Using the weight of the electrode or workpiece to control EDM working conditions

An electrical discharge machine is provided with at least one weight measuring apparatus for detecting the weight of one or both of an electrode and a workpiece, and a control unit for selecting or controlling the working conditions in response to the output of the weight measuring apparatus. The weights of the electrode and the workpiece are measured after the working operation has been interrupted temporarily, or while continuing the working operation. Well-known various accurate balances or force measuring apparatuses may be used as the weight measuring apparatus. A microcomputer including a numerical control unit may be used as the control unit.

FIELD OF THE INVENTION 
This invention relates to an electrical discharge machine, and more 
particularly, to an electrical discharge machine in which the work 
conditions can be properly controlled in response to the progress of work 
or a workpiece and to the magnitude of electrode wear. 
BACKGROUND OF THE INVENTION 
In a conventional electrical discharge machine, a working electrode and a 
workpiece are spaced at a predetermined gap and a pulse-like voltage is 
applied between the electrode and the workpiece while supplying a working 
fluid in the gap, thereby causing an electrical discharge to perform the 
spark erosion. 
In such well-known conventional electrical discharge machine, in order to 
prevent a short circuit and an abnormal arc discharge or the like from 
occurring between the electrode and the workpiece during the working 
operation, a servo motion of the workpiece and the electrode is 
additionally performed to automatically adjust the width of the working 
gap in response to pollution of the working fluid in the working gap and 
to a change in a discharge current or the like, in addition to the 
relative work-movement between the workpiece and the electrode. However, 
when the working speed (g/min) substantially drops or the electrode is 
excessively worn, the conventional electrical discharge machine described 
above cannot cope with these conditions. Thus, there are problems in that 
the working operation stops and sufficient work accuracy cannot be 
obtained. Effective means for eliminating such problems has not yet been 
proposed. 
SUMMARY OF THE INVENTION 
This invention seeks to overcome the above-described problems. It is 
therefore an object of the present invention to provide an electrical 
discharge machine in which the working conditions are appropriately 
controlled in response to the progress of work on a workpiece and to the 
magnitude of electrode wear, thereby performing a smooth and high-accurate 
working operation. 
According to the present invention, in order to accomplish the 
above-mentioned object, the electrical discharge machine automatically 
measures the weight(s) of the electrode and/or the workpiece at any time 
or in proper time periods using a high-accurate weight measuring 
apparatus; calculates the difference between the actual weight on the 
basis of its measured value and a predetermined weight, and the quantity 
of electrode wear or the like using a microcomputer; and suitably selects, 
changes, or controls the working conditions in accordance with the 
calculated data, thereby maintaining the optimum working conditions. 
The weight of the workpiece is measured together with a work tank after the 
working fluid has been drained, thereby interrupting the working operation 
temporarily. Otherwise, only the weight of the workpiece is measured with 
the workpiece immersed in the working fluid. The weight of the electrode 
is obtained by measuring the electrode which was automatically removed 
after interrupting the working operation temporarily. Or else, it is 
measured with the electrode attached at the work location in the working 
fluid. 
Other objects, features and advantages of the invention will become 
apparent from the following description of embodiments while referring to 
the accompanying drawings, which show the details essential to the 
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIGS. 1 to 4, the same elements and components having the same or 
similar functions are designated by the same reference numerals. 
In FIG. 1, reference numeral 1 indicates a main body of an electrical 
discharge machine; 2 denotes a work table movable in the direction of the 
Y-axis; 3 shows a motor for moving the table 2; 4 is an encoder; 5 is a 
work table movable in the direction of the X-axis; 6 is a motor for moving 
the table 5; 7 is an encoder; 8 is a conical guide; 9 is an electronic 
balance with large capacity and high accuracy; 10 are hydraulic jacks; 11 
is a work tank; 12 is a workpiece; 13 is a working fluid; 14 is an 
electrode; 15 is a chuck for holding the electrode 14; 16 is a stem, 16A 
is a servo-control system for moving the electrode 14 in the direction of 
the Z-axis; 17 is a column; 18 is an oil tank for supplying working oil to 
the hydraulic jacks 10; 19 is an oil pump; 20 is a four-way valve; 21 is a 
service tank; 22 is a pump; 23 is a working fluid treating apparatus for 
performing filtration of the working fluid and adjustment of the 
concentration; 24 is a power circuit for spark erosion; 25 is an automatic 
electrode exchanging apparatus; 26 is a swing arm for exchanging the 
electrode; 27 is an electronic balance for measuring the weight of the 
electrode; 28 is an electrode magazine; 29, 30 and 31 are electrodes with 
different tip shapes; 32 is a control unit including a numerical control 
unit (hereinafter, referred to as an NC); and 33 is a micro-computer. 
