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 moved against
this surface and the compound grinds
away glass to give the blank the shape and surface we want.
A 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.
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 started is:
- 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.
- 1 mixing bucket for the
- 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 diameter.
- Dam to create the mold
for the blank - a flat window 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 the folder.
- Tile nippers.
- Compass for 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 soon).
- Sharpening stone.
- Miter saw or some other
flat, thin saw (optional).
- Crisco or some other
vegetable shortening (optional).
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
- Calculate the radius.
We need to get the radius of the tiles for the tool by taking your blank's
radius and multiplying that by 95%. For example, 95% of a ten inch blank's
radius (remember that a circle's radius is 1/2 the diameter) is 4.75"
(or a diameter of 9.5").
- Trace 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.
- Cut the 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.
- Bevel 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.
- Dam the 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
- Place 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).
- Mix up 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.
- Pour the 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.
- Clearing 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 top.
- Curing. 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.
- Remove 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 off.
- Separate 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.
- Clear 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.
- You'll find 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.
- When you 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.
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 tutorial:
- Scott Rychnovsky
- Richard Schwartz
- Eli Vandevoorde