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How to avoid the runaway bur?


#1

Hi there, I have set a handful of diamonds, round and square -
learned many great tricks along the way, but I can’t seem to avoid
the occasional bur running all the way around a prong - sometimes
twice in a row - much to my dismay!

I use a hart bur, and cut prongs one-at-a-time. For typical
round-stone prongs, any one of them might run around. On a chevron
prong, It can happen too, it grabs one side and zips around before
you have time to react.

I watched the “The Art of Setting Princess-Cuts DVD” by Blaine Lewis
(a VERY good DVD). He offers a tip: hold a “spare” finger next to the
prong, and against the smooth part of the bur shaft to prevent it
from running away from you… Maybe my burs are way too long, but I
don’t seem to have a chance of reaching that far, and I have big
hands! Not to mention the friction of the shaft heating up against my
finger very hot, very quickly.

Any other ideas? Tips?


#2

Personally I will normally hold my handpiece firmly with a full grip,
all fingers wraped around it and with the bur oriented below the
pinky finger. Then I brace that wrist against the GRS benchmate
holding the ring. The free hand lower palm area will usually rest on
the knob that tightens the benchmate while those free fingers provide
more stabilisation and controll for the handpiece, not the bur itself
but the actual handpiece. Also try not to push into the prong but let
the bur cut, moderate to high speeds and slight movement.

Bil


#3
hold a "spare" finger next to the prong, and against the smooth
part of the bur shaft to prevent it from running away from you.... 

Yeah, well David - if anybody NEVER has this happen, I’d like to
know. It happens in other situations, too, if you’re not careful.
The finger thing is good as far as it goes, but your finger is no
match for the force of your flex-shaft if it really wants to go. And
yes, if it happens once there’s double the chance it will happen
again in the same spot, because the bur has made a path. I only have
three tips:

Don’t catch the far edge to begin with, work with greater precision.
It’= s just like polishing - if you catch the far edge, where the
teeth can catch it, it can grab and jump. Or cut up to that edge and
finish it off with file or graver, which isn’t always feasable. In
other situations, like texturing or roughing out baguette settings,
work towards the bottom edge, turn it over and work towards THAT
bottom edge, etc. Just like polishing…

The main thing, though, is lubrication and speed. Run the bur fast,
even the pedal to the floor. At slower speeds the teeth are
individual teeth, so to speak, and can tend to catch. At full speed
it becomes a blur, and acts much more smoothly. Think of a circular
saw - if you put a board on a table saw and just turn the blade by
hand, a tooth will catch on the board and stop the blade. If you run
at speed and power, it becomes some thing else entirely, and cuts
through. Same idea… Jo-Ann says Hi, BTW…


#4

I am sure there is more than one technique for this mine is that i
increase the speed of the flexshaft to ludicrous speed and then
lightly apply the cutting edge to the prong and am very careful not
to get in a hurry

goo


#5

It sounds as though you may be spinning your bur too fast. Not that
it can’t heat up at lower speed, but if you go slower, it’s a bit
less likely to get away from you, and will take longer to toast your
finger, as well as being less likely to get so hot it ruins the
temper and dulls your cutting surface. Also, try to use less
pressure with that restraining finger (less friction). You could even
lube it.

Noel


#6

Lubricate! I use Brown’s snake oil. Wintergreen oil isn’t as safe,
but it works well too.


#7
but I can't seem to avoid the occasional bur running all the way
around a prong - sometimes twice in a row - much to my dismay! 

Is your flexshaft full torque at all speeds? I had that problem for
years before I found out that the cc model I was using was the
problem. Burr catches and jumps out of the setting. Otherwise, you
are not holding the handpiece close to the prong and running the
burr as you go toward the prong and allowing the burr to cut slowly,
low speed.

