Apparatus for filtering machine liquid of an electroerosion machine

An apparatus for filtering a machining liquid of an electroerosion machine includes a pump for supplying the machining liquid to a filter having a membrane. The filter is constructed in such a way that the concentrate is carried substantially tangentially to the membrane wall in a circuit, and the concentrate quantity per time unit carried substantially tangentially to the filter membrane wall in the circuit is greater than the permeate quantity per time unit removed from the filter.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus for filtering a machining 
liquid of an electroerosion or spark erosion machine. 
Electroerosion machines have a working container filled with a machining 
liquid in which is immersed a workpiece to be machined by electroerosion. 
The machine liquid in the case of so-called wire cutting machines 
comprises pure water or a liquid, the main constituent of which is water, 
whereas it comprises hydrocarbons in the case of countersinking machines. 
During the erosion of the workpiece there is a reduction in the machining 
liquid purity and as a result its electrical conductivity also changes. In 
order to maintain a given quality of the erosion machining, it is 
necessary to purify the machining liquid, in order to keep its electrical 
characteristics and purity within predetermined limits. 
Attempts have already been made to use membrane filters in the filtering 
apparatus for filtering the machining liquid. Thus, e.g. JP-OS 62-24 917 
discloses a filtering apparatus with a membrane filter for an 
electroerosion machine. The filtering apparatus comprises a container for 
the machining liquid, which is subdivided into two areas by a cellophane 
membrane. The first area is connected by means of a pump, which is 
followed by a conventional filter, with the working container of the 
electroerosion machine and which, by means of a further pump, can be 
emptied again into the first area of the container. The second area of the 
container is connected by means of a further pump to an ion exchanger. The 
ionic concentration in the second container area is in the magnitude range 
below the ionic concentration in the first container area, so that the ion 
exchanger can be operated with a low ion density which is advantageous for 
its operating behavior. In this known filtering apparatus the membrane 
filter wall merely serves to bring about the adjustable reduction of the 
ion density for a cycle of an ion exchanger. 
For filtering the machining liquid of electroerosion machines, it is also 
known to use conventional filters with a large mesh size compared with 
membrane filters. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an apparatus for 
filtering a machining liquid of an electroerosion machine which would 
enable obtaining a high purity of the filtered machining liquid, without 
clogging of the filter. 
This and other objects of the invention are attained by a filtering 
apparatus in which a filter is constructed in such a way that the 
concentrate passes substantially tangentially to the membrane filter wall 
in a circuit. 
Surprisingly the apparatus according to the invention makes it possible to 
use membrane filters for cleaning the machining liquid, without clogging 
of the membrane filter wall as a result of impurities in the machining 
liquid. Thus, contrary to the expectations of experts, the apparatus of 
this invention makes it possible to use a membrane filter for the direct 
cleaning of the machining liquid, without expected problems occurring as a 
result of the contamination of the filter. The permeate cleaned by the 
membrane filter has an excellent purity and therefore also the necessary 
electrical characteristics for top quality erosion machining. 
The filter construction not only makes it easy to manufacture the filter 
with tangential guidance of the machining liquid, but it is also very 
suitable to prevent a deposition of dirt in the filter. 
The concentrate quantity per time unit carried in the circuit may be 
approximately 1.5 times to 10 times greater than the permeate quantity per 
time unit drained off by the filter. This results in particularly 
favorable flow quantities. It is pointed out that the optimum values run 
in the direction of a high ratio number (maximum 1) of a permeate to a 
concentrate. 
Very good filtering results with limited clogging tendencies of the filter 
can be obtained with the membrane filter wall pore size of 0.01 to 2 
.mu.m. 
The arrangement of the different containers in the inventive apparatus is 
particularly advantageous. A particularly efficient membrane filter is 
obtained with the hollow fibres having a pore size of 0.2 .mu.m. 
In the apparatus according to the invention, any particles which are still 
deposited and which could lead to a tendency to clog are removed. 
It is finally pointed out that, as is conventional practice in filter 
technology, the contaminated liquid to be cleaned is referred to as 
"concentrate", whilst the cleaned liquid is referred to as "permeate". 
The invention is described in greater detail hereinafter relative to a 
non-limitative preferred embodiment and with reference to the attached 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an apparatus 1 according to the present invention. The 
apparatus 1 is connected to a working container 2 of a not shown 
conventional erosion machine. The apparatus 1 comprises a collecting 
container 3, a flocculation container 4, a sedimentation container 5, an 
intermediate container 6 and a tank 7. The content of the working 
container 2 can be supplied by means of a discharge line 8 to the 
collecting container 3. The content of collecting container 3 can be 
transferred by means of a first pump 9 and line 9a into the flocculation 
container 4, to which can be supplied by means of a not shown metering 
pump a flocculant additive from a flocculant additive container 10 via 
line 10a. 
