Hi!

As a matter of personal pride and self-esteem, I wish to get
fine levels of detail into my marble carving on par with the best of
the 19th century (eventually).
They did it, I understand, with a bow drill-- hold the drill in
the left hand, a strung bow wrapped around a spiral piece in the
right. A tool-steel spade bit at the end, given the low tech of the
16th ce, perhaps? At any rate, rotary of some sort is the only
answer. I have more options today, and machining a bow drill seems
extreme.
Not many if any marble carvers are dealing with detail and I
couldn't find many into gemcarving to answer (very different, anyway--
levels 5-7 on the moh scale, very hard material), though I know it's
possible. People here will know a lot about air rotary tools, and
they will a lot about carbide quality; and they can extrapolate any
variations for a soft stone versus the more common soft metal or hard
wood; so I ask here.

I noticed that I can spend anywhere from $.25/burr on "new USA
made carbide burrs" on ebay, to $1, $2 on resharpened carbide from a
more legitimate internet store, to as much as $15 for an "aviation"
burr from one ebay seller.
I know enough to understand that grain size and machining
accuracy count for a lot, a lot more than simply the magic words
"tungsten carbide."
I probably don't want to afford retail price at whatever store
in Boston would carry them, but in shopping for ebay burrs, internet
store burrs, or resharpened and 2nd-hand burrs-- what do I need to
know? There was one ebay seller with a set of 20 2nd-hand aviation
tungsten carbide burrs that sold for $60 after 16 bids, another with
100 USA-made carbide (is all carbide necessarily tungsten? I think
so, in terms of tools-- of course there are other carbides) that
didn't sell for $40. What did those 15 bidders know that I don't;
even though the other burrs were still made in the US? How to judge
quality of the carbide and machining, from a country of origin and
maybe a brand name? Does it make a difference in lifespan; will a
cheaper carbide fragment and wear faster, because of larger and uneven
grain size in the material, as conjecture?

Marble is easily harder than mild steel (I made some chisels,
thought it was tool steel, quickly learned it wasn't), but is softer
than work-hardened bronze or even annealed tool steel (but not by
much). It has fine grains that splinter out, and is brittle. Where
it's good, it can hold up very well; but you'll run into soft spots
where you can crumble it with you fingers if you make a deliberate
effort to do so. It seems somewhat abrasive on tools, so we like to
use carbide, but steel can be made sharper and is easily resharpened
so it has many fans.
I'll need to drill holes for undercuts, they can't be chiseled;
and for very fine areas like hair, hands and feet, facial details on
2" head, say, will be very difficult to chisel without chipping out
the grains from some intricately contoured cheek and nose.
I'm thinking spade bits for drilling, glass-drilling bits welded
onto a longer steel rod for those deep undercuts?
And small, fine double-cut burrs for those details? I can use
sandpaper later, or use fine diamond coated burrs afterwards for
finishing; but use the double-cut burr for stock removal... unless
there's a burr that will leave a fine finish to begin with, without
too much sacrificing stock removal-- at that point I'd be down to
smaller details anyway.

As for rotary tools to put said burrs into,
high speed, low torque; or low-torque, high speed? Air or
electric w/ flex shaft? The smaller air tools, with only a 1/4"
plastic tube, might be easier to wield than a coiled wire inside a
rubber hose like the foredom flex shafts.

There a
-cheap dremels with flex shaft, 5k-25k rpm
-better foredoms with flex shaft, 5k-35k
-slower speed, higher torque foredoms
-dental drills, like foredoms but with armature instead of flex shaft
-(mini) die grinders, air powered, 50k
-air dental drills, like mini die grinder-- maybe lower torque, but
better built?? Just guessing; would seem that professional dentistry
would have a higher standard than the $30 mini die grinders on sale.
-turbocarvers and their kind, air, 300k

any recommendations?


thanks!!
-Bernard

Bernard Arnest wrote in news:7e14ebd7-7f04-4864-
:


There a
-cheap dremels with flex shaft, 5k-25k rpm
-better foredoms with flex shaft, 5k-35k
-slower speed, higher torque foredoms
-dental drills, like foredoms but with armature instead of flex shaft
-(mini) die grinders, air powered, 50k
-air dental drills, like mini die grinder-- maybe lower torque, but
better built?? Just guessing; would seem that professional dentistry
would have a higher standard than the $30 mini die grinders on sale.
-turbocarvers and their kind, air, 300k

any recommendations?


A few, based upon some familiarity with the use of some of the equipment
on wood, glass, and metal (copper, brass, steel - both hardened and
annealed - and quartz:

First of all, don't overlook Diamond burrs for working stone. The prices
have dropped considerably in the last few years and I've picked up
assortments of 50 burrs for as little as $5/set.

The HF "Micro Die Grinder" is a nice little thing - high-speed, quite
maneuverable, etc. - that uses quite a bit more airflow than they claim.
(You'll need a good-sized shop compressor if you plan to do very much
with one since the 2 CFM spec is an absolute joke - it's more like 5 CFM)

The flex-shaft dremel (and the imitations) will work IF you keep the
shaft well-oiled. Otherwise, you'll be replacing the shafts on a regular
basis.

Foredoms beat Dremels.

Keep in mind that Dentists don't need to "hog out" a lot of material at a
time since they're usually dealing with very small adjustments at a time.

For large-area work, you might want to consider 1/4"-shaft Diamond burrs
in a wood-working router.

Straight drilling is easy if you use carbide-tipped Masonry bits.