Quick & dirty tap burner EDM - Results!

Thanks to everybody who pitched in with ideas for a quick
and simple EDM. The tap is out! Of course I could have
probably re-made the part from scratch in the time it took
to put together the rig and burn out the tap :-(

I wound up using an 0.062 OD brass tube as the electrode.
I made up a holder shaped like a T from larger brass tubing.
One end of the crossbar was plugged and went into the drill
chuck, with a plastic sleeve around it to insulate it from
the chuck. The other end of the crossbar had the electrode
soldered into it. I hooked a length of plastic hose to
the end of the "upright" leg of the T for the fluid feed.
(The "T" winds up on its side.) I was able to attach the
other end of the plastic hose to the nozzle of an old spray
bottle (window cleaner). That gave me a lot of choices for
fluid flow. With the bottle on the bench, I could just give
it a pump whenever I wanted to flush out the hole. If I
put the bottle up on top of my toolbox (above the electrode),
then once I primed it with the pump, a siphon started and
a couple drops per second would flow constantly. Putting
the bottle higher gave a steady stream. I used distilled
water (not deionized) from the grovery store. Even though
it was distilled, there was some electrolysis - fizzing
from the electrode when not sparking. Not enough to cause
problems, though. I expect that it would have been worse
with tap water.

The part was small enough to put in a "tupperware" type
container. Initially I just set the part in the bottom
of the container and set the container on the Shoptask
mill table. There are no real cutting forces, but the
occaisional bump or electrode touch kept moving the part.
Finally I put a little spacer in the bottom of the pan,
sat the part on top, and used a clamp from outside the
pan to hold the part down. Ordinary workholding
techniques, vises, clamps, etc., don't work so well
when you have to submerge the work in liquid!

I didn't rotate the electrode (obviously - the hose would
cause a problem...) Rotation might have helped - I had
uneven erosion of the end of the electrode, which resulted
in an uneven hole in the tap. But I think it was worth it
to be able to easily flush the gunk out of the hole. With
no rotation, the only way to adjust the hole size was to
move the table in X and Y. It's hard to see where you are
when the electrode is underwater, especially as the water
gets dirty. When it's sparking well, there is a steady
flow of inky black stuff from the hole - looks like under-
water smoke. Eventually the water in the pan got too dark
to see anything at all. I could hear the spark, and went
along like that for a while, but I had to empty the pan a
couple of times and refill with clean water so I could see.

I didn't build anything to vibrate the electrode, but I did
find that if I turned on the lathe motor of the Shoptask,
the vibration from the motor made it easier to maintain
the sparking. I think the vibrator used in simple tap
burners is their way of making up for the lack of a
sophisticated feed mechanism as found in a real sinker
EDM. Keeping the sparks going was a real pain. The
best I was able to do was maybe 10 seconds at a time.
More often it was 1-2 seconds of spark, then it either
stopped or shorted, and I had to advance or retract the
quill to get it started again. With the lathe motor
stopped, I never got more than a couple of seconds of
sparking at a time.

I didn't take any pictures last night, but the stuff is
still set up (I finished at 2:30 am!). If there is any
interest, I can take a few photos and post them before
I take everything apart and put it away.

Thanks again for all the ideas!

John Kasunich