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1. * GB780197 (A)
Description: GB780197 (A) ? 1957-07-31
Improvements in or relating to spark-cutting apparatus
Description of GB780197 (A)
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BE529675 (A) CH325540 (A) DE1104089 (B) FR1098961 (A)
LU32943 (A) NL97802 (C) US2827595 (A) NL96513 (C)
BE529675 (A) CH325540 (A) DE1104089 (B) FR1098961 (A)
LU32943 (A) NL97802 (C) US2827595 (A) NL96513 (C) less
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The EPO does not accept any responsibility for the accuracy of data
and information originating from other authorities than the EPO; in
particular, the EPO does not guarantee that they are complete,
up-to-date or fit for specific purposes.
PATENT SPECIFICATION
780,197 Date of Application andfiling Complete SpecificotionJune
16,1954 No. 17755/54.
(tl / 7 Application made in France on Dec. 21, 1953.
Complete Specification Published July 31, 1957.
Index at acceptance:-Class 83(4), V4.
International Classification: -B23p.
COMPLETE SPECIFICATION
Improvements in or relating to Spark-Cutting Apparatus We, CENTRE
NATIONAL DE LA RECHERCHE SCIENTIFIQUE, a body corporate organised and
existing under the laws of France, of 13, Quai Anatole France, Paris
(Seine), France do hereby declare the invention, for which we pray
that a patent may be granted to us, and the method by which it is to
be performed, to be particularly described in and by the following

2. statement:-
The present invention relates to spark-cutting apparatus adapted to
dislodge electrically particles material of an electrically conductive
workpiece by repeated spark discharge across a gap formed between the
workpiece and an electrode.
Known spark-cutting apparatus usually comprises a capacitor which is
charged up directly from a power source and discharges periodically
causing a spark discharge across the gap between the workpiece and the
electrode. This type of apparatus suffers from the disadvantage that
the current from the power source can reach the spark gap directly and
hence, once the spark discharge occurs across the gap the current
coming directly from the power source tends to maintain this
discharge. Thus excessive electrode erosion occurs causing the
electrode to be deformed by local heating and melting.
It is an object of the present invention to provide a spark-cutting
apparatus wherein the above-mentioned disadvantage is obviated.
According to the present invention there is provided a spark-cutting
apparatus adapted to dislodge electrically particles of material of a
conductive workpiece by repeated spark discharges across a gap between
a workpiece and an electrode adjacent thereto, the apparatus
comprising a spark-cutting apparatus adapted to dislodge electrically
particles of material of a conductive workpiece by repeated spark
discharges across a gap between a workpiece and an electrode adjacent
thereto, the apparatus comprising between terminals for connection to
a source of electric current and the spark discharge circuit, a
circuit in shunt with the spark discharge circuit and including two
capacitors connected in series with one another, [Price 3s, 6d.1 rithe
capacitors being so connected to the said terminals, that at all times
at least one of the 50 capacitors prevents the direct flow of current
from the said terminals to the spark discharge circuit.
The present invention further provides a spark-cutting apparatus
adapted to dislodge 55 electrically particles of material of a
conductive wdrkpiece by repeated spark discharges across a gap between
a workpiece and an electrode adjacent thereto, the apparatus
comprising, between terminals for connection to a source 60 of
electric current and the said spark discharge circuit, a circuit
including two capacitors so connected as to be charged fromn the
source alternately and to discharge through the spark discharge
circuit. 65 Preferred embodiments of the present invention will now be
described by way of example with reference to the accompanying
drawings, in which:Fig. 1 is a circuit diagram of one embodiment of
the invention, Fig. 2 is a curve showing the variation of voltage as a
function of time, across the two condensers in the embodiment shown in
Fig. 1.

