ECM, The poor man's EDM??

I have always loved the idea of home EDM, the fact that on my small
mill I might be able to machine an electrode that was later plunged
into a block of metal so hard I could never dream of machining it
really appeals. Especially as I have an interest in making moulds.

I now have a reasonable idea of what is required to build a small EDM
machine that would be pulsed and hence quicker than the normal DIY
units that plans etc can be bought for but now I not sure if to
bother.

Enter ECM .....

Here is the basic idea:

If you put two electrodes in a salt solutuion such as NaCl(table
salt) or NaNO3 (part of some weedkillers) and then pass a current
through the solution, the positive electrode is "de-plated" or
dissolved. Not much use in machining but imagine you make the
negative electrode out of some tubing and insultate the outside of
the tube so that only the tip of the tube can conduct. As you bring
the tip of this electrode towards the postive electrode (the
workpiece) the current takes the easiest path and becomes more and
more concentrated. When the electrode is very close the dissolution
is very localised. To ensure that the material removed is flushed
away you can then pump the electroylte through the tube (at high
pressure in industry).

What you now have is an ECM drill capable of machining hard materials
with no forces (other than the pressure of the pumped salt water).

The other key thing is that for basic operation all that is required
is a DC powersupply. I decided to put the theory to the test last
night and using my adjustable 24v powersupply with a built in 1.5A
current limit (essential for occasional short circuits) I managed to
put a 1mm hole through 2mm of steel on my engineers square. All I
did was to put some salty water in a plastic tub. Attach the
positive crock clip to the square and the negative to one end of a
piece of enammeled copper wire. I do have some thin tube but no pump
so I decided to try without. The ennamled wire is obviously
insulated apart from at the tip so seemed a simple solution. As soon
as it was lowered into the salt water it started to bubble. When
brought towards the square the surface began to blacken over a
reasonably large area (3mm dia) but if the tip was put close to the
square and the powere turned on the blackening would be more
localised. Using the noise of the bubbles produced as feedback I
managed to get the tip very close to the square and the rate of
material removal was pretty high, I was holding the wire almost
freehand so it was tricky and I shorted it a few times but it had
made a noticeable crater in a few minutes. The hole took about 30min
but I removed more material than required and started a few other
holes by accident (remember free hand and the bubbles make it hard to
see)

The first modification to the test rig will be a micrometer stage to
hold the electrode and an ammeter. Plus I will try to find a pump
capable of pumping through small diameter tubing (any suggestions
appreciated!!!!!!!!!!!!!!!!!!!!!).

After than CNC on the Z should be straight forwardish (need to
control the gap and prevent shorts). Not only can you drill as I did
but also sink to make moulds and even wire cut to a certain extent.

Another even more interesting idea is the use of a hemispherical
electrode under 3-axis control, the lack of electrode wear (as per
EDM) means you can effectively do 3 axis "milling" in hard materials.

Drilling long holes (stem drilling) is another application where they
seem to use glass capillary tube with a thin wire down it as the
electrode.

Bottom line is, Friday I found out ECM existed, Sunday I found this
website:

http://www.unl.edu/nmrc/ECMoutline.htm

And the same day I did my first bit of ECM machining.

El-cheapo and safe tap remover anyone?

Cheers,

Graham Stabler
Nottingham
England