Richard Hart G.G.
Denver, Co. 80210


#8

If you are cutting a single seat in a single prong with a setting bur
as they instruct at GIA, I would make a little notch in the bench pin
on the left edge to the depth of the seat you are cutting. If the bur
catches it will run up the bench pin and not around your prong. This
technique is taught for cutting seats for oval or other stones that
are irregular.

Try it to see…

Best,
Russ Hyder


#9
Personally I will normally hold my handpiece firmly with a full
grip, all fingers wraped around it and with the bur oriented below
the pinky finger. 

Firmness (actually a monster grip) is good, but I hold mine the
other way, thumb towards the chuck. A setter I know holds his like
you would hold a pencil or a fork…

Something else occured to me on this thread that I’d never thought
of before, too. That is the size of the bur itself. One way to set,
that generally doesn’t work very well, is to use a 6mm bur to set a
6.5mm stone.

That’s messy and it’s really difficult to get straight and even. So,
probably most people are using a 3.5mm (whatever) bur to set a 6.5mm
stone, and there hangs the problem - the smaller the bur relative to
the size of the prong, the more likely it is to catch on that edge,
just because the bur itself is so small. Bigger is better, but too
big is bad, too, naturally…


#10

After reading some of the other replies to this question i thought i
should explain the reasons behind my choice of technique for
increasing the speed of the flex shaft to ludicrous and using light
pressure.

Most teeth on most brands of burrs are cut way too coarse for the
size or diameter of prong one has to cut with them to make a seat
for a stone. think about saw blades, if the blade is to big for the
sheet you want to cut then the teeth hang up and grab and piercing is
not a pleasant rewarding experience.

It is the same with stone setting or other shapes of burrs although
the quality of the steel may be good, most of what is commercially
available to bench jewelers in the way of burrs is poorly designed,
the teeth are not really fine enough, not enough teeth per
millimeter of cutting edge on the burr.

so in order to compensate for this problem of coarse cut teeth on
the burr and reduce the risk of grabbing and running away of the
burr i increase the speed of the burr to an extreme rpm and very very
lightly touch the burr to the spot i want to cut which so far for me
has resulted in more positive results than going to slow which lets
the teeth hang up on the work. Dont forget the basics of
lubricant,securing the work in a firm manner, use a good firm and
accurate grip on the hand piece, and a good quality foot pedal for
your flex shaft these all work in concert for success

goo


#11
Wintergreen oil isn't as safe, but it works well too. 

What’s the safety issue with wintergreen?

Jerry in Kodiak


#12

When I took Blaine’s stone setting class he used a small piece of
fish tank tubing around the bur to protect the finger and help
stabilize the bur.

Lynn


#13

By the way, I use needle and miniature needle files for a lot of
setting. I start start with a saw blade cut on all prongs at the
level I want the girdle to be at, and then I use round and triangle
files to get the angles for the pavilion and crown facets. For
really thin prongs or if I do not have the perfect size bur for the
stone I am setting this works wll for me.

Richard Hart G.G.
Denver, Co.


#14

It is not possible to avoid runaway bur completely, but it is
possible to minimize the chances of accident.

The first step is solving any problem is to understand the problem.
It is a bit cumbersome, without using diagrams, but I’ll give it a
shot.

A bur, while cutting, is a rotating disk. When bur just spins (no
cutting), the only forces on the scene is centrifugal and
centripetal, and they are in balance.

If it has been a long time you open a book on physics - centripetal
is towards the center of rotation, and centrifugal is in opposite
direction ( the 3-rd law of Newton ).

When you start cutting, you will apply some force in the direction
of the cut. Since you cutting is away from the center of the bur, the
force will add to centrifugal, and subtract from centripetal,
creating imbalance. However, as long as the direction of your force,
coincides exactly with bur radius at the point of the cut, there is
no problem. That imbalance is actually makes cutting more
controllable.

The problem arises when applied force does not coincides with
centrifugal force, which is almost all the time. What we are talking
about is to be able to cut exactly towards the center of a prong,
which given the size of most prongs is practically impossible. The
imbalance of forces now is your enemy and causes runaway bur effect.

