Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23H—WORKING OF METAL BY THE ACTION OF A HIGH CONCENTRATION OF ELECTRIC CURRENT ON A WORKPIECE USING AN ELECTRODE WHICH TAKES THE PLACE OF A TOOL; SUCH WORKING COMBINED WITH OTHER FORMS OF WORKING OF METAL
  • B23H1/00—Electrical discharge machining, i.e. removing metal with a series of rapidly recurring electrical discharges between an electrode and a workpiece in the presence of a fluid dielectric
  • B23H1/02—Electric circuits specially adapted therefor, e.g. power supply, control, preventing short circuits or other abnormal discharges
  • B23H1/022—Electric circuits specially adapted therefor, e.g. power supply, control, preventing short circuits or other abnormal discharges for shaping the discharge pulse train

Definitions

• the "lateral GAP” might be easily measurable, but the trouble is that it does not correspond to the "frontal GAP” which primarily concerns the machining process, and that the relations between the two dimensions are essentially variable according to the machining regime, although the two depend in particular on the different parameters that are the energy of the pulses, their voltage, the intensity of the pulse currents, as well as the characteristics of the medium or dielectric liquid.
• the minimum difference in potential for producing this ionization is proportional to the "ionization potential of atoms or molecules", that is to say to a physical quantity depending on the dielectric medium, as well as to the number of atoms or molecules to be ionized "in series”, that is to say at the dimension (d) of GAP, again taking into account the properties of the dielectric medium.
• the energy required it is proportional to the minimum difference in potential (or voltage) mentioned above, and at the same time proportional to the number of electrons involved, that is to say in principle at number of atoms or molecules concerned by ionization, that is to say ultimately by the section (area) of the channel which is ionized.
• the two polarization and ionization processes are therefore conditioned by geometrical parameters relating to the channel, as well as electrical parameters.
• geometrical parameters tend in the long term to establish themselves as a function of the electrical parameters, although the instantaneous value the length (d) of the GAP can be adjusted.
• electrical parameters that it is preferable to act, with a view to obtaining the ignition of a channel suitable for the desired machining. Indeed, if one can determine in a relatively precise way the energy con ⁇ sacred to the lighting of the channel, one can thereby determine the section of this channel, taking into account its length.
• the voltage, or potential difference, applied for the lighting of the channel is either sufficient or insufficient to allow ignition.
• the energy is used for erosion and no longer for ionization of the channel.
• the quality of the erosion work depends on the energy of the erosive discharges, but also depends on the relationship between this energy and the parameters of the channel which is on for its passage.
• the parameters of the channel can hardly be determined, that is to say controlled, in a precise manner. On the other hand, this is possible if we separate the two functions on the one hand from ignition and on the other hand from erosion.
• pre-ignition thus achieves a separation of functions, and therefore of the parameters which direct them. Leaving aside the questions of the erosive pulse itself, we focus, in the context of the present invention, on the parameters of the pre-ignition pulse. We have seen that a good quality pre-ignition requires control of the pre-ignition voltage, and, if possible, of its energy.
• this adaptation does not necessarily relate to the electrical voltage parameter, but it can also - and advantageously - be fai ⁇ you by acting on the distance of GAP which, in any case, is most often controlled by a device for automatic positioning of the tool electrode. Under these conditions, it is possible to admit a practically fixed pre-ignition voltage and wait for the electrode positioning device to establish, in relation to this voltage, the appropriate GAP distance. It is also necessary that the pre-ignition voltage has indeed a good stability because, as the pre-ignition pulse is extremely fast, the slightest disturbance of an electrical or material order can significantly modify the peak voltage of the pulse.
• an object of the present invention is to provide an electrical circuit for an electro-erosion machining machine working according to the principle of pre-ignition, in which the drawbacks of the prior art are avoided, and in particular in which the parameters which condition the quality of the ignition of the channels and of the electro-erosive machining are adequately controlled.
• the electrical circuit for an EDM machining machine in accordance with the generic definition previously stated, achieves the targeted performances due to the presence of the characters stated in one or other of the claims independent. It is noted that the characters, either of one or of the other, independent claims intervene respectively for different aspects of the problem of stabilization of the pre-ignition.