The control unit 32 including the NC serves to control integrally the 
entire working processes, namely, the positioning along the X-, Y-, and Z 
axes, working motion, vertical reciprocation of the electrode, exchange of 
the electrode, power supply for work, or the like. 
Four columns 5a are mounted at the four corners of the work table 5, and 
the conical guides 8 are mounted on each pole in order to position the 
work tank 11. The work tank 11 is mounted on the columns 5a so that it 
engages the conical guides 8. The workpiece 12 is attached within the work 
tank 11 and is immersed in the working fluid 13. The working fluid 13 is 
drained from the work tank 11 if necessary, and recycled into the work 
tank 11 through the service tank 21, working fluid treating apparatus 23 
and pump 22. 
The electronic balance 9 of large capacity has a high accuracy of, for 
example, one to 100 thousandths (1/100,000) or better, and is mounted on 
the hydraulic jacks 10 on the work moving table 5. With such a 
construction, when the working oil in the oil tank 18 is supplied to the 
hydraulic jacks 10 through the four-way valve 20 by means of the oil pump 
19, the electronic balance 9 moves vertically. When it moves upward, the 
work tank 11 is removed from the columns 5a and is supported by the 
balance 9, thereby measuring the weight of the work tank 11 and the 
workpiece 12. This measuring operation is performed in accordance with the 
program of the control unit 32. 
Upon measuring, the working fluid 13 in the work tank 11 is entirely 
drained and the inside of the work tank is completely cleaned by clean 
working fluid from a working fluid jet nozzle (not shown). 
The swing arm 26 of the automatic electrode exchanging apparatus 25 
sequentially takes out a predetermined electrode 14 from the electrode 
magazine 28 in response to a command from the control unit 32. After the 
weight of the electrode 14 has been measured by the electronic balance 27, 
the arm 26 attaches the electrode 14 to the electrode chuck 15. When a 
predetermined work operation has been completed, arm 26 detaches the 
electrode 14 from the electrode chuck 15 and after the weight of the 
electrode 14 has been measured by the electronic balance 27, the swing arm 
26 returns the electrode 14 to the electrode magazine 28. Thereafter, the 
swing arm 26 selects the electrode necessary for the next work operation 
from the electrodes 29, 30, 31, or the like, and after measuring the 
weight thereof, attaches it to the electrode chuck 15. 
The electronic balance 27 for measuring the weight of the electrode is 
attached to the main body 1 of the electrical discharge machine or to the 
electrode magazine 28. Upon measuring the weight of the electrode, it is 
preferable to hold the electrode by an electrode chuck 15' which is 
similar to the electrode chuck 15. 
In FIG. 1, reference numerals 14' and 14" denote the electrode 14 in the 
course of its being measured and stored in the electrode magazine 28, 
respectively. 
It may be possible to attach the electrode holders to the electrodes 14, 
29, 30 and 31 to hold them respectively, thereby exchanging the holder 
with the electrode and measuring the weight thereof. 
Both the values measured by the electronic balances 9 and 27, or the value 
of either of them is fed to microcomputer 33, and the quantity of work, 
quantity of electrode wear, electrode wear ratio, or the like are 
calculated on the basis of these measured values, then they are compared 
with a predetermined weight. Thus, the microcomputer 33 and the control 
unit 32 including the NC appropriately select, change, or control the 
working conditions in accordance with these data to permit the shift to 
the next working step and to optimize the working conditions, thereby 
maintaining the desired working speed and accuracy. 
In the above-described embodiment, upon measuring the weight of the 
workpiece 12, the working fluid 13 in the work tank 11 is drained and 
extraneous articles produced by the spark erosion are washed away from the 
workpiece and the internal surfaces of the work tank, whereafter the 
weight of the workpiece together with the work tank is measured. It may be 
possible, however, to measure the weight of the workpiece with the working 
fluid filled in the work tank. 
That is to say, as shown in FIG. 2 for example, force measuring devices 34 
are attached to the bottom of the work tank 11, and the workpiece 12 is 
placed on a mounting table 35 attached on the force measuring apparatuses 
34 and 34, thereby measuring the weight of the workpiece. Each force 
measuring device 34 consists of a loop-like main body 34a and a strain 
gauge 34b attached to the side thereof. The weight of the workpiece is 
measured by the strain gauge 34b which detects the deformation of the main 
body 34a caused by the weight of the workpiece 12. The detection by the 
force measuring apparatus 34 is carried out when the electrical discharge 
is stopped temporarily so that the pressure of the discharge does not 
affect the detection. The outputs of the strain gauges 34b are fed to the 
microcomputer 33 in the same way as shown in FIG. 1, thereby allowing the 
control unit 32 to control the working conditions thereafter on the basis 
of the operational result. 
