Re: [CAD_CAM_EDM_DRO] ECM, The poor man's EDM??

----- Original Message -----
From: "Graham Stabler" <eexgs@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, March 01, 2004 4:44 AM
Subject: [CAD_CAM_EDM_DRO] 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
>
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