Flexshafts for drilling and carving stones?

Jewelry Discussion
1

Dear Group,

I’ve been practicing clay sculpture for a while, and finally feel up
to carving in marble. I took a class last week, and the previous
week carved a little relief portrait.

My greatest frustration is getting spectacular detail work. Hech,
even simple details. Stone, unlike clay, is brittle and doesn’t take
well to sharp incisions or undercuts. But Bernini, my email address’s
namesake, could get fine, sculpted hair; pearls; even lacework and
foliage!

I am told that the solution is to cut, not chip-- to drill out all
undercuts before then going in with tiny chisels to clean it up.
Someone told me to get a bow drill; that’s what they used to use.
But a suitable bow drill, with the handle up rather high from the
point of the work, very hard to find and only antique, and which has
to rotate back CCW to rotate CW and cut again, is very awkward. There
must surely be a modern counterpart, commercially available, new and
improved.

I don’t really have any experience drilling stone. Marble is soft
enough that steel can cut, but it dulls within hours. I trust that I
want carbide? Or perhaps diamond? Marble will be softer than any of
the semi-precious gems you carve, but techniques should be very
similar, which is why I am taking these questions here. Most marble
carvers these days are rather abstract, with little time for detail.
Detail doesn’t pay hourly wages either; but I’m doing it for myself.

I noticed that foredom offers a 5k max rpm, high-torque/low-speed
grinder. Is this what I will want, for stone? Will I desire carbide,
or diamond; and if carbide, drill bits, mills, or router bits? What
precisely is the difference between a router bit and an end mill?
Where do you prefer to buy your bits? I’ve found drillbitcity, with
resharpened bits. Precise dimensions do not matter here. Does anyone
supply rather longer bits, to slip it into a very deep undercut?

Is water as a lubricant perfectly fine with the flexible shaft?
Cleaning up any rough marks on details is another problem. You
simply can’t get sandpaper or broken up whetstones into there. The
grinding stones I saw at harbor freight were massive, and at 20k rpm
would grind far more than polish, but maybe if I get smaller ones,
and if that high-torque/low-speed foredom can go down very low, that
would be just the thing. By polish, I mean an even, satin finish, to
look like flesh-- not a high gloss, 300-400 grit is fully sufficient.

What handpiece would you recommend? For mostly drilling marble, and
using tiny cones or balls to clean up the drilled & chiseled tiny
undercuts and detailed spots?

BTW, if you know–I can get marble with pinhead sized crystals,
though it’s hard to get. Danby marble has more like 1mm-diameter
grains, Georgian with 1/4"+ Does grain size dictate the detail that I
can achieve, or do the cutter heads slice right through the crystals,
leaving as fine a line as I want, no matter how fine the marble
grains themselves? Perhaps your experience in the context of another
stone more popular in jewelry, something that has distinct grains
like marble.

thanks for all the help!
Bernard Arnest

2 
I am told that the solution is to cut, not chip

My Bernard, we’re just full of questions, aren’t we? LOL! Where to
start… First off, stone carving is a grinding operation. Even
Botticelli ground his marble. Chiseling is a roughing operation to
remove the big things, and after awhile those chisels become more
scrapers and finally grit is used for the finish. And sculpture is
different from small scale carving. For one thing if you’re carving a
cameo type thing, one strike of the chisel can just split it in half.
It’s the fact that there’s lots of mass in the sculpture that you can
chisel it. Even though you can cut marble with steel or carbide
tools, you shouldn’t beyond a certain point. Use diamond or
carborundum. The reason for that is because of the chatter - you have
teeth rotating, and everytime a tooth hits the surface it creates
shock, and therefore the potential for chipping. That’s independent
of how true your tool is, it’s just the nature of toothed cutting
bits. In lathe work they call it intermittent cuts - like when you’re
turning a square into a circle, every time that corner comes around
and impacts the cutting tool it goes “bang”. So, yes, you can rough
out work with edged tools, but after a bit it becomes more and more
risky. And yes, water works well on a small scale for a lubricant. I
used to carve with a flex shaft in a cool whip container - cool whip
because the sides are flexible for reaching down into it. There are
two issues with cutting marble on a small scale. One is that it IS
soft - almost too soft. And they other is that it has a sandy texture
that’s prone to chipping and breaking off. They smaller you get, the
more it happens. I would suggest that you invest in some diamond
burs. I used (I got rid of my lapidary some time ago, except for my
carving kit) a 1" crystallite full circle wheel a lot, and an
assortment of diamond burs - I use plated because they last pretty
well with good care for the cost. Crystallite is sintered, and good
for the bigger tools. For polish and detail get some diamond grit - I
like the pastes because they’re easy to use - 280, 400, 600, 1200,
4000 - I use Linde A a lot for polish, or 50,000 diamond. Make wooden
wheels for a flex shaft mandrel, like 3/4" diameter, chuck a piece of
dowel with the end pointed off, or use toothpicks. All of those will
last surprisingly long with diamond grit. The issue you’re going to
run into is this: when you cut a six foot tall sculpture out of
marble, you can do hair that’s 1" across and put some lines into it.
When you’re doing cameo-size, that hair becomes.1mm across, which is
getting to the size of the grain of the marble itself. If one grain
of the marble pops out, well, there ya go. It’s the limitations of
the material itself that you run up against. Between using abrasives
for cutting and gravers or the like for scraping and lining you can
do pretty well, though. I personally like rock crystal a lot, though.
It’s hard enough to take any detail and cuts like a fine hard wax
with diamond tools. Slower than marble, but no so much. Once you use
diamond abrasives it really doesn’t matter much between marble,
agate, opal, etc. Some are just slower than others. Some things like
chysacolla are terribly hard, though. Finally, router bits and
endmills are pretty much the same, but one is designed for wood, with
straight teeth, and one is designed for metal - spiral teeth and more
stoutly built. I’ve cut wood with an endmill, but not vise versa.
Neither one is going to do much good on marble working freehand -
they’re very aggressive, and you have to be nuts to put your hand in
there with one - it’ll cut your finger right off. Both are intended
to be used under controlled conditions - a milling machine or a
router where your hands are far away and safe.

3

Hey Bernard,

I don’t do large stone but my brother Mark does. Here is a link to
his tools page

http://www.thesculpturestudio.com/stone_carving_tools.htm

If you e-mail him or call I’m sure he would share his experience
with tools.

A couple years ago I bought a NSK neumatic micro-grinder with the
water sprayer built in but this is only for very small jewelry sized
stuff.

Peace, Dave