When the weight of the workpiece 12 changes during the working operation 
and, accordingly, when the deformation of the main body 34a of the force 
measuring device changes, the workpiece 12 itself is also deflected 
upwardly; however, this deflected amount is very small and can be easily 
calculated from the measured value by the strain gauge 34b, so that the 
working depth can be corrected by the compensating calculation. 
Referring to a third embodiment shown in FIG. 3, the weight of the 
workpiece 12 can be measured with the working fluid filled in the work 
tank 11. The mounting table 35 is suspended in the work tank by 
suspension-type force measuring devices 36 for allowing the weight of the 
workpiece 12 mounted on the mounting table 35 to be measured by the 
suspension-type force measuring devices. The outputs of the 
suspension-type force measuring devices 36 are fed to the microcomputer 33 
in the same way as the embodiment described previously, thereby allowing 
the control unit 32 to control the working conditions thereafter on the 
basis of the operational result. 
In the first embodiment shown in FIG. 1, the weight of the electrode 14 is 
measured by removing the electrode from the chuck 15 and attaching to the 
electronic balance 27 for the electrode; however, it is also possible to 
measure the weight of the electrode while continuing the electrical 
discharge with the working fluid filled in the work tank. 
For example as shown in FIG. 4, a weight measuring apparatus 37 is attached 
to the bottom of the stem 16 and the electrode 14 is attached thereto. The 
weight measuring apparatus 37 consists of a casing 37a, a piston 37b which 
can slide vertically in the casing, an annular main body 37c of the force 
measuring apparatus having a curved side wall, and a strain gauge 37d 
attached to the side wall of the main body 37c. The chuck 15 is attached 
to the bottom of the piston 37b and the electrode 14 is attached thereto. 
The weight of the electrode 14 is transmitted to the main body 37c through 
the piston 37b causing the main body 37c to be deformed. Therefore, the 
weight of the electrode is measured by detecting this deformation using 
the strain gauge 37d. The detection is carried out when the working 
operation is stopped temporarily, for example, when the electrode is at 
the upper dead point in the reciprocation. The output of the strain gauge 
37d is fed to the microcomputer 33 in the same way as was the output of 
the electronic balance 27 for measuring the weight of the electrode shown 
in FIG. 1, thereby allowing the control unit 32 to control the working 
conditions thereafter on the basis of the operational result. When the 
surface level of the working fluid 13 drops and, therefore, when the 
portions of the weight measuring apparatus 37 and the electrode 14 are 
exposed over the surface of the working fluid, the buoyancy applied to 
them will be lost partially, and this causes the calculation of the weight 
of the electrode to be more complicated. Therefore, it is desired to 
perform the measurement while the weight measuring apparatus 37 and the 
electrode 14 are always immersed in the working fluid. For this purpose, 
for example, a fluid surface level sensor 38 is preferably attached to the 
outer wall of the weight measuring apparatus 37 for permitting the working 
fluid to be add into the working tank in response to the output from this 
sensor 38 when the working fluid descends below a predetermined level. On 
the other hand, the measurement by the weight measuring apparatus 37 may 
be carried out when the stem 16 and electrode 14 are raised up completely 
from the working fluid 13 so that the buoyancy effect does not apply at 
all. 
As shown in FIG. 2 or 3, the weight of the workpiece can be measured while 
the workpiece is immersed in the working fluid, and furthermore, as shown 
in FIG. 4, the weight of the electrode can be measured without removing 
the electrode. With such a construction, therefore, the weights of both 
the workpiece and the electrode can be measured while continuing the spark 
erosion and the working conditions can be controlled properly on the basis 
of these measured values, so that the electrical discharge machine is 
provided with excellent work efficiency. 
The weight measuring devices for measuring the weights of the workpiece and 
the electrode are not limited to the electronic balance 9 and 27 shown in 
FIG. 1, the force measuring apparatus 34 shown in FIG. 2, the 
suspension-type force measuring apparatus 36 shown in FIG. 3 and the 
weight measuring apparatus 37 shown in FIG. 4. Other well-known balances 
and force measuring devices with high accuracy may be also used in the 
present invention. The principle of the present invention may be also 
applied to an electrolytic working machine. 
The present invention is so constructed as described above; therefore, 
according to the present invention, an electrical discharge machine is 
provided in which the working conditions are properly selected and 
controlled on the basis of changes in the weights of the workpiece and the 
electrode to maintain better working conditions, thereby improving the 
working speed and accuracy. 
With this detailed description of the specific apparatus used to illustrate 
the preferred embodiments of the present invention, it will be obvious to 
those skilled in the art that various modifications and variations can be 
made in the present methods and apparatus described herein without 
departing from the spirit and scope of the invention which is limited only 
by the appended claims.