Re: EDM? Was: Phase Control of SCR's; Hi Curren t

--- In CAD_CAM_EDM_DRO@y..., Tim Goldstein <timg@k...> wrote:

> Bill,
>
> I see in your tagline the DIY EDM. Can you tell us what it is all

about???

Sure - Real quick - here's my poor-man's EDM experience:

Back in 1994-1995 I'd hacked together a neat 3-axis CNC Dremel tool
'mill' - made from 2x4s, cabinet drawer slides, surplus 4-phase
bipolar stepper motors, and hardware threaded rod. Used an old 8088
PC to step the three axes around. Could do three axis linear, two
axis circular interpolation. Carved out a bunch of foam positives for
lost foam metal sand castings and the like. Could also mill Balsa and
other soft woods... Software was all in C - used some tricks to get
the speed up on the circular parts. Didn't use the printer port -
instead used an i/o expander (8255?) wired into the ISA bus and mapped
at user i/o address $320 I think...

Shaft encoders attached to each threaded rod were used in conjunction
with associated hardware to display the "real" position of the
steppers, in counts. Note that this does not acocunt for backlash,
and I never completed the loop with this data. The stepper motions
were constant velocity, no acceleration profiles etc.

For EDM mode - I replaced the Dremel (which was mounted on a form of
"Quick Change" tooling (plywood w/ wingnuts). :) For an EDM
electrode I was using either thin brass rod from the hobby store, or
mechanical pencil leads (0.5 mm diam, HB). Turns out mech pencil
leads are about half/half graphite and clay - but they do conduct
reasonably well enough - and they were all I could scavenge up on my
budget. Used WD-40 and a small windshield pump motor to slosh the oil
over the part. Didn't have any filtration. It was somewhat of a fire
hazard but I never had a problem since I always kept the area being
EDM'd well submerged.

Scrounged up a 72 volt DC power supply. Ran the power through about
a 100 ohm power resistor to limit the short circuit current draw.
Next the current traveled to a bank of between (I think) 100 - 300 uF
power factor correction capacitors (low resistance, or ESR caps). The
capacitor bank was connected from the graphite to the workpiece.

In practice- the Z-axis software was modified to not step the Z axis
unless the cap voltage was over about 65 volts etc. In this way as
long as the EDM arcing action was still taking place - the feed was
stopped. Once sufficient material had been removed - feed would
resume. Feed rate was slow - I recall 3-4 steps per second max -
using probably 18 tpi rod and a (?) 200 step/rev motor... My setup
wasn't mechanically rigid - and suffered from severe problems due to
rocking etc.

I managed to EDM some holes through some pennies, sheet metal and the
like - even EDM'd a series of holes on the perimeter of a larger hole
to knockout a disk etc. Metal removal rate was SLOW - something like
5 minutes per hole in a copper penny - but copper doesn't EDM well it
turns out... Electrode wear was also an issue but since I was just
plunging a hole clean through the other side of the work it didn't
matter. The pencil leads seemed to outlast the brass wires/tubes. I
never tried anything deeper than about 1/8th inch since I knew my oil
wouldn't flush out the contaminants and I didn't want to write any
'pecking' software to allow for flushing etc.

Well that about wraps it up. My biggest problems were mechanical -
involving my sloppy XYZ rigging. I'm in the process of rebuilding
some of this using more appropriate materials - things should go
better this time. I've been really impressed at the availability of
freeware etc CNC software - there was nothing like that available 5-6
years ago.

I see this list was started in conjunction with NAMES 1999. I hope
to be able to attend this year - any other EDM'ers going to be up
there? I've heard rumors of somebody working on an actual
wire-cutting EDM - using moly wire or something. Anyone know about
this? Reminds me of the hot-wire foam cutters for model airplanes and
the like - only this time it's for metal!

My sources indicate that graphite is nearly the best general purpose
electrode for EDM. Not sure where to get the proper dimensions etc
for EDM. I might try using TIG welding electrodes - pure tungsten-
available in 0.020 diameter for about $2 per 6 inch rod...

Comments encouraged.

-Bill (Cinci, OHIO)