3. Fig. 3 is a circuit' diagram of a second 75 embodiment of the
invention, Fig. 4 is a curve showing the variation of time with
voltage across particular condensers in the embodiment shown in Fig.
3, and Figs. 5 and 6 show further embodiments of 80 apparatus
constructed in accordance with the invention.
Referring to the drawings and more particularly to Fig. 1, a direct
current source 10 is shown which charges condensers 6 and 7 via 85
rotary switches 13 and 14 respectively. Terminals B and C are
respectively connected to an electrode 1 and a workpiece 2.
Curves I (Fig. 2) on either side of the abscissa P, on which times are
plotted, show 90 the change of voltage across the capacitors with
time. If the distance between the electrode and the workpiece is
suitably chosen, the spark discharge takes place under a voltage V
which is the sum of the voltages built up across both 95 of the
capacitors 6 and 7. After the discharge one of the capacitors is again
charged and so on.
It will be appreciated that no current will reach the spark discharge
circuit directly from the source. As a matter of fact, supposing for
instance that the switches 13, 14 connect the source with A and C,
that is to say to capacitor 7, at the time of the discharge, it will
be seen that the other capacitor 6 prevents the flow of current
directly from the source to the spark discharge circuit.
Of course, the device may further include an energy storage circuit
shown in dotted lines at 3 which may comprise a capacitor or delay
network. It is clear that the circuit X surroanded by dotted lines is
only diagrammatically illustrated and may include any equivalent feed
and switching means. For instance, switches 13 and 14 may be replaced
by electronic tubes the switching of which could be controlled by
suitable means. We might also include in the charge circuit resistors
or reactances. These are not absolutely necessary to the operation of
the apparatus but may be useful in some cases.
In the embodiment shown in Fig. 3, two capacitors are charged by an
alternating current source 10t one of the terminals of which is
connected to the middle point between the capacitors 6' and 71 and the
other terminal of the source is connected to the-two other terminals
of said capacitors through a rectifier whereby each of said capacitors
is alternately charged by the source in the course of two successive
half-cycles.
In Fig. 3, electrode 1 and workpiece 2 are connected with the
terminals of the energy storage circuit 3' which may be substituted
for a delay network. The said terminals are themselves connected with
terminals 4 and 5 of the system of capacitors 6' and 7' connected in
series. However, between the lower terminal of capacitor 3', which is
connected with the workpiece 2, and terminal 5 it is advantageous,

4. although not necessary, to interpose a self-inductance coil 8 and a
rectifier 9.
Capacitors 6' and 7' are preferably identical to each other and they
are successively charged by the source 101 through the two rectifiers
11 and 12. If Vfl is the effective voltage of said source, said
capacitors are each charged at a voltage close to Vff /V2, whereby
between the terminals 4 and 5 there is a voltage of constant polarity
but of wave form, the maximum value of which does not exceed 2 V/2x
VY,: and the mean value of which, account being taken of the voltage
drops through the rectifiers 11 and 12, is close to 1.5 V2xViff.
Fig. 4 shows the variations of voltage with time that are obtained
across the condensers 6' and 7'. Curve 1 (partly in dotted lines')
shows the voltage variations of source 10' as a function of time t
plotted as abscissa. Curve II in solid lines shows the variations of
voltage across the terminals of capacitor 6', the ordinates of this
curve being above the abscissa, and curve III (in solid lines) shows
the variations of voltage across the terminals of capacitor 7', the
ordinates ot this curve being below the abscissa. It follows that the
value of voltage V 7( across terminals 4 and S is given by the
distance between curves II and III. It may be shown that, for
practical purposes, the delay network is loaded to a maximum voltage
averaging 1.8 X 1.5x > 2 x V f,, that is to say 3.8 7' V.,f, which
discharges quickly in the form of sparks in the dielectric fluid that
is chosen.
The frequency of discharges / is approximateiy given, in kilocycles,
by the expression [=l/- V/LC, L designating the value of
selfinductance coil 8 in miilihenrys and C the value of the capacity
of means 3 in microfarads.
In the embodiment shown in Fig. 3, as that in Fig. 1, practically no
current from the 85 source reaches the spark gap directly. The current
is suppressed by the reactances of capacitors 61 and 7; and by the
self-inductance coil 8. This suppression occurs with a minimum
dissipation of energy and hence without slowing douwn the rate of the
discharges. Thus no limit is imposed upon the speed of machining.
In the embodiment shown in Fig. 3, the rectifiers 11 and 12 may be
constituted by either dry rectifiers e.g. copper, selenium, germanium
95 or by vacuum valves cr valves containing metallic vapours e.g.
mercury, caesium, lithium.
The embodiment shown in Fig. 3 may have the following characteristics:
- 10( Source 10':a single phase alternator having a frequency of 2500
cycles. Voltage 250 volts.
Power 1.2 kilowatts.
Internal self-inductanice:- L 1 microhenry.
Valves 9, 11i and 12:the rare gas type (xenon).

