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[Source] Opal cylinder


#1

I have a project in the works. I need two opal round cylinders
measuring approx 20 x 4-5 mm.

Any suggestions as to how one would cut such a form would be welcome.
I have pretty much the standard lapidary equipment. I’ve cut and
polished a variety of stones and shapes, but I’ve not cut a round
cylinder. The first question: how do you hold it?

The material is Australian crystal opal from Mintabie. It’s clear
with all colors floating in the transparent base.

Thanks
Kevin


#2

Kevin,

Centerless grinding is useful for this kind of precision rod shaping.
For production work we have a Danish GRIT GF75 wet belt centerless
grinder. These are often found in metal finishing shops, for example
working stainless handrails to a satin finish. We use ours
exclusively on minerals and polymers.

For very small work, we embed a square-cut strip of stone in hard
epoxy, inside a short length of rigid plastic tube. Choose a tube
whose inside diameter just exceeds the diagonal dimension of the
square-cut stone bar. You could make the square bar maybe 6mm x 6mm
x 50mm long, slip it into a plastic tube with over 6 x 1.4142 = about
9mm inside diameter, plug one end of the tube and fill the
interstitial spaces with pre-mixed epoxy resin and hardener. We use
acrylic tube so we can see we have a solid epoxy fill. This means we
start grinding from a very round length of composite rod, which
causes no rough-running difficulties on the blade-like work rest of
the grinder. The grinding process is smoothly done, water-wet
(recycle flood cooling is standard on our big GRIT centerless
grinder) with 3M Trizact abrasive belts. Taking the diameter
reduction slowly, we are first grinding away the skin of plastic
tube, then grinding away at a rod like a “four-point star” of stone
in epoxy, progressively reducing the diameter to eliminate the last
of the epoxy, grinding at last just pure stone.

The lovely thing about the centerless grinding process is the
work-piece is trapped and controlled between three elements:

  • the grinding belt, supported by a rubber contact wheel - or a
    grinding wheel in some machines;

  • the control wheel, made of grippy rubber, running at slow
    peripheral speed and slightly tilted to progress the work through the
    grinding zone;

  • the work rest - a smooth, wide support blade underneath the
    work-piece.

You do not need to hold the work-piece at all. No ‘centres’ at each
end (hence the name centerless grinder) and no contact apart from the
three surfaces listed. Finer wet-running belts can quickly be fitted
to leave a polished finish on truly round and parallel stone rods.

If there is a metal workshop nearby that satin-finishes stainless
tube, talk with them. Show them an epoxy-encapsulated square bar of
your material.

I have seen rougher centerless work done on a simple belt sander
with a water drip and a wooden push bar. Here the three surfaces to
control the spinning rod are the sander’s horizontal fixed table, the
abrasive belt and lastly the push bar, which must be tilted and
pushed with great dexterity to grind without losing control. It can
be done with a $30 one-inch belt sander after a lot of practise, but
with rather poor control over parallelism and diameter. You still
need first to embed the mineral inside a round rod of epoxy, using a
plastic tubular mold. Check out Google for references to “centerless
grinder” and you will understand the layout.

It is possible to hold a rough bar in the slow-spinning chuck of a
lathe and use a so-called toolpost grinder, too, but the slenderness
of your very small target size sways me towards the centerless
process.

Mark
http://www.fourth-axis.com


#3

Hi Kevin,

I have successfully cut cylinders in that size range from all sorts
of materials, including corundum and opal. I used my faceting
equipmwnt, with the stone attached to a standard brass dop with
super glue. My preferred brand is Loctite Professional Grade, from
Lowe’s or Home Depot or wherever.

The whole process is very simple, but use caution in applying
pressure only to the stone, not the dop, as the glued contact area
is small.

Wayne


#4

Kevin,

The way I’d tackle making your opal cylinders would be to glue the
rough with cyanoacrylate to a 1/4" dop, carefully rough it out to an
approximate cylinder on your cabbing gear as close as you can get it,
then chuck the dop into the Foredom (the handpiece that takes 1/4",
the exact product designation for that one escapes me). Clamp the
handpiece firmly to the workbench, (Foredom has a device for that
also) set the speed control to a comfortable working speed and
gently run diamond files over the stone. Basically you’re working a
lathe, turning down the cylinder. When it is close to diameter follow
with 400 grit diamond, then 800, then 1200, then polish. 3M makes
strips of sticky backed diamond on mylar or similar film. Stick these
down on whatever comes to hand, a ruler, what have you, to make a
series of files. Or cut strips from your worn diamond sanding pads
and glue these on a stick. It is possible that the 1/4" diameter
sticking surface between dop and opal cylinder may be too small to
take the stress. In that case I’d use a dop with a flange, leave a
similar flange on the dop end of the opal and saw that off at the end
of the operation. To avoid leaving grooves around the cylinder move
the files and sanding sticks back and forth while working.

