On Sat, 15 Aug 2015 17:00:52 -0500, Pete Keillor
wrote:

I broke a 4-40 tap in my filing machine casting. It was for the top
cap, which is there to hold packing as a seal for the file rod, so not
that big of a deal since there are five good screws. But it feels
like a berry seed in my tooth. Aggravating.

I got Langlois' book on building a hobbyist edm, but have a few other
thoughts. I have a very nice tig welder, Miller Dynasty 200DX. In
tig mode, it'll go down to 1A DC or 5A AC, plus a pulser on dc up to
250 Hz and lots of control over ac frequency and wave form. Seems I
might be able to use it in lift start mode for edm.

I'd still need to build the carriage, stepper drive, controls, etc. I
have a Beaglebone Black which could be applied to that job. I can get
a reasonable stepper for $11 from the salvage place in town, plus some
linear bearings and a bellows coupler for not much.

Any known reasons the welder is a bad idea?

Thanks.

Pete Keillor

It's an interesting idea. Basic EDMs deliver current from a big
capacitor, however, so the energy in that spark is a lot greater than
you think.

We had a tap buster (a proto-EDM) in my shop back in the '70s. I think
we paid $25 for it, used. The power supply was a simple RC circuit and
it was the size of a lunch box. It had a hand-cranked feedscrew
rather than a servo. It worked very well. I spark-eroded a stud out of
the head of my Honda motorcycle with it (a frequent need in those days
-- Honda's bolts and studs were made of frozen ****), and eroded
several taps out of pieces we were working on. The hand feed made it
kind of slow, but the real electric servo systems in EDMs are kind of
special. Steppers are FAR easier to implement for this job than servo
motors, which is why Sodick used them into the 1980s. If you want to
know why, I'll explain.

What I don't know is if a welder power supply will do it. You might
want to take a look at this description of early EDMs from Poco
Graphite, which is close to being accurate:

http://edmtechman.com/about.cfm?pg=2&chap=1

I think you could try it out by hand, to see if you get the results
you want. Take a screw and try repeatedly lift-starting on the end of
it. See if it actually eats the screw quickly enough. If so, it will
be worth taking the next step.

BTW, tap busters did not use liquid dielectric, with few exceptions.
They just work in air. Be careful; hot globs can spit out of the hole.

Good luck!

--
Ed Huntress (former marketing manager for Sodick)