[TheForge] Drilling stone

Sat Jun 22 13:01:01 2002

Yes. Petrified wood is agate and not only very hard, it's very brittle with
(usually) no well defined fracture planes. This last means it'll fracture in
almost any direction, like glass.

Those being the general properties of the material you need a drill that is
ultra hard and gentle at the same time. This spells diamond bits and liquid
coolant with a steady rotation speed and pressure.

A carbide abrasion bit will work but you'd be pushing it mighty close to the
limit of it's ability to cut. As with any tool pushed to it's limit you run
the very real risk of a catastrophic failure. In this case it means carbide
either shattering or just coming loose and jamming the bit in the hole.
Drilling anything as hard and fragile as agate this almost invariably means
shattering the stone as well.

I'd recommend a liquid cooled, diamond bit at high speed with a gentle
steady feed pressure and a vigilant ear for squeal. If it starts squealing
let off the feed and EASE back into it. NEVER stop the rotation unless it
actually jams. And never, NEVER let the water stop. If the return water
stops, (water coming out of the hole) kill the feed, retract the bit, then
kill the roatation.

Before you try drilling the piece you want to keep I'd suggest you practice
on as closely matched pieces as you can find from a rock shop, etc.

You can also make drill bits from tubing and grit (diamond, etc.) Copper
tubing works very well as the grit imbeds in the copper and cuts the stone
better. The downside of copper is it's sensitivity to being twisted off in
the hole by an over exuberant operator. <grin> A tubing bit has the
advantage of being able to deliver coolant in an open circuit. Basically
feed it down the tube and return it up the hole around the bit. This really
helps prevent cuttings from jamming the bit in the hole.

There are compressed gas cooled bits as well but I've never cared too much
for them. Not because they don't work but they're noiser, dirtier and fail
with less warning.

Frosty
------------------------
If it ain't forged
it ain't real.
Wrought iron is.
The FrostWorks

Meadow Lakes, AK.

----- Original Message -----
From: "Larry" <[email protected]>
To: "theforge" <[email protected]>
Sent: Thursday, June 20, 2002 2:59 PM
Subject: [TheForge] Drilling stone

> Has anyone tried to drill a piece of petrified wood for a
> tang?  I have a beautiful piece that I have wanted to drill
> for years but have no idea where to start.  The last thing I
> want to do is damage it.
>
> Any ideas?
>
> Larry
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