It does not matter how you hold your handpiece. John says

the smaller the bur relative to the size of the prong, the more
likely it is to catch on that edge, just because the bur itself is
so small. Bigger is better, but too big is bad, too, naturally... 

It is somewhat correct. The stated reason is wrong, and the last
sentence is incorrect as well. To re-state it - there is an optimal
combination of bur size, rotational speed, bur coarseness, amount of
force used, bur profile, and probably many others, which will
minimize that imbalance. Most setters come to know this combination
through years of experience.

How one can minimize the chance of an accident ? by minimizing
forces involved and try to learn from every occurrence. When it
happens, try changing bur size, or speed, or profile, and etc…

Leonid Surpin
www.studioarete.com


#15

Hello All,

For what it’s worth here’s my experience with burs and flex shafts -
and I don’t do stone setting so this may not apply to the specific
situation under discussion.

I used to do small carvings with my flex shaft - working directly in
metals and bone. Not every day, but enough to develop some moderate
skills. But I never got good enough to avoid the danged thing running
wild every once in a while and cutting a bloody track around some
finger or, at least, a great gouge out of some bit of the sculpture.
Firm grip, loose grip, work in a vice and two-handed grip - whatever
I tried I’d rarely get through a job without some minor accident or
close call.

I finally concluded I was a hopeless klutz.

But a few years ago I got ahold of some dental burs which I use in
my flex shaft now. Some are carbide with various shapes and some are
diamond-impregnated compositions. They are mostly quite small but
there is a tremendous variety of shapes; points, teardrops,
cylinders, cones, round ones, etc etc. There is a world of difference
between these and the ones I used to buy from jewelry supply or hobby
shops… The dental tools have been very easy to control and I rarely
have any accidents to myself or to the work with these. And they do a
bang-up job of carving in copper, silver, brass, bone. Fast material
removal.

Unfortunately my skills still rate as only moderate at best, but at
least I now do no harm.

I’m no expert on the fine points of designing these little tools but
I wonder if it isn’t just that a dentist has to be even more careful
of his “workpiece” than a goldsmith does. Imagine how long a dentist
would stay in business if he did to some patient’s tongue or gums
what I have done to my own fingers. So maybe their tools are just
that much better for that reason? I dunno.

Here’s another cool thing about them. They are free! Dentists don’t
work with dull tools or maybe don’t want to go through the trouble
of re-sterilizing the dozens of pesky little bits and burs after each
use, so they discard them at the first sign of wear or even before
any wear that any of us crude crafters would notice. So they often
have used ones which they are only too happy to give away, along with
lots of the hand tools, all those little scrapers and chisels and
things their assistants use to scale teeth, which are also very handy
though not the subject of this thread.

Anyway - talk to your dentist, ask him / her to put aside some used
burs for you and see what you think. I love 'em. Their only drawback
I have found so far is that they have relatively short shafts so
might not be easy to use in some situations.

Marty


#16
I use needle and miniature needle files for a lot of setting. I
start with a saw blade cut on all prongs at the level I want the
girdle to be at, and then I use round and triangle files to get the
angles for the pavilion and crown facets.' 

This is a very safe way of working and completely adequate. I never
use burs on prongs. It is probably not the fastest way of doing
things, but I like it. For me, it takes much of the stress out of
setting a stone.

Leach


#17

I use a separating disk on most large prongs. It’s fast and works
really well. I have much more control too.


#18
I wonder if it isn't just that a dentist has to be even more
careful of his "workpiece" than a goldsmith does. Imagine how long
a dentist would stay in business if he did to some patient's tongue
or gums what I have done to my own fingers. So maybe their tools
are just that much better for that reason? I dunno. 

Dentists have different requirements. It is relatively easy to avoid
runaways by applying a bit of pressure in opposite direction. The
problem is when you undercut a 0.9 mm prong, the depth of cut should
be between 0.3 and 0.4 mm. There is just no room for applying extra
force to counter bur movement.