Cheers,
Hans Durstling
Moncton, Canada


#5

Hi Kevin.

Saw these in LJ Feb '07. John Kennedy uses cylinder cut stones
(Cylettes) and has worked out those difficulties. He uses mostly
chalcedonies.

Email and phone contact info on the website:
http://www.jkennedydesign.com

Pam Chott
www.songofthephoenix.com


#6
Any suggestions as to how one would cut such a form would be
welcome. I have pretty much the standard lapidary equipment. I've
cut and polished a variety of stones and shapes, but I've not cut a
round cylinder. The first question: how do you hold it? 

Very carefully. That’s expensive stuff. I’d probably chuck it in a
slow-speed drill or handpiece and grind and polish against a wheel
or belt. Delicately. I’d be very afraid that 4-5mm would break
easily.

Al Balmer
Sun City, AZ


#7

Hello Kevin

If your piece of rough is large enough you could use a core drill
with the inside diameter about equal to the size you need and use
the core.

Another option would be if you have a faceting machine is to set up
the rough parallel to the lap and set your index to freely spin,
keep lowering the mast until you create the round cylinder in the
thickness that you need.

The method I use is to hold the rough between your index fingers and
spin with your thumbs, count 1-2-3 while grinding then spin 1/4
turn. beats your fingers up a bit, but it’s fast and no worries
about coming off the dop.

Once the pieces are roughed out take a nail with a good head and
insert it into the flexshaft. spin and file down the head of the
nail to the diameter you need, then epoxy the cylinder to the head
of the nail. after a good cure, insert the nail into the flexshaft
and spin while sanding with wet/dry sandpaper in the usual sequence.
Then polish, I use french cerium on leather

Hope this helps

Stan McCall
Custom Creative Gem Cutting
http://home.earthlink.net/~custom-creative


#8

Thanks for all the responses. Every response provoked a reflection on
my part; and I learned from each.

Mark at ‘fourth axis’ suggested the ‘Centerless Grinding’ process. I
had never heard of “centerless grinding”. I followed through on the
suggestion and went to MSSOnline and found a straight forward
description and a very helpful visual.

That and Mark’s detailed description have inspired me. No hands
needed!

I have a 'Cabmate" and an fixed arbor from Diamond Pacific. I’m going
to try to set up a centerless lapidary grinding unit using a 6" x 1 &
1/2" wheel and the fixed arbor. The wheel rotates downward or
counterclockwise while the arbor surface rotates clockwise rotation
on the fixed arbor I would think. That should keeps the stone in
place. First, of all, I’ll preform the cylinder on my faceting
machine.

Of course I’m going to use another material before I go to the opal.

I’ll post the stages of this venture as I go along. But I’m also
preparing for two upcoming shows.

Thanks again
KPK


#9

Hi Kevin,

Mark at 'fourth axis' suggested the 'Centerless Grinding'
process ..........First, of all, I'll preform the cylinder on my
faceting machine 

You may like to take a little care in preparing the cylinder before
attempting to centreless grind it as the process of centreless
grinding does not necessarily produce a perfectly round cylinder.
Because of the way the process works, it only produces a piece of
material in which the ‘diameter’ is constant - however, this may
easily be a tri-lobed shape. The effect is sometimes measureable in
centreless ground steel even though the manufacturers start with
round wire - I think it depends upon how well the machine is set up.
Could you not use a lathe, however crude, to grind your piece? This
could be as simple as an electric drill set on a slow speed and held
in a vise to rotate the work and a dremel or foredom with a mou=
nted grinding wheel to act as a cutting tool. You could probably
arrange some form of simple guide to traverse the grinder along the
stone ( I have in mind using a Dremel and making a wooden guide by
nailing a couple of lengths of wood to form an ‘L’ shaped tray and
then running the Dremel a= long this. The Dremel could be advanced
towards the work by using shims between the tool and the tray). This
way would probably be easier to set up than a centreless grinding
system and would produce a true cylinder.

