as the surface
on which grinding compounds are placed. The mirror blank is
this surface and the compound grinds
away glass to give the blank the shape and surface we want.
few years ago it was common for those in the know to simply
use another plate glass blank of the same diameter as the tool.
Sometimes ceramic tiles would be glued on, and sometimes it
would just be glass on glass. Willmann-Bell still provides a
second identical Pyrex blank in their mirror kits just for this
purpose. Heck, that's another mirror if you can come up with
a way around using it as a tool.
another method folks began to use. This method was to place
a dam around the blank to mold plaster-of-Paris to the same
diameter as the blank. After it was cured the plaster would
be sealed with something like fiberglass resin to waterproof
it. After the plaster and resin had cured the ATM'er would epoxy
ceramic tiles to the top of the newly created tool. Potential
problems with that technique included tiles popping off at random.
with today's fantastic materials it doesn't have to be that
complicated, and a "new" technique can now be employed.
This technique avoids resins and epoxies completely by embedding
the tiles into the tool. I've seen it successfully used multiple
times, including my own 10" project. What we need to get
1 box of
"yellow dental stone" or "buff labstone" (both
are a strong water-resistant plaster-like material). The stone
is sold in 25 lb. boxes at dental supply businesses and in San
Diego you can get it from Pozin Dental
Supply) or you can try FixAll from Home Depot but this has
not been extensively tested. The important part of this is that
you use a plaster or stone that is relatively waterproof. We are
not going to use any sealants.
bucket for the dental stone.
1 pair of
latex gloves (optional - just to keep your hands clean).
1 sheet of
ceramic tiles (Home Depot has sheets
of 1"x1" tiles
that are great. Other tiles can be used - just make sure that
they are solid throughout. DON'T get the tiles with the soft plaster
filling). Make sure that you have enough to cover your blank's
Dam to create
the mold for the blank - a flatwindow blind works great
for this. Other dam suggestions include using manila folders (thin
cardboard) cut into 4" wide strips and taped together to
create one long strip capable of going around the blank's diameter.
The dental stone should cure before it has a chance to go through
writing or creating circles (or a string that you can cut and
trace around a pivot point to write a circle - you'll see why
or some other flat, thin saw (optional).
some other vegetable shortening (optional).
Basically,the idea is to cut the ceramic tiles prior to making the mold
and place them on to the top of the blank before pouring the dental
stone. This embeds the tiles into the dental stone, avoiding the
epoxy. This has two terrific benefits. One benefit is that it
is rare that a tile would pop off (I haven't seen it happen yet,
but I suppose that its possible). Another benefit is that it directly
molds the tiles to the shape of the curve you may already have
generated. Since the hogging process results in thinning tiles
we are recommending that grinders in the class use pipe flanges
or some other iron disk (for example a sub-diameter barbell plate)
to hog out the depth and then make this tool which almost immediately
conforms to the shape already there. So let's get started:
out the circle. Set your compass (or string length) to the
radius length and trace around the tiles to produce a circle of
tiles that would be about 95% of the diameter of the blank.
Note: You want to draw this circle so that the middle tile
is offset to the side of the center of the ceramic tiles disk
you will be cutting. Don't get this confused with centering the
cut disk of tiles on the blank when you pour the dental stone.
You still want to do that, but you don't want the middle tile
in the center of the tool. Offsetting the middle tile from the
center of the tool gives you randomness. Having the middle tile
in the center can result in rings, etc. You'll see what I mean
when you do it.
tiles. After tracing out the disk, separate the extra tiles
that are outside of the disk and then use the tile nippers to
cut the sheet of tiles into a disk. Do not be overly concerned
with tiles on the outer edge that will be cut to small triangles.
You can stand to lose some of these, so go ahead and pull them
off. Probably a good rule of thumb is that if the resulting cut
tile is going to be less than 1/3 (or maybe even 1/2 if there
aren't too many together) the size of a full size tile just clip
it off. The randomness of your stroke will completely erase any
negative effects these missing sections might cause.
the edges. Once the disk of tiles is cut out you'll need to
use the sharpening stone to make a bevel (AKA chamfering) around
the edge of the tiles. This doesn't have to be that dramatic.
Just take off the edge. The fact that we are using a somewhat
sub-diameter tool will keep us from having to worry about the
edge of the tiles and their bevel.
blank. Form the dam by wrapping the window blind around the
blank and taping it off (duct tape works great). Make it tight
to avoid losing too much dental stone down the side. You can also
use something like the manila folder cardboard strips noted above.
The dental stone will soak through, but not compromise the integrity
of the dam or mold. Simply wet it once cured and it should peel
the tiles on the blank. Center the disk of tiles on the blank
(the side of the blank to be shaped and polished is UP; the side
of the tiles that will be doing the grinding is DOWN; just in
case you missed that).
the dental stone.NOTE: Dental
stone cures fast. Don't waste any time. The amount and consistency
you'll need is an art more than a science. For the tool's thickness
I guess a good rule of thumb is to divide the diameter of your
blank by 6. For example, a 10" blank would warrant a 1.6"
thick tool (10 / 6 = 1.6). I usually recommend around 2"
for 10" blanks or less. The consistency range is pretty wide.
I've mixed it very thin (by accident) and very thick. Both have
worked great. Mix it so that you can pour it out relatively easily.
dental stone in the mold. I usually pour into the center only
to avoid moving the tiles with the force of the pour, but this
is probably just superstition on my part.
bubbles. Once it is all poured tap and shake the mold to bring
any bubbles to the surface. Smooth out the dental stone on the
Give it about 20 minutes to settle and firm up. You will note
that the tool will heat up with the curing process of dental stone.
This is normal. Thicker tools might need more time, but dental
stone cures very quickly and is very uniform in its curing.
the dam. After it is cured enough to firm up you can remove
the dam. After removal I usually let it continue to cure and cool
the tool and mirror. I have seen this happen easily in some
cases and hard in others. If it sticks then simply use a utility
knife around the edge between the tool and blank to generate a
space, breaking the seal. Continue to do this until they separate.
It shouldn't be that dramatic. I've also had it suggested that
freezing them in a freezer works as well, but try that at your
own risk. Optional - its been suggested that a thin layer of Crisco
on the mirror blank would completely remove this potential problem.
I'll probably try that on the next run. It should not affect grinding
or setting of the stone.
out the channels. Once you've separated the tool and blank
you might need to clear out the spaces between the tiles. You
can do this by running a miter saw between the tiles, or you can
also try using the edge of the sharpening stone to accomplish
this as well. If you used the suggested tiles above then you'll
have about an 1/8th of an inch of space between the tiles.
that many pockets will develop in the channels. These pockets
can hide grit. A good, solid scrubbing with a vegetable brush
can clean these out without a problem.
get to the figuring stage it is recommended that you make a new
tool as the base for the pitch lap. Use the above steps except
for the ceramic tile. Remember to scrub the surface of the tool
with a stiff brush to help give the pitch sticking power. You
are doing this just in case you have to go back to the original
tool later. <click here
for more ideas on preparing the pitch lap tool>
There you have
it. Your tool should be set and ready to go after a day or even
a couple of hours later (but it will continue to cure slightly).
I've usually waited 3 or 4 days to make sure its fully cured, but
again, that's probably just superstition. If you have any questions,
or if I missed anything DON'T hesitate to e-mail
me and let me know.
P.S. I'd like
to thank the following people for their ideas and additions to this