The same goes for speed. The higher the speed, the less chance,
providing bur is sharp, but it is so easy to over cut, so is not a
good solution either.

Goldsmith needs control above all, and that means slow speed, and
that is when flexshaft-bur-hand combination is the least stable.

Leonid Surpin
www.studioarete.com


#19
sometimes twice in a row 

Yup, that’s what happens because the first escape creates a groove
for the next one

I use a hart bur, and cut prongs one-at-a-time. 

To each his own but I don’t care for this method. Aside from the
slippage problem you also must be exact with seat height 4 or 6 times
in a row and each cut has to be placed on the radius. Neat trick
freehand. A setting bur does a round stone quick, neat, and usually
precise. I’ll use a setting bur also for some fancy cuts. For a
marquis or oval I make a light cut to set the seat location on the
four central prongs all at once and then cut the ends. The reason to
make a light cut is that the diameter of a setting bur that fits the
central prongs is somewhat different than what’s required for a
fancy’s larger radius on the side so if you left it as cut the seats
will be off center. Just make an adjustment afterwards.

A hart bur by design has a knife edge. No wonder they dig in and
escape, despite strong hands even. In most situations, when cutting
with one side of the bur its handy to apply a little counterforce to
the side which if I think about it would be counter clockwise from
the cutting edge(you’d want to double check that, I could be
remembering wrong, its not something I consciously think about
anymore, but experiment if you like). Things like inverted cones and
wheel burs have much the same propensity to escape. Part of the
reason is that they are single cut. Crosscut burs are much easier to
live with but I have never seen a crosscut hart bur.

Maybe my burs are way too long 

Could be. The further the cutting edge is from the handpiece the
more leverage it has against your grip.

I’ll agree with high speed when called for, lube when needed.

On a chevron prong, It can happen too, it grabs one side and zips
around before you have time to react. 

Try this. Instead of using say a 2mm hart(which might be the size
actually required to span the two flanks, these are random numbers
just to illustrate the point) start of with a much smaller
hart…maybe 1.3mm. This size keeps you away from the leading edges
which really is the core of the problem. After you get a good cut
simply go back in with larger burs til you are where you need to be,
your first cut gives the larger bur a place to nestle and a place to
bear against.

but I don't seem to have a chance of reaching that far 

What kind of handpiece are you using? A jacobs chuck is maybe the
worst style for setting. Try a quickchange sometime. I don’t care
much for pencil tips, its harder to get a good purchase on the
handle.

One thing about a hart for a round prong setting… a hart doesn’t
not cut the area above the seat on a vertical. It basically cuts a V
notch which means you’ll have to either go back in and file a
vertical or bend the prong outward and then set the stone which would
leave you a very small tip. Sometimes you want a small tip, but
mostly a bigger one lives longer.

One last thought (hears a unanimous sigh of relief from around the
orchidsphere)…save your dull burs. Sometimes they work better than
brand new burs. I’ve got literally pounds of dead and dying burs
although that might be equally from cheapskatedness.


#20
I use needle and miniature needle files for a lot of setting. I
start with a saw blade cut on all prongs at the level I want the
girdle to be at, and then I use round and triangle files to get
the angles for the pavilion and crown facets.' 
This is a very safe way of working and completely adequate. I
never use burs on prongs. It is probably not the fastest way of
doing things, but I like it. For me, it takes much of the stress
out of setting a stone. 

I remember my mentor relating a story about a jeweler newly arrived
in this country from Russia (the Ukraine?) He hired on at one of the
larger jewelry stores here in Memphis, where the store manager was
astonished to find that the new guy did not use a flex shaft for
stone setting. Instead, he had a handle similar to a graver handle
in which he could chuck a standard burr. No “traveling 'round the
prong” there! He used this and a graver to cut all his seats and
notch prongs. Alas, the first store let him go because he worked too
slowly.