Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2000-06-21
2002-05-21
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069180
Reexamination Certificate
active
06392183
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process and device for machining by electroerosion, with which an electrode-piece is machined by means of an electrode-tool separated from each other by a working space by applying electrical pulses between the electrode-tool and the electrode-piece by means of an electrical circuit comprising at least one low impedance voltage source and a regulation circuit.
DESCRIPTION OF THE RELATED ART
Machining by electroerosion requires a generator of intermittent discharges, which is connected between the piece and the electrode, the latter having for its purpose to penetrate progressively into the piece thanks to the erosive action of the discharges.
As the electrode is subjected to only low wear, there is obtained a faithful hollow reproduction of the shape of the electrode in the case of a penetration along a single axis, thanks to a servomechanism which maintains an optimum distance between the piece and the electrode, a so-called sparking distance, and which is the order of several dozens of microns. Other servomechanisms can ensure the relative translatory and/or rotative movements along other axes and one takes account of the amplitudes of these movements to predetermine the deformation of the imprint. These movements and these servomechanisms will not be described nor even shown here, as they relate to well-known and mastered techniques. However, precise imprints cannot be obtained unless the initial dimensions of the electrode are maintained in the course of machining, and if this is not the case, it is necessary to provide several electrodes, which gives rise to supplemental costs and renders electroerosion less economical. The problem of providing a generator with very low wear is always present, although the performances in terms of volumetric wear are remarkable, because there are nowadays obtained, with current generators, certain favorable conditions, and values as low as 0.1%. Unfortunately, this wear is not equally distributed, it is in fact concentrated on the pointed portions and edges of the electrode, such that perfect reproduction of shape is not ensured.
The generators of the type with limitation of current by resistance are known from the time that it has been known to use transistors for generating intermittent discharges.
FIG. 14
shows in a very simplified way the principle, particularly by symbolizing the transistor by a switch S. E
0
is the internal voltage or vacuum voltage of the source, the current is limited by the resistance R, by the parasitic or distributed inductance of value L, and by the impedance of the space between the electrode and the piece. During an erosive discharge, this impedance is represented in an idealized way by an element which maintains the voltage U
g
at its terminals, no matter what the current i. Of course, the physical reality is much more complicated, particularly by virtue of the fact that the electrode-piece voltage is substantially higher at the beginning of discharge, and develops toward a certain value U
g
defined as being the mean statistic of the voltages at the end of discharge. For an understanding of what follows, it will be recalled that, at the beginning of a discharge, the current increases with a slope di/dt=(E
0
−U
g
)/L and which tends asymptotically toward (E
0
−U
g
)/R. This type of generator gives acceptable wear, but unfortunately obtaining very low wear requires, as will be seen later on, an adjustable voltage E
0
which is very near U
g
, such that the control of the asymptotic current requires again finely adjusting the value of R, which would be very difficult and costly.
The generators of the current source type form a second category which offers much flexibility as to the control of the current, because switching transistors require the current to follow a standard. They are thus capable of maintaining a practically constant current during discharge, no matter what the voltage at its terminals, or at least toward a certain limit.
But this type of generator has the drawback of producing current pulses with an initial slope that is much too steep, which is a cause of wear.
SUMMARY OF THE INVENTION
The object of the present invention is to obtain a process and a device which eliminate the mentioned drawbacks, and which permit reducing the wear of the electrode-tool to a minimum and to provide a perfect shape reproduction, whilst ensuring easy control of the current during and after the rise of the machining current.
The invention is characterized to this end by the fact that there is carried out an adjustment of the current rise of the electrical pulses as a function of the time after triggering a discharge between the electrode-tool and the electrode-piece such that a parameter connected to this current rise will be substantially equal to a reference size corresponding to a minimum wear value of the electrode-tool.
These characteristics permit obtaining a machining process and device ensuring very low wear of the electrode-tool. The initial dimensions and shape of this latter are maintained during the course of machining, which renders electroerosion very precise and economical. There is thus carried out upon triggering a discharge according to a first mode in which a transitory current arises under the influence of a low impedance voltage source, which source is connected between the electrode and the piece, the impedance being sufficiently low that the increase of the current will be determined not only by the electrical characteristics of the circuit, but also by the discharge voltage, the current rise having been adjusted to a minimum wear value predetermined by technological tests and, as soon as the transitory current reaches a desired value, one operates according to a second mode in which the current is essentially determined by the electrical characteristics of the circuit.
The principal advantage, which is the obtention of very low wear, has a great economical importance, because, as mentioned in the introduction, it permits not only machining with precision, but also reducing the number of electrodes required.
Another advantage is that the process can be introduced into various known types of generators: current limitation by resistance, current limitation by inductance and switching transistors arranged to create a current source and also a DC-DC converter. Moreover, the advantages inherent in the generator serving as a platform can be preserved. For example, a generator with a current source according to the known technique is transformed into a new generator permitting controlling the initial slope of the current increase of the discharges, which permits achieving adjustments ensuring very low wear. Moreover, the advantages of the discharge with a current source are maintained, particularly the absence of any additional element such as a resistance or inductance adapted to limit the current, this latter function being ensured by the rapid switchings of the power transistors. The flexibility of control of this generator permits it to generate a wide variety of forms of current pulses.
Preferably, the process is characterized by the fact that there is carried out the adjustment of the current rise of the electrical pulses during a first transitory phase and that the machining current is maintained during a second phase at at least a reference value as soon as the current increase has reached this reference value.
There is thus obtained a very low wear of the electro-tool associated with an effective electroerosion and easy control.
According to a preferred embodiment, the adjustment of the rise of current is carried out by means of a loop such that a statistical size representative of said parameter coincides with said predetermined reference size.
Given the fact that the adjustment of the rise of machining current is very critical, a mechanism permits ensuring reproducibility in the field of regimes with very low wear obtained in the laboratory, in spite of the disturbances such as differences in line l
Bovay Gilbert
Gamboni Aldo
Martin Roland
Tognolini Maurizio
Charmilles Technologies SA
Evans Geoffrey S.
Young & Thompson
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