5. Capacitors 6 and 7:each 4 microfarads, this value being liC chosen so
as to comply with the relation:
(2r f)LC=1 with, for frequency f and internal s-lf-inductance L, the
above mentioned values.
Self-inductance 8:- 115 1 microhenry.
Energy storage circuit 3':delay network may have a total capacity of 8
microfarads, and a characteristic impedance of 0.05 ohms. 12(1 Figs. 5
and 6 show two further embodiments of the invention.
These modifications, which are particularly advantageous, include in
particular the combination of a transformer 17 and switching 125 means
such as 15, 16 respectively by which the working frequency of the
apparatus may be adjusted. The said switching means may be triggered
by an auxiliary voltage impulse.
Such an arrangement may be advantageous 130 780,197 780,197 when the
voltage across the spark gap is large.
In Fig. 5, a switch 15 co-operates with a transformer 17, forming part
of the spark discharge circuit and interposed between the charge
circuit portion and the discharge gap.
The switch 15 which is connected in shunt across the primary of said
transformer, comprises an electronic tube which may be of the high
vacuum type. In this embodiment the current impulse which creates the
spark and passes through the primary 17P of transformer 17 results
from the sudden discharge of capacitor 3" through electronic tube 15.
The latter then acts as a switch because its control grid maintained
by a suitable bias 18 and through a resistor 19 at a cut-off voltage,
periodically receives voltage impulses from a generator 20 which make
tube 15 conductive.
At this time the capacitor 311 discharges. The time interval between
two successive actions of tube 15 must be at least equal to the time
of charge of the capacitor 311 through coil 8 and primary 17P. If L is
the self-inductance of the circuit C its capacity, the switching
frequency will be given by f=11/7r7LC. It is of course possible to
dispense with coil 8 provided the transformer has a sufficiently high
self-inductance.
In the embodiment shown in Fig. 6 there is provided a switch 16 in
series with the primary of a transformer 171 forming part of the
discharge circuit, the switch being the electronic type comprising a
thyratron. Across the terminals of the tube 16 there is provided a
circuit including a self-inductance coil 21 and a capacitor 22
because, with this kind of tube, the grid ceases to have any action
upon the plate current once this current has been started. Capacitor
22 then charges, through self-inductance coil 21, to an instantaneous
voltage substantially twice that existing across the terminals 4-S and
discharges in an oscillating manner into tube 16 once this tube has

6. itself been started. The plate voltage may become zero and even
negative due to the oscillation thus created, and at this time the
grid can reassume control. The auxiliary generator may be eliminated
provided that bias 18 is given a value such that tube 16 starts
operating only for a voltage substantially twice that across terminals
4-5. In these conditions the frequency of the current oscillations in
primary 17P, and therefore that of the 5c spark discharges, will be
Ie=1/7r VL'C', L' being the self-inductance of coil 21, and C' the
capacity of capacitor 22.
It should be noted that in this embodiment it is possible to dispense
with an energy storage circuit 3 and use a secondary 17S as the means
for storing the energy. The device thus obtained is such that the
means for storing energy which discharges between the electrode and
the workpiece to produce the machining sparks, is an inductive
reactance, an arrangement which also constitutes a feature of the
present invention.
Whatever be the embodiment that is chosen, our invention makes it
possible to eliminate undesirable arcs between the electrode and 70
the workpiece to be machined. The current coming directly from the
source may be eliminated without using the insertion of an energy
dissipatng element.
The apparatus according to our invention 75 makes it possible to
obtain high machining speeds and smooth surfaces. It is known that to
reduce roughness it is necessary to eliminate arcs, a result which is
obtained with the apparatus according to the invention. go With the
apparatus according to the invention, it is possible to use high
frequencies with low capacities for the said energy storage circuit
which makes it possible to obtain both a good surface and a high rate
of machining. 55
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