In article ,
Electric Comet wrote:
On Tue, 01 Nov 2016 20:08:17 +0000 (GMT)
Stuart wrote:

Electrical discharge machining.

yes

Also know as spark erosion. A well known technique for removing broken
taps from metal.

so that is how they do that

Depending on the capability required, they can be easy home-shop
projects.

are there off the shelf units

Commercially yes but not at prices you or I would want to pay.

i had never heard of it but now will have to look some time

in this competetion the operator could not see as it progressed
but instead listend and watched the bubbles

seems to work in a variety of material too
steel aluminum and pencil lead

wonder if it could be used in wood

I think it requires that the material be electrically conductive. I'm
surprised it works with graphite (not because of it's conductivity but the
nature of the material).

I saw a simple one demonstrated at a model engineering show and the plans
to build it were on a single sheet of A4 paper, I bought a set, and it
wasn't tightly drawn. A second much more elaborate one was featured in
"Model engineers workshop" (UK magazine) over two monthly parts.

I've seen nice, intricate, patterns cut into razor blades. Hardened
stainless steel and difficult to cut any other way because it's so hard
and thin.

--
Stuart Winsor

Tools With A Mission
sending tools across the world
http://www.twam.co.uk/