1. * GB780139 (A)
Description: GB780139 (A) ? 1957-07-31
Improvements in or relating to machines for the electric machining of
conductive materials
Description of GB780139 (A)
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BE541079 (A) CH320029 (A) DE1031906 (B) FR1135851 (A)
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The EPO does not accept any responsibility for the accuracy of data
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up-to-date or fit for specific purposes.
PATENT SPEClfICATION
780,139 -2 9 Date of Application and filing Complete
Specification: Aug. 5, 1955. No. 22564/S5
Application made in Switzerland on Sept. 10, 1954.
Complete Specification Published: July 31, 1957.
Index at acceptance:-Classes 38(4), R(4:20); and 135,
P(1A:9A6:10CX:24X:25F).
International Classification:-G05f.
COMPLETE SPECIFICATION
Improvements in or relating to Machines for the Electric Machining of
Conductive Materials 5.
We, ATELMERS DES CHARMILLES, S.A., a Company organised under the laws
of Switzerland, of 109, Route de Lyon, Geneva, Switzerland, 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

2. particularly described in and by the following statement:-
The invention has for its subject a machine for the electric machining
of conductive materials by means of spark discharges, said arrangement
including a condenser connected through its terminals, on the one hand
with an electrode forming the operative tool and on the other hand
with the work to be machined, a supply of direct current being
provided for charging said condenser and the electrode being
controlled by a servo-mechanism adapted to hold said electrode at a
predetermined distance from the work during the machining.
In such machines, it is generally difficult to cut out any
short-circuit between the work to be machined and the electrode and
each short-circuit produces a small crater of molten metal on the
machined surface of the work so that it is very difficult to obtain a
smooth machined surface. It is well known that the electrode should be
held at a very small distance from the work to be machined, say a few
hundredths of a millimeter without ever coming into direct contact
with the surface to be machined.
In prior machines, the manufacturers have used the condenser-charging
current to control the servo-mechanism adapted to keep the gap between
the electrode and the work constant. The charging current increases
when the electrode moves too near the work to be machined and
decreases when it moves away from the latter; however, these
variations are very small and furthermore it is essential that the
res[Price 3/61 prce 45 ponse of the servo-mechanism may be obtained
within a lapse of time as short as possible so that a high power is
required with a view to shifting the electrode at a sufficiently high
rate. The servo-mechanism 50 of known machines includes generally a
large electronic arrangement of a critical nature, adapted to amplify
the variations in the condenser charging current and to produce the
power required for shifting 55 the electrode.
The invention has for its object to cut out the necessity of resorting
to such a costly and critical electronic arrangement.
To this end, the machine forming the 60 subject of the invention
includes a servomechanism comprising an electromagnetic device
subjected to a voltage tapped off the average voltage between the
electrode and the work to be machined, said electromagnetie device
actuating a hydraulic distributor, controlling a hydraulic system
controlling the shifting of the electrode.
One form of construction of the machine forming the subject of the
invention is 70 shown diagrammatically and by way of example in the
accompanying drawings, wherein:Fig. 1 is an electric wiring diagram of
the machine; 75 Fig. 2 is a lateral view of the machine; Fig. 3 is a
cross-section of the servomechanism eontrolling the shifting of the
electrode.

3. The electric circuit of the machine in-80 eludes a condenser C the
terminals of which are connected respectively with the electrode E
forming the machining tool and with the work to be machined P. Said
condenser is charged by a voltage supply 85 connected across the
terminals A and B, the terminal A being connected with the
corresponding condenser terminal through a resistance R and an
induction coil L inserted in series. The work P is immerged 90 780,139
in a body of dielectric liquid such as kerosene carried inside a vat
20. The electrode E should be located at a small distance from the
surface to be machined so that the gap therebetween may be of a
imagnitude of a few hundredths of a millimeter. When a supply of
direct current is connected across the terminals A and B, the voltage
of the condenser increases gradually and when it has reached a
sufficient value, a spark jumps between the electrode E and the work P
so as to produce the discharge of the condenser. Said spark draws a
very small amount of material off the work P during the discharge of
the condenser C. The latter is then recharged and the cycle of
operation is repeated, the frequency of the discharges being very
high, say several tens of thousands per second.
To reach such frequencies, it is of advantage for the inductance of
the induetion coil to be large enough for the hal]period of the
condenser-charging circuit to be equal to at least twice the period
separating two discharges, as disclosed in full detail in my
co-pending Specification No.
21546/55 (Serial No. 780,138). It is also essential for the auxiliary
mechanism to respond speedily to all rnodifications in the conditions
governing the discharge between the electrode E and the work P.
The improved arrangement includes, as stated, a servo-mechanism
including an electromagnetic device subjected to a voltage tapped off
the average voltage between the electrode and the work to be machined,
said electromagnetic device actuating a hydraulic distributor
controlling a hydraulic circuit controlling in its turn the shifting
of the electrode.
In Fig. 1 there is illustrated diagrammatically only the
electromagnetic device which ineludes a movable core 1 subjected to
the magnetic attraction of a stationary core 2 round which are wound
the two coils 3 and 4. One end of the eoil 3 is connected with the
electrode E while its other end is eonneeted with the slider of b0 a
potentiometer 5 connected across the input terminals A and B. It is
apparent that the voltage feeding said coil 3 is equal to the
difference between the voltage across the gap between the electrode
and the work and a constant reference voltage which may be adjusted to
the desired v-alue through operation of the potentiometer 5. The
voltage tapped off the electrnde E assumes the shape of a saw-tooth

