The
flushing process during spark erosion
Every experienced spark erosion
expert knows that the flushing process is of utmost importance, when metals are subjected
to this procedure. The dielectric must flush away the eroded particles from the gap
between electrode and work piece, otherwise they may form bridges, which cause short
circuits. Such arcs can burn big holes in the work piece and in the electrode. Modern
spark erosion plants therefore have a builtin power adaptive control system, which
increases pulse spacing as soon as this happens and reduces or shuts off the power supply
completely. The more thin-bodied a dielectric and the lower its surface tension, the
better it is able to meet flushing requirements.
Open flushing
Open flushing is the most common form of flushing and is used when it is impossible to
do additional flush boring.
Fig. 1. Open flushing |
Pressure flushing
Next to open flushing, pressure flushing is the most important form. The dielectric is
either pushed through a flushing hole in the electrode from above, or through a flushing
hole in the work piece from below. The amount of dielectric flowing through is more
important for effectivity than the pressure of flushing. When calculating the smaller than
specified dimension of the electrode, it must be remembered that in this type of flushing
particles rising up through the lateral gap are continuously causing additional erosion.
This leads to minor defects in precision.
Suction flushing
In suction flushing the eroded particles are sucked out of the gap between electrode
and work piece. This type of flushing is best in those cases, where a fine finish and
paralell walls are required in the work piece. When using this method with narrow gaps and
small amounts of dielectric flowing through, care must be taken that enough dielectric
gets into the spark gap, so that the spark erosion process will remain stable.
Combined flushing
In very complex jobs it may be advisable to combine suction and pressure flushing.
Interval flushing
In interval flushing the erosion process is interrupted after a while and the
electrode is retracted. This improves the flushing out of the eroded particles. The
retraction and return of the electrode has the additional effect of suction and pumping
respectively, which improves the effectivity of the flushing process. This method is
particularly suitable, when deep depressions or thin electrodes are involved, and also
during finishing work.
Filtering
the Dielectric
In order for the dielectric to
perform its flushing function in the best possible way, eroded particles from the
workpiece and the electrode, as well as the cracked parts of the dielectric itself, must
be removed. In addition the dielectric, which heats up during spark erosion, has to be
cooled down again to a normal working temperature of 20° C - 300° C. If it is too hot,
there will be inaccuracies in the work and much of the dielectric will be lost through
evaporation. For this reason every spark erosion machine has a filtering plant, which has
the following functions to perform:
a) Storing the dielectric
b) Cleaning the dirty dielectric coming from the work tank
c) Providing the required amount of clean fluid and the necessary pressure for rapid
filling as well as for pressure and suction
flushing
d) cooling the dielectric (by air, water or cooling plant)
e) processing backwashed fluid and filtrate
Cartridge filter system (see Fig. 1)
In practice cartridge filter systems have proved very effective for filtering dielectric
in smaller spark erosion plants, in which up to approx. 450 MM3 /min are eroded. Cartridge filter systems are simple, and, as far as the cost
of acquisition is concerned, inexpensive apparatuses. In the main they consist of a
storage tank, filter pump, machine pump, cartridge filter, cooler and the requisite
piping. The plant is operated manually. The filter element itself is housed in a pressure
resistant container and consists of a piece of paper, folded like a star and arranged
around a central pipe. The filter cartridge is not reusable. Once it has attained its
maximum capacity for retaining dirt, it has to be replaced by a new one. The fineness of
the filtering effect of such a plant lies between 1 and 5 um, depending on the paper used. Under normal conditions the dielectric IME
can be used with a paper filter plant for about 1-2 years.
Precoated filter system (see Fig. 2)
In big spark erosion plants it is advisable to mount a so called precoated filter system.
In these systems the filter elements are coated with an even layer of filter aid, before
filtering begins. This layer may consist of diatomite, Rixid or cellulose. After
precoating is completed, the filter cycle of the plant is started, either by hand or by
machine. After a maximum differential pressure has been reached, the entire filter system
is flushed back and all the dirt on the filter elements, plus the filter aid, are expelled via a mud valve into the after-filter. After
the flushing back process is completed, the filter plant can be precoated anew and the
filter cycle restarted. The filter area should be large enough, so that all the dirt
accumulating during one shift can be absorbed, before flushing back becomes necessary. A
fineness of up to 1 um can be achieved with precoated filter systems. On the average 1 kg
of diatomite or 0.5 kg Rixid are required for 1 M2 filtering area. The residual
moisture of a dry sludge cake discharged from a precoated filter system lies between about 20 % and 30 % of the weight, depending on
the type of dielectric used. The service life of the dielectric in precoated filter
systems is very long, since diatomite and Rixid not only have a mechanical cleaning
effect, but also filter out acid components from the dielectric to a certain extent. In
precoated filter systems bleaching earth may also be used as a filter aid, in order to
clean the dielectric even more thoroughly. There is data available from precoated filter
systems, which were filled twenty years ago with a quantity of the dielectric IME, which
is still fully operative today. Merely the amounts lost through drag-out and evaporation
had to be replaced.
The Transor filter system (see Fig. 3)
The Transor filter system is able to produce a filtering effect of 1 um without the use of
filter aids by employing the edge filter principle. Filtering rods, on which thousands of
extremely fine special paper discs are mounted, are installed in a pressure tank. The
dirty dielectric is pumped into the pressure tank and pressed through the filtering rods
from the outside to the inside. As this system works without filter aids, no precoating is
necessary. The gaps between the paper discs are so narrow, that all particles that are
larger than 1 um are deposited on the surface of the filter rods. When the rods are dirty,
flushing back occurs. and the dielectric. which has already been filtered, is pressed back
through the filter rods in the opposite direction.
The dirt layer on the filter rods is blasted off and can be taken out of a sludge tank.
There is little sludge in comparison to the precoated filter system, because no filter
aids are used. The service life of the filter rods is on the average about 8,000 working
hours. In a Transor filter system one must make sure that the viscosity of the dielectric
does not excede 4.0 cSt at 20° C.