Best wishes,

Ian
Ian W. Wright
Sheffield UK


#10

Kevin

Just in case the centerless grinding setup doesn’t workout for you I
thought that I would pass on another method. When I first saw your
post I visualized your opal as expensive and beautiful. Something
that I wouldn’t want to either waste or damage. I develop a healthy
paranoia when working dangerously on rare material and want as much
care and control as possible. That means that I want the work it in
my hands and close to my eyes.

First block the stone out on a flat lap until you have perfectly
square rod with the thickness of the finished cylinder. If you are
trying to recover as much weight from your material as possible the
corners can remain a little rough, Glue round brass rods about 1 inch
long (turned to size of the cylinder on a lathe) dead center on
either end. These are going to be your grinding guides and handles.

Paint the four flat sides of the opal with a blue permanent marker
and lightly scribe a centerline down their length. This will help you
keep your grinding parallel. Use a wet lapidary machine to rough
grind the corners off of the opal and pre-form the cylinder. Don’t
grind the brass guides! You will now have round corners with four
narrow flat sides that still have a fine line running down the center
of each.

Now use a vertical flat diamond lap to finish the shaping, Supporting
the work between your index fingers and thumbs rolling the piece
against the lap, keeping the guides parallel to the lap gradually
working the high areas closer to the brass guides and creeping up on
the center lines. You can roll with the lap or pull against it
depending on how you feel it cutting. Don’t take heavy cuts, keep the
lap wet and work continuously around the piece attempting to reduce
each corner gradually by the same amount with each pass. When done
the brass guides should just about be in continuous contact with the
lap and the scribed line almost beginning to disappear. Switch to a
finer lap to finishup and then polish. Sounds time consuming but in
fact it goes pretty fast. If you have experience at this type of work
you could skip bluing and scribing, and just roll lap it by eye to
match the brass guides.

I have an 8" lapidary setup but use the accu-finish tool grinder
with a wet sponge for this type of work, the worktable makes a
perfect hand rest and the speed is about right, 300rpm. The link
below will show you the machine. Any flat diamond lap should work. I
also use the accu-finish for grinding lathe tools, gravers, brass,
nickel silver, soft gemstones and even the occasional piece of wood.
I
true up gold and silver castings, and square flat work on it as well.
It is one of the most useful and used tools in my studio.

http://www.accu-finish.com/seriestwo.html

Good Luck
Dennis Smith - thejewelmaker


#11

Hi, Kevin

The Dremel could be advanced towards the work by using shims
between the tool and the tray). This way would probably be easier
to set up than a centreless grinding system and would produce a
true cylinder. 

As Ian points out, turning it is the best and maybe only way, really.
Centerless grinding is great, but it HAS to be a precision machine,
and the wheels all have to be precision dressed, etc. That’s why
they’re expensive. The problem you’re going to have, I think is
workholding. That’s where centerless grinding would be great, but —
well, if you can build it economically, good for you. Or if you can
farm it out…My question to you is, How precise does it really need
to be? Doesn’t it really just need to LOOK like a cylinder?
Especially since material is at a premium, I’d probably use a core
drill, which is obvious, and then dop it and spin it carefully in my
fingers on a fine wheel - even get a piece of sandpaper and twist it
round in your fingers. Workholding: any sort of lathe, dremel or
flexshaft arrangement is going to need a tailstock to get any kind of
precision, and since you can’t use center drills and a steel center
with opal, you’d have to somehow dop both ends on the same center,
and mount it all, on and on, and then you’ll still need a lathe with
a fine wet grinder and sander. I like the idea of encasing it in
plastic tubing, myself. We used to encase rocks in cement in a
similar way for the big slab saws. Personally, I think you will spend
$1000 to get $200 worth of opal that’s more precision than necessary,
and just spinning the dop in your fingers on a fine wheel very
carefully would probably give perfectly satisfactory results. Or
grind off the drill marks, hand sand it, and throw it in a tumbler.
Now, if you needed 1000 pieces, that would be different. Then talk to
the marble and granite workers - they have all that kind of equipment

  • rock lathes and such. The problem would be getting that
    comparatively tiny work done by them.

http://www.donivanandmaggiora.com


#12

Ian, as you state the results depend on how well I set up the unit.
The main concern for me is how to hold the material. It’s extremely
rare opal; I don’t want to waste any. I may first try turning a
cylinder using my faceting machine. I can use wax to hold the preform
and simply rotate the dop. I say, simply, but I don’t think so; there
are lots of variables and adjustments. Sometimes the laps are not as
flat as they might be.

