Page 74 - Fall 2023 EDM Today

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range from 40 µS for municipal water from a surface

municipal well water.) However, as the resin bottle is
Excessive DI Recovery Time reservoir to as much as 200 µS for either domestic or
used, the conductivity of its output water will steadily
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decline. Thus, the recovery times of the DI system will
After topping off my Wire EDM with city water to make
gradually increase due to the increasing conductivity of
up for evaporation and drag-out losses, I have noted
the bottle output water. The most common conductiv-
increasingly excessive times for the machine to return
ity lights have a switch point of 20 µS (or 50 KOhms).
to its conductivity set point. As you have recommend-
Therefore, it is entirely possible that you could observe
ed in the past, we have equipped our resin tank with
long recovery times with a green light as the conductiv-
a red-green conductivity light, and the light is still
ity of the bottle output water approaches the set point
showing green. Do you have any idea of what could
be the cause of our observed long conductivity
recovery times?
For example, just recently when I drained all the water
from my lab machine for semi-annual maintenance
- Fred conductivity of your machine.
and then refi lled it. After six hours the machine water
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Roger Kern’s reply:
conductivity was only 25 µS even though I had a green
light. Suspecting the resin bottle was near the end of
There are possibly a number of causes for the condi-
its life, I replaced it, and the conductivity was restored
tion you’ve observed:
to the 15 µS set point in less than twenty minutes.
It is possible that you have a low fl ow condition in your
resin circuit:
Carefully observing recovery times is the best way to
determine remaining resin bottle capacity. If you really
Some resin systems employ a fi lter prior to the
•
resin bottle inlet. The purpose of this fi lter is to
purchase one with a lower set point (such as 10 µS
remove any contaminants that may have passed
or 100 KOhms), but you’ll then be replacing your
through the primary dielectric fi lter in order to want to depend on the conductivity light, you could
bottle more frequently before you have utilized its
improve resin life. At some point this fi lter will get
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total capacity.
clogged and reduce the fl ow to the bottle.
•
Most Wire EDM users add a “safety fi lter” to the
outlet side of the resin bottle. This fi lter consists
Diffi culty Re-dressing Copper Tungsten Electrodes
of a fi ne mesh screen encased in a clear housing.
In a conversation with the machinist who does our
The purpose of this fi lter is to protect the machine
milling on copper tungsten, he had mentioned that
from contamination by resin beads in the event
after a few times of milling and after the RAM EDM
of either a failure of the resin bottle lance screen
or the possibility that the operator inadvertently
becomes harder to recut. So I was wondering if this
reverses the resin tank hose connections. In the
was an accurate statement by the machinist, and, if
event of either of these occurrences, the screen process the copper tungsten
it is true, then if there is a process to normalize the
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will catch the errant resin beads, protecting the
material after ram EDM to make it easier to machine?
machine, but will also stop the resin circuit fl ow
once it has clogged.
- Pat
However, the most likely cause is much simpler to
Brian Pfl uger’s reply:
rectify. A freshly regenerated resin tank will produce
The machinist is correct. The properties of electrodes
water with a conductivity of less than 1 µS (or resistiv-
can be different when re-machining them after the
ity of one megohm). Since most machines operate in
EDM process. I would bet that your machinist encoun-
the range of 15 µS, the 1 µS water from the resin bottle
will quickly restore the machine to its conductivity set
cut electrodes. The roughing electrodes see the high-
point when make-up water is added. (Depending on ters the most severe issue when re-milling rough
est level of power and achieve the highest material
the source, the conductivity of the make-up water can
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