4. curve, since the voltage of the condenser C increases gradually and
then drops suddenly to zero at, the moment of the discharge. However,
as the frequency.- of the discharges is comparatively high, the
current passing through the coil 3 is substantiallv constant and is
equal to the current which would flow throughi it if said coil were
connected to a point the voltage of which is equal to the average
voltage between the electrode and the work. 70 The second coil 4 is
fed with a current tapped across the terninals A and B and passing
through a resistance 6. a hydrogen and iron resistance 7 and an
induction eoil 8 inserted in series and adapted to75 provide constancy
of the current flowing through it.
Fig. 3 is a eross-seetional view of the electromagnetic device which
is secured to the movable arm 22 of the arrangement. 80 Said
eleetromagnetic device is enclosed inside a cylindrical easing 23
secured to the arm 22 and inside which may slide a cylindrical member
24 which is held in position by a flanged ring 25 the outer flange of
8S which engages edgewise the upper end of the easing 23 while the
threaded section of said ring 25 engages a tapping in the cylindrical
member 24. Said member 24 is held against roctation by a screw 26
the90 end of which engages a longitudinal groove in said member 24.
The magnetic circuit controlling the hydraulic distributor described
hereinafter, includes a stationary core 2, the95 upper end of whiih
assumes the shape of a disc 27 hearingi on a cylindrical yoke member
28 of ferro-magnetie material; the lower inturned end of said
cylindrical member 2'8 surrounds the lower end of the 100 core 2 so as
to formra a gap between it and said core. The movable core 1 includes
an annular section. the shape of which matches that of said gap so
that said movable core may be attracted inside said gap. 105 The
movable core 1 is suspended elastically through spring blades 9, the
ends of which are secured to two pins 10 rigid with the cylindrical
member 28. The spring blades 9 provide also for the centering of the
movable core I with reference to the stationary c-are 2.
The magnetic circuit is secured inside the cylindrical member 24 by a
plug 29 screwed inside the latter and bearingS 115 against the
terminal disc on the core 2.
The movable core 1 has its lower end in contacting relatio-nship with
the slide valve 11 of a b::draulie distributor 12. a spring 13 lbeing
adapted to urge the slide 120 valve 11 into enta.ring relationship
with the core 1. The hydraulic distributor or throttle 12 eontr,
dIv-,auli,, ir,-uit inheluding a punmi feedii il under pressure into
the pipe 14 c, i;m.,i] output pipe 15. 125 A spring-urged tla valve 16
is provided letween thle pipes 14 inki 11, so as to ensure constancy
of the rre:sure in the pipe 14.
Said pipe 14 opens into- the lower end of a chamber 17 inside which a

