Basically the wire is highly charged and the electricity burns
the material away.
It used to be called spark machining. Nearly 50 years ago I was
associated with a Cutlery Research Association, which began to
introduce EDM to the highly conservative cutlery trade in Sheffield
England. The concept is simple. An electrode, usually brass, can be
any shape and is electrically charged to a high potential. A piece
of metal, like steel (which may previously be completely hardened) is
the opposite electrode. A very hot fat spark is made to jump between
the two whilst completely immersed in a circulating kerosine bath.
The intense heat of the spark vapourises metal at the point of
contact, and the kerosine instantly carries away the vapour and cools
the metal. The spark is not continuous as it is supplied by a very
large bank of electrical capacitors which are being charged and
discharged through the electrodes at a fairly high frequency. On top
of this, the brass electrode is very close to the workpiece, but is
moved up and down as the spark is discharged. Using this principal
one may make holes or depressions in hardened steel and stainless
steel of any shape or size.
At an open day, the Association showed a brand new 12" bastard file
which had a piece of brass curtain rod of weird section making a nice
slide-fit through it. Another piece of hardened steel had a curved
square hole machined through it from a face to an edge - a square
hole curving through 90 degrees! They’d made several pairs of half
scissors in brass, and used these as electrodes to make new hardened
and tempered steel stamping dies for turning out steel scissors -
otherwise a craftsman would have had to laboriously carve out the
dies. It is, in a way, similar to arc cutting.
There is a drawback; electrical discharge machining is very slow, so
they set it up and leave it to work all day and night, and the brass
forming electrode has to be changed every so often as it too wears
away, but more slowly than the workpiece. Thus, a University
research establishment where I worked built one of these machines.
Only something went wrong in the middle of one night the building was
very badly damaged by the consequent fire. (Murphy’s Law states that
if anything can go wrong it will, and a corollary states that it will
occur at the worst possible time; e.g. water pipes burst at 3am on
Christmas morning) I don’t know for sure, but I have the impression
that spark machining doesn’t work too well wirh very conductive
metals, like silver, copper or aluminium. But it is marvellous for
making steel dies.
John Burgess; @John_Burgess2 of Mapua Nelson NZ