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?


Probably not. My experience is you'll never get a fat spark from a TV anode
power supply over 1". More like 1/8" or so. You may get some
ionization, but
not enough to create a full conduction channel in air. So, I think you
need a LOT
more voltage per inch, maybe about 70 KV/inch, to get full breakdown.

Anyway, sparks in air just make noise and light, but don't do much to metal.
Running EDM processes under a liquid seems to greatly enhance the metal
removal effect. You can use water or hydrocarbon fluids, although the
latter
do pose a fire hazard. I have had good results using about 30 V at an amp.

You need a capacitor to deliver a burst of current, and then to let the
spark
die out as the cap recharges from the power supply. A light bulb can do
pretty well for that.
See my el-cheapo EDM at http://pico-systems.com/edm.html

Jon