5. piston 18 130 780,139 slides, said piston carrying a tube 19 coaxial
therewith. A rod 30 surrounded by a tube 31 and extending therewith
inside the tube 19 is adapted to feed current to the electrode E
carried by said rod and for this reason it is separated from the outer
tube 31 by an insulating layer which is not illustrated. The tube 31
which provides for the connection between the condenser C and the work
P, is connected with the latter through a yielding cable 49 and is in
its turn insulated with reference to the enclosing tube 19. It is
apparent that the tube 19 as illustrated has a smaller cross-sectional
area above the piston 18 than underneath the latter.
Above the piston 18 is a capacity 32 the modifications in the volume
of which for a predetermined movement of the piston 18, are equal to
about twice the modifications in volume of the lower compartment of
the chamber 17. In other words, the upper surface of the piston 18 has
a free area which is about twice larger than the free area of its
lower surface. The oil entering through the pipe 14 is adapted to pass
through a diaphragm 33 into a branch pipe 34 opening into the
distributor 12. The oil passes then, through an opening of adjustable
cross-sectional area defined by the slide valve 11, into the output
pipe 15 through which it is exhausted.
The adjustment of the electromagnetic device is such that when the
electrode E is located at the desired distance from the work P to be
machined, the cross-sectional area of the adjustable opening in the
throttling distributor 12 is equal to that of the diaphragm 33. This
adjustment is obtained by simply screwing or unscrewing the ring 25,
which operation shifts vertically the whole electromagnetic device
with reference to the distributor 12.
It is thus apparent that the pressure of oil in the pipe 34 is equal
to one half the value of the pressure inside the pipe 14, since the
drop in pressure produced by the loss of pressure in the diaphragm 33
is equal to the loss of pressure in the distributor 12 while the
pressure inside the pipe is equal to atmospheric pressure.
A by-pass 35 starts from a point between the diaphragm 33 and the
distributor 12 so as to feed oil into the capacity 32 above the piston
18. Since the pressure in said capacity is equal to half that
prevailing inside the chamber 17 while the operative upper surface of
the piston 18 is twice larger than its lower operative surface, it is
apparent that the thrust exerted on the two surfaces of the piston are
normally equal and of opposite sign so that the electrode E remains
stationary.
If the distance between the electrode and the work increases as a
consequence of the removal of material from the work, the average
voltage across the gap between said electrode and work also increases.
which leads to a modification in the current passing through the coil

6. 3. The direction of winding of the latter is such that this
modification of the current redyees the attraction exerted on the
movable core 1 inside the gap of the magnetic circuit and this has for
its result to shift downwardly 75 the slide valve 11 of the hydraulic
distributor.
The cross-sectional area afforded for the passage of the oil inside
the distributor 12 is thus reduced and this leads to an increase in
the pressure inside the pipe 34, said increase in pressure being
transmitted through the branch pipe 35 to the capacity 32. The thrust
on the upper surface of the piston 18 is then higher than that exerted
on its lower surface and this shifts the piston downwardly and
produces a movement of the electrode E rigid with the piston towards
the work P.
In contradistinction, if the distance between the electrode and the
work were to decrease, for instance, if as a consequence of the metal
depositing between the two said parts, the average voltage across the
terminals of the condenser C were to sink, 95 the consequent
modifications in the current flowing through the coil 3 would increase
the magnetic attraction exerted on the movable core. This leads to an
upwardly directed movement of the slide valve 11 loo and to an
increase in the cross-sectional area provided for the passage of the
oil through the distributor 12 and this results in a reduction in
pressure inside the capacity 32. In such a case, the difference
between the pressures exerted on both sides on the piston 18 will urge
the latter upwardly so that the electrode E is urged away from the
work P. Since the masses moving inside the distributor 12 are very 110
small and-since the only movable members are constituted by the slide
valve 11, the spring 13 and the movable core 1, while on the other
hand, a very small displacement of the slide valve 11 leads to a
substantial 115 modification of the cross-sectional area through the
distributor 12, it is apparent that very rapid modifications in
pressure in the capacity 32 are obtained for very small modifications
in the current flowing 120 through the coil 3.
The servo-mechanism thus designated produces very quick movements of
the electrode E, which allows in practice cutting out any risk of a
short-circuit since 125 the electromagnetic device is responsive as
soon as a very slight tendency to shortcircuiting appears.
In the branch pipe 35 feeding the capacity 32 is inserted a ease
containing a 130 780,139 sliding member:6 the inoperative position of
which inside the case is defined by two antagonistic coil springs 37
and S. Said sliding member 36 is provided with a port the
cross-sectional area of xwhich is smaller than that of the branch pipe
35, withl a view to limiting the speed of progression of the piston 18
in the case of a movement of a large amplitude.

