On Wed, 26 Aug 2015 11:43:18 -0700 (PDT), SteamboatEd Haas
wrote:

On Saturday, August 15, 2015 at 3:00:47 PM UTC-7, 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

Well if you're anywhere near Santa Rosa, CA I've got an old Hansvedt tap buster that we recently rebuilt... ;-)

Unfortunately nowhere near. I tried the welder. Made a cylindrical
back cap out of delrin and brass, chucked in the drill press with the
test piece well insulated and the chuck grounded to the column. It
didn't do much. I tried ac, dc, varied frequency, pulser, etc.
Interesting, but I'm not going to live long enough to get anything
useful done.

Oh, tried welding the tap too. finally got some ss welded on the tiny
tap, but it broke off deep in the hole when I tried it. Then even
with a sharp .040" tungsten, the arc just raced around the cast iron
in a little circle.

By the way, +2.5 diopter cheaters in the hood plus my 18" focal length
safety glasses gave me a good close view of the arc and work area.

Looks like Ed was right. Langlois' book talks about switching in
capacitors for more spark, up to 700 microfarads. So I'm going to
start that way, putting together a list of stuff the surplus place in
town has available.