I’m not sure I’ve totally digested your post, but a crude lathe may
result in a crude cylinder. Again, it’s how to hold the material.
I’ve got equipment that would serve as a lathe if the faceting
machine doesn’t serve. I’ll just take as much time as it needs. And
wait for the moment.

Thanks for the input.
KPK


#13

Ian’s perfectly correct. Centerless grinders, badly set up and
operated without care or experience, produce dreadful work daily
around the world! Intelligently set up and operated, they excel on
slender work. Depending on how you set up any other kind of grinder
instead, you may possibly suffer from taper, diameter, finish, shape
and other, different errors. As a jeweler, you will develop the
necessary “feel” and you can practise on inexpensive marble or such.

Centerless grinding best lends itself to long, slender work that
would be too difficult to hold conventionally. The three-line
contacts gently cradle the work, like a pencil between three fingers.

In the case of small diameter mineral rods, I strongly suggest tube
"potting" to provide a quite round starting object, rather than
multi-faceting by hand or otherwise. Vibration due to spinning an
out of round starting object on the work-rest will cause fractures.
We use an electronic potting grade of epoxy, as it has very low
viscosity (releases bubbles readily), a low exotherm (does not do
heat damage) and grinds away to dust, without gumming whatsoever.

Mark
www.fourth-axis.com


#14

Thanks Dennis. you bring up some interesting points. With all the
technical points that posters have made this is a great learning
experience.

Switching to a flat lap seems odd; why not just continue with 8"
wheels. That way one has full contact with the wheel. I have a 6"
flat lap as well as an 8" flat lap. The speed varies in proportion
to the distance from the spindle. It’s easy to catch an edge.

Somewhere in the dim mists of the past I remember someone suggesting
using a length of bamboo to cradle the material being cut. Perhaps a
60 section would be cut out for the opening to reach the cutting
surface.

Thanks for taking the time to outline your method.

KPK


#15
In the case of small diameter mineral rods, I strongly suggest tube
"potting" to provide a quite round starting object, rather than
multi-faceting by hand or otherwise. Vibration due to spinning an
out of round starting object on the work-rest will cause fractures.
We use an electronic potting grade of epoxy, as it has very low
viscosity (releases bubbles readily), a low exotherm (does not do
heat damage) and grinds away to dust, without gumming whatsoever. 

You’ve convinced me of the wisdom of ‘tube potting’. Dreadful work
can be produced using the most precise equipment. I find it
interesting that although I use mm gages and a ‘Leveridge’ gage I
finally rely on eyeballing as the final judge. Thanks again for the
helpful advice.

KPK


#16
How precise does it really need to be? 

I have certain standards. It has to be precise enough to satisfy me.
I’m a perfectionist which makes life difficult.

“… whether by ambition, blood or lust
Like diamonds we are cut by our own dust.”

KPK


#17

Kevin,

I’ll say it again.

I have done cylinders many times on my Gem master II faceting
machine. You are correct in saying that laps may not be flat and
true, but mine are because I made them that way (different subject).
Crystalite makes a better grade of steel lap with bonded diamond
that has a reputation for being flat. If you have runut or wobble
with them, you have a machine prblem, get it fixed or get a better
machine.

Making cylinders is quite easy, just take your time. I set the quill
to free-wheel and rotate by hand on a lap at moderately slow speed.
With opal work moves quickly, but I use a GOOD cyanoacrylate, not
wax, with opal. In my experience, the cyano bond is stronger IF YOU
SHIELD IT FROM WATER. You can use fingernail polish for that. I
happen to use only oil as a coolant or lube in my faceting, s the
cyano remains unaffected. Acetone will break the bond, I wouldn’t
risk heat with high end opal.

It’s simple, really, and, at least on my machine, I can duplicate
diameters accurately to within 1/100th mm.

Wayne Emery


#18

I know of what you speak concerning steel laps by crystalite. I got
one in Tucson at Crystalite’s tent accidently. I’m a self taught
faceter using Raytech’s most rudimentary machine. I find it
interesting that you use oil as a coolant - what kind of oil?