• the dependent claims define embodiments of the subject of the invention which are particularly advantageous, in particular from the points of view of the voltage stability, of the detection of tendencies in the increase of pre-ignition energy, of the constitution of circuits allowing a stabilization of the energy, manually or automatically, of the simplicity or the convenience of use of the circuits generating the pre-ignition pulses, etc.
• clipping means which are mentioned can be either with voltage limiting characteristic, but not voltage stabilizer (pe) Zener diode in series with significant resistance) or with truly current stabilizing characteristic (Pe diode Zener without notable ohmic resistance in series).
• FIG. 4 is a diagram of a form of execution of a circuit according to the invention, similar to that of FIG. 2, but also having automatic means for limiting the pre-ignition energy
• FIG. 5 is a partial diagram of a variant which can advantageously in certain cases be used to replace the circuit part lying about the primary winding of a transformer for generating the pre-ignition pulses, fig.
• the pre-ignition pulses being entirely dependent, as regards their voltage and current, that is to say their energy, the machining power and the characteristics of the erosive pulses, the parameters of the pre-ignition pulses do not could hardly be controlled, even getting out of hand. Significant variations in the erosive pulses produced very harmful parasitic effects.
• parasitic phenomena (such as those mentioned above or others still) provided the servo-control system with automatic advance (or automatic positioning) of the tool electrode with often incoherent information from so that the servo motor was unable to properly maintain the machining GAP value, making the entire machining process unstable.
• the duration of the pre-ignition pulse that is to say of the ionizing effect, must be as short as possible; the pre-ignition pulse should drop as soon as the channel is ionized and the erosive pulse current begins to flow.
• the material device 2 for machining by electro-erosion comprises a tank filled with dielectric liquid and in which an electrode-tool El is located up to the top of a workpiece Pi.
• the circuit 1 for delivering electro-erosive pulses is connected to the terminals of the two electrodes that constitute the electrode-tool on the one hand and the workpiece on the other hand, only by means of 23 diary of a diode 7 necessary for the pre-ignition pulses to be applied.
• a diode 10 prevents any reversal of voltage polarity across the terminals of the GAP, that is to say on the electrodes Ei and Pi.
• the diagram in fig. 2 il ⁇ only shows the pre-ignition circuit in detail. It can be seen that it comprises a transformer 6, voltage booster, in which the pre-ignition pulses are generated. An adjustable resistor 5, a capacitor 4 and a Zener diode 3 are connected in series with the primary winding of the transformer 6, all of this series branch being connected to the output terminals of the supply circuit 1. Furthermore, the secondary winding of the transformer 6 is connected, via a limiting resistor 8 and a non-return diode 9, to the terminals of the diode 1, previously mentioned.
• the effect of the series connection of the condenser 4 is thus advantageous in that it provides a pre-ignition pulse of adequate power.
• the variations in the conditions in the machining zone itself that is to say in GAP
• due to fluctuations in the characteristics of the dielectric medium - such as an accumulation of machining residue in the form of conductive particles, gas formation, carbon deposition, etc. - can cause variations in the resistance of this medium, which makes it difficult to control fluctuations in the voltage of the pre-ignition pulses.
• an indicator 17a will provide an indication allowing an operator to know the desired information as for the voltage drop in the resistor 12.
• This indicator 17a could be for example a lamp or an optical device, indicating that a threshold is exceeded or not by the tips of the clipping current, it could also be a digital indicator, providing the very value of this current, or an analog needle indicator, etc.
• the current which the transistor 14 allows to pass is not, in all cases in the general case, the total current which passes through the GAP, since a part of this current passes through the clipping branch, 11 , 12.
• the distribution of the pre-ignition current between the clipping branch and the GAP is conditioned by the distance d of the GAP, which must be adequately established. In most of the cases,
• Fig. 5 shows a possible modification, and at least in some cases advantageous, according to which the elements which control the flow of current can be arranged in the primary winding of the transformer which supplies the pre-ignition pulses.
• the elements which control the flow of current can be arranged in the primary winding of the transformer which supplies the pre-ignition pulses.
• a transistor 25 is provided, provided with a resistor for neutralizing the leakage current 26, which is controlled by its base by the branch- ment-series of a Zener diode 3 ', corresponding to diode 3 of fig. 2 to 4, and a capacitor 4 ', corresponding to the capacitor 4 of FIGS. 3 to 4.
• a resistor for neutralizing the leakage current 26 which is controlled by its base by the branch- ment-series of a Zener diode 3 ', corresponding to diode 3 of fig. 2 to 4, and a capacitor 4 ', corresponding to the capacitor 4 of FIGS. 3 to 4.
