Hi Judy, and all
The reason you are having difficulty is…what you are doing takes a
long time and many errors to learn. It’s not you.
The brand new, very sharp undercutting burr is the hardest to use
for a relative novice. If the setting is a commercial one, it is
probably kind of thin, and may torque, or twist if the burr catches
and binds a couple of the claws while spinning out of control. Try
to be patient, and you will eventually be successful. If there is
one tip that I may offer you about using undercutting burrs while
you are learning, it is to use one that is considerably smaller than
the stone diameter to start the seat, or to use a small ball burr to
mark the location of the cut. Once you have established a bearing
where the larger undercut burr will spin without catching, you will
find some relief.
I use undercut burrs to cut the seat in one claw at a time. Sounds
weird, but I seldom overdo a cut. I’ll try to explain this clearly.
Use a burr that is about 50% of the diameter of the stone. Slip it
inside the head, and cut the bearing into the claw which is on the
left…that is, the one at 9 o’clock that you can see from its side,
and look at it while cutting. Hold the piece firmly, and rotate the
burr at a relatively slow speed until it establishes a mark, and
then , in increments, cut the bearing to a workable depth and
profile. Think of it as 'carving". Then rotate the setting until you
have an unworked claw on the left again, and cut a matching bearing.
Repeat this until you have cut all four, or six if that is what you
Why the one on the left? This is important.
Because of the rotation directionof the motor, the burr wants to act
like a tire and travel clockwise from where it first catches. It
finds traction, and travels, or “walks”, often cutting or marring
where you don’t want it to. it goes out of control. if you hold the
handpiece very firmly, and the ring clamp very firmly, and only make
contact with the one claw on your left, you are taking advantage of
the strongest muscles in both your hands which will resist the
burr’s inclination to walk away from you, from the nine o’clock
position to the 12 o’clock position, which is where it ALWAYS goes.
Your right hand can offer enough resistance to the burr goiung
forward, simply by gently applying some pressure toward you, or
toward the six o’clock claw, without touching it. Once the cut is
established, go to the next.
Eventually, you may need to try to drop the stone into the setting,
and eyeballing its fit. Is the available diameter in your fully-cut
bearing the same as that of the stone? If it is close, or right on,
bend two of the claws outword from ABOVE the bearing in order to
allow you to drop the stone right into the bearing. If the seat is
correct, and the claws can be bent forward without pinching the
girdle of the stone, you will have little trouble. If the stone
rocks within its bearing, the bearing will require some correction.
Be patient, and you’ll get it right. Don’t cut corners or take
chances with a bearing which doesn’t match the profile of the stone.
If you want to practice cutting a bearing in the way that I have
described without wrecking anything, try this. Use a pin vise, and a
piece of heavier wire, about 1 mm. Secure the wire in the pin vise,
and try to make a bearing cut with your undercut burr on the right
side of it, as if it were the claw on your left that I described. It
will walk, and catch, and make you crazy until you find a way to
feel the fine control necessary to hold both the vise and the
handpiece steadily and securely enought to prevent the burr from
going out of control. Gentle pressure toward you, with the burr
spinning at a moderate speed. Use it like a saw, establishing a mark
and chipping away at it until you have a pleasing depth, as you
would on a claw. You will develop a "feel’ for what you need to do
when working on an actual setting.
Do you use regular bearing cutters as well, or “setting” burrs? They
are the ones with the vertical sides, and a cone-shaped bottom which
mimics the pavillion of a diamond. These are less aggressive in
their cutting, and if you use the size which matches the (diamond?)
diameter, you will generally get an accurate, workable bearing. Not
idiot-proof, but better for the novice.
You really need to study the profile of the stone as well. Is it a
well-cut diamond? Thick girdle? Thin? Coloured stones require more
careful study, and often I find that each claw must be cut to match
a particular area of an uneven girdle.
You will find that your success depends on your willingness to
accept failure, and to learn from your mistakes.If you need more
help, feel free to call me at my studio, and I will try to help in
any way that I can.