Method of filtering the effluent from a wire EDM process

The present invention is directed to a method of removing impurities from the effluent of an EDM process wherein the improvement comprises steps of: providing a filter apparatus; providing the effluent from an EDM process to said apparatus; filtering impurities from the effluent thereby forming a filtrate; removing said apparatus from said effluent; and back flushing said apparatus with said filtrate, said back flushing conducted at a pressure greater than atmospheric pressure, thereby removing the impurities from said apparatus.

SCOPE OF THE INVENTION 
The instant invention is directed to a method for filtering the effluent 
from a wire EDM (Electrical Discharge Machining) process. Specifically, it 
has been found that the impurities collected on the filter elements can be 
effectively and quickly removed by back flushing the filter elements, at 
an elevated pressure, with the cleaned filtrate or cleaned filtrate and 
compressed air. 
BACKGROUND OF THE INVENTION 
In EDM processing, a potential difference, i.e. voltage, is applied between 
a moving wire and the material being machined. The voltage is increased to 
an amount sufficient to overcome the dielectric breakdown strength of the 
medium, e.g. deionized water or other dielectric fluid, which separates 
the wire and the material. When the voltage overcomes the dielectric 
strength of the medium, a spark is generated, and current flows from the 
wire to the material or vise versa, depending upon the polarity. Two 
things result from the current flow. First, the current ionizes the medium 
between the wire and the material being machined at the point at which the 
electric field intensity is a maximum and creates a cylindrical envelope 
of ionized gas, thereby electrically coupling the wire to the material. 
Second the gas expands in the medium because of the associated elevated 
temperature and then collapses, much as a cavitation bubble would 
collapse. The electric energy in the spark, therefore, initially heats to 
a melting temperature, a very small spot on the surface of the material 
being machined, and this molten material is then rapidly ejected by the 
collapsing force of the ionized gas bubble. 
Perhaps this is a somewhat different method of defining the mechanics of 
electric discharge machining. One can imagine, for example, that if an 
electric current can be developed through an ionized gas, as occurs in 
tungsten inert gas welding, then virtually all the current supplied is 
utilized to heat and melt the material away or contribute to bonding, such 
as in welding. In this case, the material tends to remain at its original 
site, although it may become molten. In order to remove such material it 
is necessary to use other mechanical means. 
In the case of electric discharge machining, the action of the expanding 
gas and subsequent collapse of the gas bubble provides the mechanical 
means for ejecting this micro (very small) puddle of molten metal at the 
site of the spark initiation. 
By-products of the process comprise numerous impurities which may include, 
among other things, fine particles of the material machined, and metal 
hydroxides or metal hydroxide salts formed when the medium is ionized. The 
exact composition of these impurities is not known with certainty. 
Somethings are, however, known. The principle one of which is that much of 
the debris is conductive, so that its random distribution in the medium, 
in effect, alters the distance between the wire and the material being 
cut, thus reducing the voltage at which dielectric breakdown will occur 
and thereby limiting the amount of energy that can be carried by the 
spark. Thus, the impurities effect the dielectric strength of the medium. 
These impurities, which build up with time during operation, reduce the 
medium's effectiveness as a dielectric. Thus, it is necessary to filter 
out these impurities so that the EDM process can be effectively 
controlled. 
In U.S. Pat. No. 4,740,315, a filtering system for an EDM process is 
disclosed. Therein, an edge filter is utilized to remove the impurities 
from the EDM process effluent. The impurity coated filter elements, 
however, are cleaned by air drying the impurities to a powder. At the time 
that method was proposed, it was not believed that water back flushing 
would cleanse the filter elements of the impurities because of the 
tenacity with which the impurities (in sludge form) adhere to the filter 
elements. This air dry cleansing method is not only time consuming, but it 
is expensive because the air blown across the elements is typically 
heated. U.S. Pat. No. 4,740,315 is incorporated herein by reference. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method of removing impurities from 
the effluent of an EDM process wherein the improvement comprises the steps 
of: 
providing a filter apparatus; 
providing the effluent from an EDM process to said apparatus; 
filtering impurities from the effluent thereby forming a filtrate; 
removing said apparatus from said effluent; and 
back flushing said apparatus with said filtrate, said back flushing 
conducted at a pressure greater than atmospheric pressure, thereby 
removing the impurities from said apparatus. 
This method is an improvement over the method set forth in U.S. Pat. No. 
4,740,315. The instant method is quicker in operation: back flush 
cleansing time can be reduce to about 1 to 12 seconds. The instant 
invention is less expensive to operate: the necessity for heated air is 
eliminated. The instant method is simpler to operate: the air blowing 
apparatus is eliminated and the dual filter arrangement is reduced to a 
single filter.

