Re: [CAD_CAM_EDM_DRO] EDM GENERATORS

john_glynn57 wrote:

> I WISH TO BUILD OR BUY AN EDM GENERATOR. I UNDERSTAND THAT THERE ARE
> TWO TYPES OF GENERATOR:
> 1. BASED ON A CAPACITOR\INDUCTOR CIRCUIT.
> 2. BASED ON HIGH POWERED SEMICONDUCTOR TECHNOLOGY.
> THE SECOND TYPE IS MY PREFERENCE AS I WISH TO PRODUCE A TRIANGULAR
> WAVEFORM WHERE I CAN ACHIEVE THE MAXIMUM PEAK POWER YET WITH THE
> MINIMIUM PULSE DURATION AND THE HIGHEST POSSIBLE FREQUENCY. WHILE THIS
> TYPE OF GENERATOR IS READILY AVAILABLE ON MODERN COMERCIAL MACHINE
> TOOLS COULD ANYONE GIVE ME AN IDEA OF WHERE TO START TO RESEARCH
> BUILDING\BUYING MY OWN

Here's a description that Ed Huntress wrote on
rec.crafts.metalworking in 2003 :

Here's the basic concept of Sodick's power supply ca. 1980. It
was designed
to get around the patents of Charmille's Robofil power supply,
which used a
more complex (and slightly more effective) approach.

The simplest, standard circuit (there were about four separate
ones, IIRC,
for different cutting conditions) was actually three power
supplies in
parallel. The first was a high-voltage, high-impedance circuit
intended to
polarize a plasma channel. It was about 300 V, but I don't
remember the
impedance.

In parallel to that was a 90 V, medium-impedance circuit that
initiated the
plasma and got some current flowing.

In parallel to that was a high-amperage circuit that could
deliver something
like 50 average amps, at around 10 - 12 volts.

The first circuit prepared the way for the second, and the
second for the
third. When the third circuit fired, the plasma channel opened
up and
carried an extremely dense current, which melts the little glob
of metal
that each spark produces.

The sequence was repeated at about 4 kHz. At the end of each
spark, the
circuit has to shut the current flow down, HARD, in order to
prevent a
self-perpetuating arc. Once an arc starts, current will flow
even from the
high-impedance circuits, and the workpiece will develop a weld
spot and be
wrecked.

All of this has to occur with a minimum of inductive reaction.
If that
sounds easy, remember that you have peak amperages of around 500
A flowing
during the discharge, if you're running at a 10% duty cycle
(roughly average
in those days). Current density within the plasma channel is
immense and
uncontrolled reactions ring like a bell.

Controlling the initiation of each spark is a mean-voltage
sensing circuit
and a current-sensing circuit that actuates the servo and that
overrides the
shut-off circuitry to prevent contact-welding. If you get
contact, the
electrode often will weld to the work and blow the power supply
to hell.

This is an extremely simple system by today's standards,
although it was
very effective on the ordinary run of work, with ordinary
projected-area
EDMing. When you have pointy electrode projections or when
you're trying to
do contouring and have minute projected areas, this circuit
slows down to a
crawl. Still, it's many times faster than an RC circuit under
the same
conditions.

--
Ed Huntress