• the pre-ignition voltage is at least notably greater than the voltage of the electro-erosive pulses.
• the transistor 6 ′ will nevertheless be at least slightly voltage-boosting, but this is not an absolute necessity.
• a coupling capacitor 31 also connects the collector of the reaction transistor 29 with the base electrode of the ignition control transistor 25.
• the transis ⁇ tor 29 is also conductive and, via the condenser 31, this conduction state is maintained at least during the charging time of this capacitor 21, even if the current through the Zener diode 3 'and the capacitor 4' was no longer sufficient to control the conduction of the pre-ignition control transistor 25 on its own.
• circuit of fig. 5 both with the addition drawn in dotted lines and without it, is a circuit of the type which does not add any current at rest.
• the pre-ignition control means the characteristic function of a monostable rocker, which can make it possible, for example, to calibrate the pre-ignition pulse in time independently of the control supplied by the diode Zener 3, 3 'and capacitor 4, 4', control which, in turn, is dependent on the evolution of the voltage delivered to the output terminals of circuit 1, which will provide the erosive pulse . It is also necessary to consider figs.
• FIGS. 6 and 7 These two figures each consist of a schematic diagram (upper part) and a diagram of the evolution of voltages and currents (lower part).
• Fig. 6 illustrates the fact that, as a circuit for delivering erosive discharges, a circuit of the "relaxation" type can be used, such as, without regard to pre-ignition questions, the prior art knew of it, in particular by the description of the previously cited USA patent.
• the pre-ignition arrangement is connected, as shown in FIGS. 2 to 4.
• I terminals of the main pulse delivery capacitor first increases to a value U_, at which the pre-ignition process is triggered.
• the voltage then passes through the value U which constitutes the pre-ignition peak voltage, then, with the channel on, this voltage drops to a relatively low value, of the order of 30 to 40 V, where it is maintained, with an evolution substantially flat, until the end of the pulse.
• the current begins to gradually increase when the channel has been turned on by the voltage spike.
• the means for positioning the tool electrode include a motor which is controlled to bring the electrode closer to the part or to move it away from it.
• This engine is controlled by conventional means; in order to determine its functioning, we generally have means which react
• JTJRE WIPO necessary commands of the electrode positioning motor consists in first of all entering as information the signal qx (fig. 2 to 4) or mw (fig. 1). If this signal is constantly positive, it means that there is very probably a short circuit or an arc in the GAP, and it is necessary to command a distance of the electrode. If this signal is constantly zero or negative, this means that there is never a significant current in the GAP and it is advisable to order an approximation of the electrode. Finally, if there are positive voltage pulses, separated by time intervals with zero or negative voltage, it is advisable to consider, as a second criterion, the signal xm (fig. 1-4).
• this signal has pulses whose tip reaches a value close to a setpoint (designated as V), this means that the distance from GAP is suitable and the motor must remain stationary. If this signal xm has voltage spikes which nevertheless remain significantly lower than the said setpoint value V, this means that the distance of GAP is too small and that it is necessary to actuate the motor to move the electrode away. On the contrary, if the voltage peaks of the signal xm are notably higher than the said setpoint value V, it is because the motor must be controlled so that it causes a reduction in the distance of GAP.
• Another control method practicable according to the prior art, consists in using only the signal xm and in carrying out only the determination that the first method made in the alternative, in
• Zener of diode 11 and the xm signal can never significantly exceed this setpoint.
• this second method had the advantage of not having to measure a voltage drop on a diode such as diode 7 (fig. 2 to 4) or diode D_ (fig. 1).
• the circuit according to the invention nevertheless provides a very advantageous possibility of practicing a method close to the second method previously considered, but using another criterion to know the situations where the electrode must be distant.
• a resistor 12a which is in series on the supply of the pre-ignition voltage and current. This additional resistance has approximately the same value as resistance 12.

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• Mechanical Engineering (AREA)
• Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)

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• 1984
  • 1984-06-29 EP EP84902502A patent/EP0187738A1/de not_active Withdrawn
  • 1984-06-29 WO PCT/CH1984/000108 patent/WO1986000248A1/fr not_active Ceased
• 1985
  • 1985-06-27 EP EP85902950A patent/EP0187770B1/de not_active Expired
  • 1985-06-27 DE DE8585902950T patent/DE3561377D1/de not_active Expired
  • 1985-06-27 WO PCT/CH1985/000103 patent/WO1986000249A1/fr not_active Ceased
  • 1985-06-27 US US06/848,379 patent/US4788399A/en not_active Expired - Fee Related

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