There's also a post in the RCM archives (Google groups) on how the make
a crystal using air pressure to form a heat-softened blank. I've tried
it twice: I shattered it once with too much pressure and not enough
heat, and blew a hole the 2nd time with too much heat. I'm hoping the
3rd time will be better.
I did this a few years ago. I had a dial indicator with a damaged
crystal. The "manufacturer" had never heard of that particular model
when I called. I'm guessing that it was 40 to 50 years old.
I cut a disk from some thin, 1/16th(?) inch Lexan. To put a crown
in it, I found a coffee mug of the right size and with an appropriately
dished bottom. I put the coffee mug in the oven upside down and
slowly heated it until the new crystal sagged and matched
the curvature of the coffee mug.
IIRC, I scribed a circle of the right size, i.e., slightly larger, ~1/8th
inch larger in diameter than the final size. I carefully cut the Lexan
with an Xacto knife or single-edged razor blade
Here's a start:
The rest is a total mystery to me.
I'm still waiting for another sublime, transcendent flash of adequacy.
I thought these might be standardized sizes under the AGD specs but a quick
check shows there is still a min max range for each AGD group.
Reputable brands should have exact replacement parts available in exchange
Next simplest is probably to find an import dial that you can harvest the
lens from and trim as necessary. Any dial worth fixing is worth
sacrificing a $10 HF import for IMHO.
Watchmakers have selections of these in graduated sizes, and a thing
with a suction cup and fingers to deform the outer edge to slip into the
groove in the "watch". But, you can make a crystal out of sheet
Plexiglas, and relieve the inside face in a step, and bevel the outside
edge. then, just glue it into the bezel. That's what I did for a
Federal indicator that was missing the crystal. I actually like the
flat crystal better than the curved ones, there's no reflection of
lights. I super-glued the front of the crystal to a freshly faced piece
of steel, did all the relieving of the inner face and OD, then just
snapped it off the arbor, and peeled the glue off with a pocket knife.
I polished the inner face while on the arbor, the front after it was
Actually -- capture it between a round piece held in the lathe
chuck and another held in place with a live center. Relieve most of the
diameter of both, so they press only near the needed OD.
Turn the material (1/16" Lexan if you are talking about a 2-1/4"
diameter dial indicator or so, thinner if a runout indicator) to just a
few thousandths of an inch larger than the ID of the hole in the bezel
ring. Bevel about 1/2 of the thickness for appearance. (Angle is not
Turn a ring about 75% the diameter of the fitted crystal. Make
a smooth turn on the end and polish it smooth. Turn up another piece
(perhaps 50% the fitted diameter) and crown it on the lathe and smooth
and polish it as well. Turn a 1/2" diameter pin on the back of this.
Place the ring on the table of your drill press, or on an arbor
press. Fit the crowned piece in the drill press's chuck, or to the
bottom of the arbor press's ram.
Place the crystal, bevel side down, on the ring (it is good if
you leave the protective film in place on both sides until you have it
mounted in the bezel ring, to minimize scuffing of the crystal). Place
the bezel ring on the top side of the crystal, lower the crowned piece
in the drill press chuck until it presses the center of the crystal, and
apply force while trying to fit the bezel ring on the crystal. As you
press the center down, the outer rim will move up, and the OD will
decrease slightly. When it is small enough, the bezel ring will drop in
place, and when you relax the pressure from the drill press, the OD of
the crystal will expand and grip in the bezel ring. Now peel off the
protective film. (Proably you should partially peel the film on the
back side and trim the film just enough so it does not get trapped in
the bezel ring.
Anyway, this produces a very nice looking domed crystal for most
Well ... the above is how I make crystals for dial indicators
and small runout indicators. The main trick is figuring out how the
bezel ring removes from the indicator. There are quite a few ways, and
if you mention the brand I (or someone else here) can probably give you
a hint or two.)
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