When I dop opal I preheat everything including the holding jig and
finally melt the wax over a alcohol lamp. I’ve had opal so hot it
would burn your fingers with no untoward results. Getting opal that
hot was the result of inattention, not intent.

KPK


#19
I find it interesting that you use oil as a coolant - what kind of
oil? 

Almost all of my actual cutting, from 200 to finer, is done with
charged tin alloy laps, plus free diamond boart on the lap, all in a
very small amount of WD-40. Too little, the boart gums up, balls up
and is thrown off the lap; too much and the thin slurry is thrown
off the lap. Experience qnd attention quickly brings you to a place
that works. I originally learned before the time of bonded laps
(plated), using copper charged laps for everythig. Thr plated laps
were presented as a boon…water was less messy than oil
(especially the popular kerosene, paraffin or olive oil used then)
and you didn’t have to fiddle with loose diamond powder. We all
bought in…but the plated laps wear quickly, so for a production
cutter, they are expensive…and as they wear, the effective grit
size changes; and water makes things rust and gets into electrical
parts sooner or later; and water is a LOUSY lubricant for diamond
tools (unless you are the manufacturer, of course). So, many
professionals go back to boart on copper or tin alloys. Faster,
smoother, constant surfaces, etc.

On the occasions that I do use water, I use it with KoolMist as an
additive. reduces spray, lubricates and cools very well; is cheap
and re-usable!

For oil, it’s WD-40, applied with a small brush (or my finger). I
buy it by the gallon, put it in a baby food jar, dip my finger in
it, dip my finger in the diamond powder, apply near center post of
spinning lap, and that’s it.

I don’t like the spray, it will irritate your eyes and sinuses
eventually. No permanenet damage, but who needs it? MSDS and years of
usage with no problems indicates WD-40 to be one of the safest
lubricants available. Traditionally, olive oil (refined, not virgin)
or thinned Vaseline also work well.

I have to work to a very high standard of quality, while time is
still of the essence, and these methods provide predictabilty and
quality, and speed, every time.

I find wax to be slow and sometimes unpredictable, although if you
are working onesy-twosy, it’s fine. I dop up 6-8 pieces at a time,
using cyanoacrylate, sometimes with 2 part epoxy over it for very
large pieces and let it cure for 24 hours…I’m always working,
always behind, so when I get to it it is CURED. Likewise, placing it
in a solvent works for me, as it is just part of the work flow, and
when it releases, it releases. No heat, no damage.

Although the resident idiot did flush a large Tanzanite down the
toilet recently, a true sinking feeling…ugh.

Wayne


#20

Kevin

I agree that if you have the experience for it you could shape the
cylinders completely on an 8" wheel, but I find that I get closer to
perfection on a vertical flat lap. I want to be able to see a narrow
straight line of light reflection running the length of my finished
gem as I rotate it.

I think the important idea here is that the lap is vertical and that
you can closely look over the stone at the lap as it is cutting,
manipulating the material between your fingers and watching the play
of light on the stone as the shape gets closer to perfect. The reason
for making the brass guides fairly long is to give you a way to see
that the work is parallel to the cutting surface. I think that Lap
speed is important, the unit that I use is 300 rpm and it seems about
right. My concern with the 8" wheel is that the surface is fairly
narrow so you have to make all of your judgments within the width of
the stone you are cutting and it is fairly easy to position the work
either above or below center on the wheel. I also find that the flat
lap lets me tilt the work around if I want to work one side a bit
more or alter the cutting speed. I’m constantly moving the work, not
just grinding away. I thinking of a very light touch here, almost
like caressing a baby’s cheek. It might be a case of what one is use
to. I’ve cut tens of thousands of stones on an 8" wheel and believe
that I could get pretty close to a perfect cylinder on one, and
probably in 15 minutes or less, but for something special I would
switch to the vertical lap. Heck, I think that I could file it at the
bench if I had to.

Trying to explain how one works intuitively is a bit difficult. Like
trying to explain how not to fall off of a bicycle. With care and
good magnification you should be able pull it off in any number of
ways.

Oh, and don’t worry about the fingers, the skin will either grow
back or you will have permanent tapers on your forefingers that will
just make it easier to do this kind of work in the future.

Again Good luck, I can see those beautiful finished opals in my
minds eye.

Dennis Smith - thejewelmaker