If you want to hand-make a crystal, I’d recommend using a dental
technician’s Redwing lathe. This is a ball-bearing motor which runs
very smooth, has two speeds, and a chuck which takes 1/4" or (I
believe) 3/32" diameter mandrels. Use the 1/4" for its added firmness
and freedom from vibration.
Or you could use an electric drill clamped FIRMLY to the workbench.
In any case what you’ll end up with is a more or less improvised
lathe, in which you can chuck up the workpiece (the crystal) and turn
it down with diamond tools to the shape you want.
Get a piece of 1/4" brass rod, cut it to a convenient length, say 2"
or 2 1/2" and silver solder a brass plate onto it approximately the
diameter of your watch crystal. Chuck this up and turn it down to
centre and level it so that the plate surface is flat and
perpendicular to the shank. You’ll need a good solid steady tool
rest. I use a big cube of brass I got from the scrapyard, about 3x3x3
inches. Something as massive as that you can hold a tool on it and
the tool won’t vibrate or chatter as you spin the work.
Also you’ll need one side of the crystal blank flat, because this
will be the reference when you switch to grinding out the concave
inside surface. You can do that by hand grinding - wet - with silicon
carbide grit on a flat metal or glass plate.
Warm some dop wax, and the crystal blank, stick the crystal blank
flat side down onto the brass plate by means of the wax. Make sure
there is a good flat contact between the crystal bottom and the dop
surface. Now adjust the blank while the wax is still hot and pliable
(or reheat the wax, gently, in an alcohol flame or even a candle) so
that the blank is centered. Put it in the lathe & give it a spin
holding a grease marker or magic marker against it. This will leave a
mark on the off centre spots. Adjust again, by warming the wax and
pushing the crystal around in it. It helps here if you have a piece
of rough glass to start with that has a lot of extra material. You
just cut that away and it will be centered.
Anyway you have your crystal blank centered on the brass dop. Now
you turn it down to the profile you want by spinning it while holding
a diamond tool against it. Keep it wet. This gets messy. Again the
tool should be solid, even massive. For coarse shaping like this I
use a single “segment” (the diamond bearing metal platelet) salvaged
from a tile saw blade and silver soldered onto the head of a long
3/8" machine bolt.
With that you can spin it down to profile. Keep it wet and check
frequently. The surface will be rough & scratched. Switch to a finer
diamond tool to take the coarse scratches out, always working wet.
Then finer and finer still. The progression should be approximately
like this…roughing out, 100 grit diamond, refining and taking the
rough scratches out 240-280 grit. You can make your own diamond file
by putting a dab of olive oil on a file-shaped piece of copper (beat
a copper tube flat but leave one end round to serve as handle, then
file it flat & smooth) and applying just a minute pin-head amount of
diamond grit. Spinning the work against the file will press the
diamond into the copper.
When you’ve done the fining stage you can switch to using silicon
carbide wet-or-dry paper glued onto something like a paint stick, say
about 400 grit, followed by 800, still working wet. Diamond works
better though.
You could do this stage also by taking the dop out of the chuck and
holding the crystal against a rubber backed sanding pad (wet, always
wet) on a lapidary wheel. The usual caveat applies: cut and look, cut
and look.
By 800 grit you should have a fine satiny sheen. Check for scratches
remaining from previous stages and re-sand where necessary to get
them out. Once the you’ve reached the 800 stage you can go to polish.
Do this on leather with cerium oxide (wet, always wet) or any other
stone polishing compound.
That done, gently heat the crystal, take if off the dop stick, flip
it and re-attach from the other side so that you can grid out the
inside. Here’s where you’ll need to very carefully center and level
the piece. Spin it against the grease pen or magic marker to see
where it’s off center and off level, adjust by warming and pushing.
The inside is done in the same way, except rounded tip tools are
helpful. Again, you can make your own with copper, grinding the
copper to the profile you want and embedding diamond grit with olive
oil and pressure against the work.
For polishing the inside glue some leather on an appropriately
profiled stick, apply compound, keep it wet, etc etc. Or you can use
a leather polishing disc in the foredom…or similar…still working
wet. Polishing builds up heat, and you’ll have put so much work into
the piece by now that you don’t want to have it heat-crack at the
very last stage.
Oh yes and you should always carefully carefully wash the thing and
your hands and under your fingernails between each step so as not to
carry any stray particles of coarse grit onto the finer stage.
That’s long, but that’s how I’d do it. You see why buying a
ready-made crystal would be easier…S
Cheers
Hans Durstling
Moncton, Canada