Re: [CAD_CAM_EDM_DRO] EDM Fluid

Hi Hoyt:
This is getting to be a very interesting discussion of how the whole EDM
process is supposed to work.
I admit that my recollection of 2nd year inorganic chemistry is no longer
what it could be, but I'm having a bit of difficulty with some of what you
say.
A half remembered blurb by Agie or Charmilles(I can't remember which)
seemed, to me, to state that the spark vapourizes or melts the substrate,
but it is the gas bubble collapse at the end of the cycle that actually
blasts the molten bit free, and that the molten remnant that is left behind
forms the recast layer.
The ionization of some of the metal is not too far fetched when you consider
the energies involved, and of course, the presence of water should hold them
stable in the ionized state...(unless I've completely misunderstood that
Chemistry course way back when).
I was always operating under the belief that ionization was not exclusively
a consequence of solution but could also be achieved by high energies, such
as those used to create plasmas (ion clouds??)
The deionizing resin bed, from my understanding, substitutes H+ for metal
ions and OH- for non-metal ions.
I assume that the remaining ionic species from the dissolution of the acids
and bases used in the resin bed, remain bound to the carrier beads.
I do know that the conductivity sensors in the water system operate a valve
that directs water into the resin bed when the conductivity rises above the
threshold set by the valve and sensor.

To drag this whole discussion back to the practical; I contend that the
practical problems in using water as the dielectric in a home brew system
are not trivial, and center around keeping the dielectric sufficiently
non-conductive to maintain reasonable properties.
With our rather crude home made electronics for control of all the other
things that need to happen for the system to work well, it seems to me, that
ignoring this aspect of the process invites poor results.
Distilling the water in the family teakettle and then using it in the belief
that it has been optimized for use and will stay that way long enough to do
useful amounts of work doesn't strike me as the best way.

Don't forget, as Mark Fraser pointed out...you will consume a fair bit of
expensive wire to make your cut.
It's worth tuning the system as best you can.
Cheers

Marcus

----- Original Message -----
From: <bjammin@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, June 10, 2002 7:58 AM
Subject: [CAD_CAM_EDM_DRO] EDM Fluid


> At 08:14 AM 6/9/02 -0700, you wrote:
> >concentration of any ions other than H+ and OH-,
>
> Actually, water itself has nearly no ions. The concentration of those two
> is so low it's probably way less than 1 in 10^8 or so.
>
> >continually producing a lot of metallic ions from the workpiece and the
> >wire.
>
> Really, it's producing lots of metallic fines but very few metallic ions.
> In order to produce metallic ions in water, the metal has to react with
> other chemicals, to which each metal atom loses an electron. In pure
water,
> there are no other chemicals.
>
> >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.
>
> Sensing the conductivity is a grand idea. But strong acids and bases, in
an
> aqueous medium, will neutralize each other, usually to produce a dissolved
> salt and high conductivity. Also, plastic ion exchange resins do not need
> those in order to remove ions, if any ions exist; in fact, in say water
> purification systems, they are pre-impregnated with NaCl. That is
> substituted for ions that harden water, leaving it soft but salty. And
> conductive to precisely the same degree. If indeed ion exchange resins are
> used in EDM and exchange ions, they would do exactly the same thing. Not
to
> say that water-based systems do or don't use resin beds, but if so for the
> several reasons above they don't exchange.
>
> >The creation and subsequent collapse of the ion channel between electrode
> >and workpiece is what blasts a bit of metal away from the workpiece.
>
> Not quite: the spark itself erodes the material. Ion channels do not
> themselves 'collapse', they just go neutral when electric stress is
removed.
>
> >Since water is such an efficient solvent, many more ionic species will
stay
> >in solution with water,
>
> That's true of water and ions, but there are few ions if any produced by
> EDM in pure water. Or, where do they come from?
>
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