The content of the working container 2 can also be directly introduced into 
the flocculation container 4 by means of a suction line 11. Flocculation 
container 4 is connected to the sedimentation container 5 by a passage 12a 
arranged close to the bottom of containers 4 and 5. 
Sedimentation container 5 contains a plurality of sedimentation guide 
surfaces 13 which are slightly inclined with respect to the vertical. The 
sediments of the sedimentation container 5 can be removed by means of a 
first disposal valve 19a. An overflow 15 of sedimentation container 5 is 
used for transferring the precleaned machining liquid to the intermediate 
container 6. 
The precleaned concentrate is removed from the intermediate container 6 by 
means of a second pump 16 and is supplied to a membrane filter 17, which 
will be explained in greater detail hereinafter relative to FIG. 2, in 
circuit or cycle manner via a return flow line 18 to the intermediate 
container 6 again. 
In the case of an excessive accumulation of dirt in the intermediate 
container 6, the latter can be emptied by means of a second disposal valve 
19b and namely into a not shown settler, which can be emptied as required. 
Although not shown in FIG. 1, the concentrate can be supplied directly from 
flocculation container 4 and whilst obviating the sedimentation container 
5 and intermediate container 6, to the membrane filter 17. For this 
purpose, on the suction side of pump 16, a further suction line leads to 
the flocculation container 4 and a changeover or switchover valve is then 
inserted in the two suction lines of pump 16. 
The permeate is supplied to tank 7 via a feed line 20 and from the tank it 
is passed via a third pump 21 and a rinsing line 12 to the working 
container 2. 
As seen in FIG. 1, the concentrate is removed by means of the second pump 
16 from intermediate container 6 and is supplied to the membrane filter 17 
and then to the circuit again via return flow line 18. The permeate is 
removed from the membrane filter 17 via feed line 20 and is supplied to 
tank 7. 
Reference will now be made to FIG. 2, which shows details of the membrane 
filter 17 shown in FIG. 1 and the associated liquid circuit. 
As can be seen in FIG. 2, in a preferred embodiment, the membrane filter 17 
has a tubular membrane 22, which separates a concentrate area 23 from a 
permeate area 24. The filter is enclosed in a cylindrical wall 25. 
The drawing shows a single membrane filter element 17. In a practically 
realized, preferred embodiment a plurality of identical filter elements is 
provided in flow parallel connection. The flow rate of the concentrate is 
dependent on the dimensions of the filter and the requirements of the 
permeate cleanness. In principle, by setting different parameters, every 
effort will be made to bring the ratio between the permeate quantity and 
the concentrate quantity as close as possible to 1. However, at present, a 
ratio of 0.5 is considered to be good. 
A pressure of e.g. 2 to 3 bar is applied on the concentrate side to a 
membrane filter element. 
Preference is given to a filter, whose membrane wall is made from hollow 
fires constituted by polypropylene. The hollow fibres have a diameter of 
approximately 1.5 mm and a pore size of 0.2 .mu.m. 
Between tank 7 and membrane filter 17 is located an electrically operable 
valve 27 with which the membrane filter 17 can be shut off for a return 
rinsing from tank 7 and can be connected to a feed line 27 for an 
air-water mixture. 
At given intervals the membrane filter 17 is cleaned by rinsing in a 
reverse direction. It is theoretically possible for this purpose to carry 
out a rinsing back at fixed, predetermined time intervals. However, it is 
more advantageous to carry out said rinsing in a pressure or 
flow-dependent manner. As soon as the pressure on the concentrate side 
exceeds a given value, this constitutes a measure for a given clogging of 
the filter an this can be detected by a pressure sensor. So-called 
back-rinsing or rinsing in the direction counter to the filtering 
direction could also be initiated after a predetermined permeate quantity 
is reached, which quantity can be detected by means of a flow meter. For 
back-rinsing purposes valve 26 is switched over and then the membrane 
filter 17 is subjected to the action of an air-water mixture, to which can 
optionally be added cleaning chemicals. During a back-rinsing cycle, which 
can e.g. last 2 minutes, valve 26 is preferably stochastically operated 
and the individual operating phases can be very short (up to a few 
seconds). As a result of this irregular, pulse-like back-rinsing, an 
optimum cleaning effect is obtained. Naturally the second pump 16 is 
disconnected during the back-rinsing operation. 
There has been disclosed heretofore the best embodiment of the invention 
presently contemplated. However, it is to be understood that various 
changes and modifications may be made thereto without departing from the 
spirit of the invention.