7. As a matter of fact, when, the location of the electrode has to be
modified during the machining procedure, the electrode is subjected to
only small displacements when a sudden modification in pressure
appears in the branch pipe 35. the flow of oil inside the latter
produces a shifting of the member 36 against the somewhat small
resultant action of said spring system 3738. The speed of displacement
of the electrode may consequently be very high when said displacement
has a small amplitude.
In contradistinction, when the electrode is to execute a movemrent of
greater amplitude, for instance when it is to move away from the work
at the end of tile rnachinino period, or again when it is brought into
contact with the work at the beginning of tile machining period, the
flow of oil in the pipe 35 shifts the member 36 until the q0 latter
abuts through one of its ends against the end of the ease containing
it, and at this moment the flow of oil inside the pipe is limited by
the port of a restricted cross-sectional area provided in the mermher
36. Said cross-sectional area of this port is selected so as to obtain
the desired speed of displacement for the large amplitude movements of
the electrode.
The arm 22 is mounted slidingly on an upright 39 rigid with the base
40 of the whole arrangement (Fig. 2). The upright 89 carries a rod 41
acting as a piston inside a cylinder 42 (Fig. 3) rigid with the arm 22
of the machine.
The upper end of the cylinder 42 may eomnmunicate through a pipe 48, a
two-way valve 44 and the co-operating pipes 45 and 46 selectively with
the pipes 14 and 15.
The two-waay valve 44 is controlled by a lever 47 illustrated in Fig.
2. When it is desired to shift the arm 22 upw.ardly as a unit it is
sufficient to raise said lever 47 sc as to allow the oil under
pressure entering the pipe 14 to flow into the chalmber -2. This is
the position illustrated in Fig.
3. A spring which is not illustrated is adapted to return the valve
44= into its inoperative position for which it disconnects the pipes
48., 45 and 46 with referenee to one another.
When it is desired to low-er the arm 22, the lever 47 is depressed so
as to intereamconnect the pipes 43 and 46 and this allows the oil to
pass out of the chamrber 42 into the discharge pipe 15.
Turning again toc Fi,.:. it is apparent that current is fd, , the
e]ecirode E through a two-wire 4-ble 48 the ends of which are
connetedi respectively with the rod 30 and with the tuhe 31. The
cable70 48 is enclosed isid- metal sheath.30 entering the upri ght:,
Fig. 2,.
This two-wire,:mne tion enclosed inside a metal sheath is aiapteid on
the one hand to prevent the production of parasitic disturbance which

8. oiduld lie o)bietionable to the reception of l;a.]ast waves and on the
other hand t, re4l'-e as much as possible the natural iduxtance oif
thle discharging circuit otf tbe condenser C: but 80 obviously it is
possible to reach the same result by replacing th,-e two-wire cable)by
a co-axial cable.
The front wall of tlhe upriht 39 carries the control system of the
machine as illustrated in Fig. 2. Th.is control system includes as
shown a mini switch 51 adapted to occupy three pcsi(ion O. I. II. For
the position 0. the voltage supply is not conneeted with the
ternfinals A. B -(Fig. 1). 90 When the switch 51 is in the position I.
the supply of voltaie is ionneefbd with the tei'nminals A and B B,i
that the auxiliary mechanism is suppied while the condenser C is not
connected with the electrode E. 95 as illustrated in Fig.. A tell-tale
lamp 52 which ma 1 he fni instance green, is ignited so as to indiMite
that the switch 51 is actually in the p-sition I.
Wlien the main s-! 51 is shifted into 100 the position II. the. .
netion between the electrode E and fi-, -ondeiser' is obtained and the
m1-f 1ining may lbe executed. This latr p -.iti- n corresponds to
ignition say f a 1,d tell-tale lamp 58. 105 Above the s -it-h 1 is
located a lever 354 w.hich allows sele,:tiig nne of a plurality of
imadlininig' co,mlnii. and to this end the lever 54 is eniected with a
switch which is not ilhsi.i, - anld which allows 110 inserting in
the.-ir, -t different capacity values for the cndercl C. In practice.
condensers of dieent apa,-ities are provided which are. tel to he
connected selectively or in r e1 1 relationship betw;een the elecTrne
E and Ilhe work P.
A knob 35.5 is ado+:/e tn adjust the variations ofc the resist:e P.
The latter is illustrated as a -,- riae resistanee in Fig.
1. blut it include-.-fe mbly a pluralitof resistanees ad,] ii:-,, lhe
conneeted in succession in pa1ar-J:.elationship tlro'uch a
switch-controlled _:._S.
Lastly, a voltmeter. located above the knob li is inserted t.eitveen
thle point eonneetine' the,esistai n, - R ith tlhe induction coil L
and the terminal B of the supply of voltage. this teilininal B forming
a point at a constant voltage. Said voltmeter is adapted to ascertain
whether the machine 130 780,139 is actually operating in a stable
manner.
Underneath the main switch 51 are also arranged two levers 56 and 57
of which the former allows circulating the kerosene contained inside
the vat 20 through any suitable known means not illustrated while the
other lever 57 Is adapted to control the periodical receding movements
of the electrode, which are necessary for the renewal of the mass of
kerosene lying between the end of the electrode and the surface to be
machined. The means serving for said removal are described in full

