In article , John Albers wrote:
I am thinking about building an EDM (electric discharge machine) which
remove tiny bits of metal by sparks. I have found plans and
schematics on the web and in various magazines like Home Machinsts
Workshop. I could just blindly copy their designs, but I wanted to
understand how they work first. Most are build around a relaxation
oscillator circuit which charges a capacitor and the capacitor
discarges across a spark gap. Most of the EDM machines that I have
plans for work with voltages around 80-100 volts. Expreimenting helps
me understand things better, so I tried the following experiment:

I took a couple of old HP deskjet 40V DC wall warts and wired them in
series to produce 80V DC. I checked this with a DVOM and I am getting
80V DC output. According to various sources the break down voltage of
air is around 20V per .001 inch. I have a block of wood (pine) with a
notch in it holding two machine screws facing each other to create a
spark gap. Using a piece of .001 inch brash shim stock, I adjusted the
gap to be .001 inch. I am thinking that if 80V is supposed to jump a
.004 inch gap, then 80V should have no problem jumping a .001 inch
gap.

I am not getting a spark. Should I be getting a spark with this set
up?

I believe it takes around 200, maybe 300 volts to break through air no
matter how little, and where the voltage and/or electrode geometry do not
favor corona and where the electric field within the spark gap is evenly
distributed, it takes about 75 volts beyond that 200-300 volt figure per
..001 inch (3 volts per micrometer), give or take a little depending on
temperature and air pressure.
I suspect EDM requires actual contact or some sort of assistance
("ignition pulses", UV, photoelectric effect, whatever) to spark at 80
volts.

- Don Klipstein )