Re: [CAD_CAM_EDM_DRO] Re: EDM Fluid

Thank you Jon:
My point exactly!!!
The reason this is all relevant to the EDM process, is that the maintenance
of cutting performance depends absolutely on maintaining a very low
concentration of any ions other than H+ and OH-, in an environment that is
continually producing a lot of metallic ions from the workpiece and the
wire.
The ion burden is sufficiently important that commercial machines have a
sensing system to continuously monitor the conductivity of the water, and a
resin bed consisting of porous plastic beads impregnated with strong acids
and strong bases to suck up the unwanted metallic ions.
As you can easily imagine, the ability of a dielectric fluid to resist
current flow will drop off drastically as the conductivity goes up.
EDM depends critically, on the ability of the system to jack up the current
to a threshold value before it crosses the gap between the electrode and the
workpiece.
The creation and subsequent collapse of the ion channel between electrode
and workpiece is what blasts a bit of metal away from the workpiece.
The dielectric fluid must recover its insulating properties extremely
quickly, because the current pulses happen very quickly: finishing
operations occur at 100KHz and more.
Oils do this by creating mostly insoluble or slightly soluble precipitates
as the end products of ionization...this makes oils self recovering in a
different way than water is.
Since water is such an efficient solvent, many more ionic species will stay
in solution with water, and progressively degrade its dielectric constant to
the point where it becomes a uniformly conductive ionic soup.
Discharges will occur at lower and lower threshold values, and the cut will
slow down and then stop.
So water requires a continuous active means of getting rid of the ions
floating around in solution, whereas oils are much more "self cleansing"
from the perspective of their conductivity.
Of course, all those insoluble precipitates make for a bigger "crud" burden,
which is why filtration is needed for oil based systems, and why the oil
goes "dirty" after a while.

Cheers

Marcus

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, June 08, 2002 11:43 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: EDM Fluid


> bjammin@... wrote:
>
> > At 10:31 AM 6/8/02 -0700, you wrote:
> > >Distilled water, on the other hand , refers only to water that has been
> > >evaporated and then recondensed.
> > >It implies nothing about the ion burden in the water
> >
> > Actually, it's about the exact same thing as deionized. Evaporation
removes
> > the dissolved minerals.
>
> It depends on how the distillation is done, and what condition the
> still is in. If the water in the boiler is boiling rapidly, particles of
> dirty water or scale can be blown into the condenser. For really
> pure water, you have to distill 3 times in glass, with the last still
> running at a low heat so it is just evaporating from the surface
> without boiling. I don't know how ion-free the water needs to be
> for EDM, I suppose it depends on the voltage used.
>
> Jon
>
>
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