9. detail hereinafter.
In order to start the arrangement, the switch 51 is first set in the
position I, after, of course, the work P to be machined has been laid
inside the vat 20 which is filled with kerosene, and after the
electrode E of the desired shape has been secured to the end of the
tube 30. Since the electrode E is earthed through the switch 51 in its
position I, the current flowing through the coil 3 is the'same as that
produced by a short-circuit between the electrode and the work. The
servo-mechanism shifts consequently the electrode away from the work P
and holds it in its position of maximum spacing with reference to the
latter.
The switch 51 is now shifted into its position II so that the
electrode is connected with the condenser C and the voltage across the
terminals of the latter is higher than its average operative voltage
since there is no discharge. The servo-mechanism controls consequently
a movement of the electrode in the direction of the work to be
machined and this movement is performed at a comparatively slow speed,
as soon as the member 36 has reached its upper extreme position, for
which the spring 37 is compressed, as explained hereinabove. When the
electrode E enters a position at a very small distance from the work
P, the condenser discharges intermittently and the servo-mechanism
keeps the electrode at the desired distance as already disclosed.
The machining is thus performed under conditions which have been
previously selected through operation of the lever 54.
In principle, the knob 55 is first adjusted so that the resistance R
may assume its maximum value. The knob is then turned so as to reduce
the value of the resistance and consequently increase the
condensercharging current, which leads in turn to an increase in the
frequency of the discharge of the condenser. The limit frequency of
the machining depends pn nLmerous factors and in particular on the
shape and size of the electrode. If, by turning the knob 55, the
operative current is increased sufficiently for the frequency to rise
beyond this limit frequency, there are produced first small
short-circuits of a limited intensity which lead to modifications of
the condenser charging current and consequently of the drop in voltage
70 across the resistance R. Thus the hand of the voltmeter V
oscillates, which allows ascertaining that the conditions are beyond
those corresponding to stability of operation. It is then sufficient
to shift the 75 knob 55 back by one sub-division so as to reduce the
machining current and to make the machine operate under stable
conditions.
When the machining is at an end, the 80 switch 51 is returned into the
position I, which cuts out the connection between the condenser C and
the electrode E, which latter is now grounded. The servomechanism

10. controls thus automatically a 85 receding movement of the electrode
with reference to the work P.
In order to control the above-referred to periodical receding movement
of the electrode during the machining, the lever 57 is 90 mechanically
connected with a switch 58 which allows connecting in parallel with
reference to the circuit element 6, 7, 8 a system of two adjustable
resistances RI and R2 inserted in parallel with each 95 other, a
condenser Cl and a relay 59 with rest contact.
It is apparent that at the moment of the closing of the switch 58, the
condenser Cl is charged with the drop in voltage corresponding to that
appearing across the system of elements 6, 7 and 8. Said charging
current and the current passing through the resistance R2 and the
relay 59 are added to the current passing through the 105 coil 4,
which leads to a shifting of the movable core 1 controlling the
receding movement of the electrode. The voltage across the terminals
of the condenser Cl increases gradually under the action of the 10
resistance Ri and when said voltage has' reached a sufficient value,
it causes the relay 59 to attract its armature 60 as provided by the
current flowing in the circuit constituted by the resistances RI and
R2 115 and the winding 59 of the relay. When said armature 60 is thus
attracted, it breaks the circuit controlled by the switch 58 and the
current flowing in the coil 4 is restored to its normal value and
consequently the electrode E is returned into its machining position
with reference to the work P.
The machining continues thus during a time defined by the period of
discharge of 125 the condenser Cl into the resistances RI and R2 and
by the curve giving out the conditions of release of the relay 59.
When the relay releases its armature 60, the condenser Cl is charged
again and the cycle 130 780,139 of operations is repeated.
In a modification, the voltmeter V, which serves for ascertaining
whether the machine operates under stable conditions, may be connected
in direct shunt relationship across the terminals of the coil 3 of the
electromagnetic device.
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