DESCRIPTION OF THE INVENTION 
Referring to the drawings wherein like numerals indicate like elements, 
there is shown in FIG. 1 a filter apparatus 10 made according to the 
instant invention. 
Effluent 11 from a wire EDM process is introduced into effluent chamber 12 
of filter apparatus 10. Edge filter apparatus 18 is immersed in effluent 
11. Edge filter apparatus 18 comprises of plurality of filter sticks 20 
connected to a manifold 22. Individual filter sticks 20 may comprise a 
plurality of annular filter disks 23, see U.S. Pat. No. 4,710,402 
incorporated herein by reference, stacked upon one another about a tube 
(not shown) through which the filtrate may pass to the manifold. See U.S. 
Pat. No. 4,664,814, which is incorporated herein by reference, for a 
non-limiting example of the apparatus which holds the disks 23 on the 
tube. Apparatus 18 is pivotal between the immersed position shown in FIG. 
1 and an elevated position shown in FIG. 2. 
Manifold 22 is connected to a pump 26 via line 24. Pump 26 is impeller 
pump. Such pumps are well known to those of ordinary skill in the art. 
Pump 26 is driven by a reversible electric motor 28. Pump 26 and motor 28 
are preferably contained within a pump chamber 16. 
A source of compressed air 25 is in communication with line 24 via line 27 
which may have an inside diameter of 8 mm. The compressed air, at a 
pressure of about 7 bars, may be combined with the filtrate during back 
flushing. The combination of compressed air and filtrate provide the 
necessary back flushing pressure and flow-through velocity to obtain 
effective filter cleaning. 
A filtrate chamber 14 which receives filtrate 15 is preferably located 
adjacent to effluent chamber 12. Filtrate chamber 14 is connected to pump 
26 via conduit 30. 
Wall 34 which separates effluent chamber 12 and filtrate chamber 14 may 
include a plurality of overflow holes 32 which allow filtrate to spill 
over into effluent chamber 12. Filtrate is generated faster than effluent. 
At the base of manifold 22 is a lip 36. When filter apparatus 18 is in its 
elevated position (see FIG. 2), lip 36 protrudes through a port 38 in a 
side wall of apparatus 10. During the back flushing, the impurities or 
sludge 42 formed on the filter sticks cascades off the filter sticks 20 to 
lip 36 and into a sludge box 40. 
In operation, effluent from the EDM process is charged into chamber 12 and 
filter apparatus 18 is immersed in the effluent 11. See FIG. 1. Pump 26 
draws the effluent to the filter sticks 20 and removal of the impurities 
occurs at the outside of the plurality of disks and in the interstices 25 
between filter disks 23. Note U.S. Pat. No. 4,810,380 is incorporated 
hereby by reference. Filtrate 15 is drawn into the manifold 22 and through 
the pump 26, then discharged into chamber 14. After a period of time 
during which impurities are built up on the filter disks 23, filter sticks 
20 are raised out, any conventional method may be used, of the effluent. 
See FIG. 2. Pump 26 is then reversed so as to draw filtrate from chamber 
14. The back flushed filtrate is pumped into the filter sticks 20 at a 
pressure of approximately 2.5 bar. The cleaning action takes place in 
approximately 10 seconds although times between one second and twelve 
seconds are also contemplated. Alternatively, the combination of filtrate 
at about 2.5 bar and compressed air at about 7 bar are introduced into 
manifold 22 from line 24. This also performs a good back flush cleaning of 
sludge from the filter disks. It is also within the scope of the back 
flushing scheme to pulse the back flush of filtrate alone or in 
combination with the compressed air to the filter disks. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specifications, as indicating the scope of the invention.