GB2111081A - Electrochemical machining - Google Patents

Abstract

An installation for improving the quality of the machining of workpieces in electrochemical machine units by suppressing the arcing between the machining electrode and the workpiece by means of a short-circuit suppression device which acts parallel to the working gap of the installation, and comprises either a current generator which acts counter to the short- circuit current, or short-circuit means which in the event of a short circuit, takes over the short-circuit current, is characterised in that in addition to the short-circuit device (6) which acts parallel to the working gap (5), an additional direct- current pulse-generator (19) is provided which is connected in parallel to the working gap and which successively generates for the duration of a few micro-seconds, currents counter to the working current which modulate the unidirectional electric field at the working gap, which field controls the machining process. <IMAGE>

Description

SPECIFICATION Installation for improving work-quality in electrochemical machine units The subject of the invention is an installation for improving the quality of machining of workpieces in electrochemical machine units, by suppressing the arcing between the machining electrode and the workpiece by means of a short-circuit suppression device which acts parallel to the working gap of the installation and which comprises either a current generator which acts counter to the shortcircuit current, or a short-circuit means which in the event of a short circuit takes over the short-circuit current.

Installations of the above type are known.

In one such arrangement, the short-circuit current which, in the event of a short circuit can flow across the working gap of the installation, and in some circumstances destroy or damage the machining electrode and the workpiece, in compensated by a current generator which is connected in parallel to the working gap, in a manner such that the difference of the currents achieves a value which is not harmful. In a second arrangement the short-circuit current which can flow across the working gap, is led in the event of a short circuit, through a short-circuit means which relieves the working gap from carrying the short-circuit current. In this latter arrangement the short-circuit means also is connected in parallel to the working gap.

In the first instance the number of shortcircuiting events at the working gap can be reduced, namely by reducing the number of those events which proceed in conjunction with arcing, this being effected by suppressing the short-circuiting events as they originate. As a result the machining-speed is increased. In the second instance, the workpieces are protected from being damaged without any decrease in the number of shortcircuiting events.

According to the invention it is possible with installations of the abovementioned type, to bring about a significant reduction in the number of short-circuiting events which occur, and hence to improve the quality of the workpieces which are to be machined, in that in addition to the short-circuit protection device which acts parallel to the working gap, an additional direct-current pulse-generator is provided which is connected in parallel to the working gap and which successively generates, for the duration of a few micro-seconds, currents counter to the working current which modulate the unidirectional electric field at the working gap, which field controls the machining process.

By this procedure the current from the additional direct-current generator is expediently activated by means of a control instrument which is controlled in accordance with the voltage at the working gap and which reacts to slight voltage-fluctuations between 100 KHz and 2 MHz. Experience has shown that these voltage-fluctuations correspond to voltages which occur in the working gap before arcing takes place.

The invention is hereinafter further explained and illustrated inAhe accompanying drawing, of which Figure 1 is a diagrammatic illustration of one embodiment of the invention, and Figure 2 illustrates the construction of the associated current generator.

In Fig. 1, a rectifier 1 is connected to a three-phase mains system and supplies an electrochemical machining appliance by line .2. Said appliance comprises machining electrode 3 and is associated with workpiece 4.

It is possible in the event of a short circuit, for a short-circuit current to flow through the working gap 5 between the electrode and workpiece, the boundaries of this gap, which is filled with a liquid electrolyte, being formed by the machining electrode 3 and the workpiece 4, either or both of which can be damaged by a short-circuit current. In order to guard against damage of this nature a shortcircuit protection device 6 is provided parallel to the working gap 5, this device taking over the short-circuit current, which in the event of a short circuit flows across the working gap 5.

The short-circuit protection device 6 can for example be a thyristor switch which takes over the short-circuit current flowing in the supply line 2.

In order to detect this short-circuiting event, a detector 7 is provided, which signals the short-circuiting event, for example as a function of the voltage at the working gap (or by some other method), and actuates the shortcircuiter 6. At the same time the rectifier is switched off through line 9, and the supply is cut off.

In order to reduce in a successive manner the short-circuiting events developing at the working gap 5, which proceed in conjunction with arcing, a second detector 10 is provided, which is connected to the working gap 5 via the signal lines 5a and which contains a highpass filter on the input side, this filter allowing voltage-fluctuations occurring between 1 00 KHz and 2 MHz, to pass through, and generating a corresponding control signal at the output side. The control signal is fed to a current generator 20, which is likewise connected on the output side to the working gap, and which generates for brief periods, lasting for example, a few micro-seconds, currents counter to the working current in the working gap 5.Whenever the voltage-fluctuations at the output from the detector 10 exceed a certain threshold, the generator 20 is activated for a period of micro-seconds, and the unidirectional field in the working gap 5 is thus modulated.

The arrangement 10, 1 9 is represented, diagrammatically in Fig. 2, this arrangement being connected to the working gap 5 of the installation according to Fig. 1.

Fig. 1 shows the machining electrode 3 of the electrochemical installation and the work piece to be machined 4 and the working gap 5 therebetween. The working gap 5 and the associated supply lines are replaced, symbolically, by the ohmic resistance 29 and the inductive load 30. This resistance and the associated inductance 30 form in association with the capacitance 20 an oscillating electric circuit, which can be discharged via the thyristor 26, at the frequency which is developed by the oscillating circuit. The capacitance 20 can be recharged via the line 21 and the diode 23. An associated charging unit is not represented in detail. The capacitors 21, 22 are connected in parallel to the capacitor 20, it being possible to charge these capacitors by means of the diodes 24, 25 and to discharge them via the thyristor switches 27, 28.

The discharging operation is controlled by the control unit 5b. It is connected to the signal line 5a, and contains high-pass filters which allow the passage of the voltage-fluctuations, between 100 KHz and 2 MHz, previously described. The switches 26, 27, 28 are successively actuated by appropriate voltagefluctuations, and, following the discharge of the associated capacitors, the latter are again recharged.

The capacitances 20, 21, 22 and the associated inductances 30, and/or the ohmic resistances 29, are selected so that the associated oscillation electrically modulates the unidirectional electric field at the working gap 5 in a manner such that arcs which form are extinguished and suppressed.

Claims (4)

1. An arrangement for improving the quality of machining of workpieces in electrochemical machine units by suppressing the arcing between the machining electrode and the workpiece by means of a short-circuit suppression device which acts parallel to the working gap of the installation and which comprises either a current generator which acts counter to the short-ciruit current, or a short circuit means which in the event of a short circuit, takes over the short-circuit current, characterised in that in addition to the short-circuit device (6) which acts parallel to the working gap (5), an additional direct-current pulsegenerator (19) is provided which is connected in parallel to the working gap and which successively generates, for the duration of a few micro-seconds, currents counter to the working current which modulate the unidirectional electric field at the working gap, which field controls the machining process.

2. An arrangement according to Claim 1, characterised in that the output current from the additional generator at the working gap is controlled by means of a detector which contains a high-pass filter on the input side, this filter allowing the passage of voltages between 100 KHz and 2 MHz.

3. An arrangement according to Claim 1 or Claim 2, characterised in that the additional direct-current pulse-generator contains a plurality of capacitors which can be discharged through switch means, the said capacitors being rechargeable by means of associated charging devices.

4. An arrangement according to Claim 1, substantially as hereinbefore described and illustrated in the accompanying drawing.