US2951142A - Method and arrangement for spark erosion by alternating current - Google Patents

Description

Aug. 30, 1960 w. ULLMANN 2,951,

METHOD AND ARRANGEMENT FOR SPARK EROSION BY ALTERNATING CURRENT Filed March 19, 1957 4 Fig.7 i

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INVENTOR. WERNER ULLIMN/V United States Patent METHOD AND ARRANGEMENT FOR SPARK EROSION BY ALTERNATING CURRENT Werner Ullmann, Locarno, Switzerland, assignor to Agie, A.G. fiir industrielle Elektronik, Locarno, Switzerland, 2 Swiss company Filed Mar. 19, 1957, Ser. No. 647,163

Claims priority, application Switzerland Mar. 19, 1956 Claims. (Cl. 219-69) The present invention relates to a method of and arrangement for detaching material by means of spark erosion using alternating current.

The invention is characterized by the fact that a half cycle of the alternating voltage applied to the spark gap is at least partly suppressed for the purpose of altering the voltage course and, accordingly, the gap configuration.

In carrying out the invention an electrode and a workpiece are bridged so that polarity may be reversed by a control member having a variable forward resistance which is dependent on the direction of current.

A number of embodiments of the invention are illustrated on the attached drawings, in which:

Fig. 1 is a basic circuit diagram of an embodiment of the invention;

Fig. 2 is a circuit diagram of the electrical devices used in spark erosion with a relaxation generator;

Fig. 3 is a circuit diagram of the electrical devices used in spark erosion with an oscillatory circuit;

Fig. 4 is a circuit diagram of the electrical devices used in spmk erosion with an impulse generator;

Fig. 5 is a circuit diagram of the electrical devices used in spark erosion with another impulse generator;

Fig. 6 is a section of a workpiece manufactured by means of a method according to the concepts of the invention, and

Figs. 7a and 7b are sections of another workpiece manufactured by means of a method, illustrating the electrode disposed in the workpiece.

The circuit shown in Fig. 1 comprises a generator 1 which supplies the spark gap 2 with arrangements whereby the voltage and frequency may be altered. The spark gap may be surrounded by a liquid dielectric. A first polarity reversal switch 3 is arranged in the circuit. Arranged parallel with the erosion gap via a further reversing switch 5 is a valve or rectifier 4 which is connected to the input circuit and suppresses part of the total energy in the one or the other half-wave cycle of the current. A variable resistance 6 is connected in series with the valve 4, by means of which the action of the valve on the spark gap can be influenced.

This basic arrangement may be applied to all generators so far employed for electric erosion; the generator 1 in Fig. *2 is a relaxation generator, in Fig. 3 an oscillatory circuit generator, and in Figs. 4 and 5 an impulse generator. The prerequisite for the operation of these generators is that the consuming member passes current in either direction; this condition is fulfilled in the am rangement disclosed.

Almost any of the known rectifier types may be used as a valve, in particular, inductances having an acute saturation bend and DC. bias; magnetization by an additional winding.

In addition, the voltage between the tool and the work may be altered. The dimension of the spark gap varies depending on the magnitude of the voltage.

The amount of material detached at an electrode in spark or are operations is determined by the polarity of the spark gap. If the cathode of the spark gap in the application according to Fig. 1 is formed by the work to be' treated, and the anode by a working electrode, the amount of material removed from the work is proportionately smaller than that removed from the electrode. If switch 5 is actuated and the rectifier polarity thereby reversed, both the work and electrode will change their polarity; a larger amount of material is then removed from the work than from the electrode. The ratio between the detachment per unit current in both directions depends on the type of material; however, it corresponds to approximately 1:4.

Fig. 6 shows an example of a workpiece produced with the method described. A bore with a recessed end is to be produced in a workpiece. With the present method, procedure is as follows:

The electrode 7 is connected with the positive pole of the voltage, workpiece 8 with the negative pole, and the electrode then inserted in the workpiece in the known manner. As stated above, this arrangement causes more material to be detached at the electrode than at the workpiece so that the neck of the bore is obtained. When the electrode has reached the location of the shoulder 9, the two switches are actuated to reverse polarity; a larger amount of material is then detached at the workpiece and the widened blind end thereby produced.

It is known that, according to the use of different metals for the tool, e.g. different metals, semiconductors or insulating materials with metallized surfaces on the one hand, and for the workpiece on the other, the eroding action at the anode and cathode will be difierent.

Selection of polarity, voltage at the workpiece, electrode material and the portion of energy conducted through the shunt, the quantity of the material detached may be varied in the manner known.

If the positive voltage of 220 volts is applied to the workpiece, a bore of 5.99 mm. will be produced with a certain electrode diameter, while a voltage of 800 volts with identical polarity will .produce a bore of 6.06 mm. and, after reversal of polarity, one of 6.09 mm.

By means of the variable valve forward resistance of which the polarity may be reversed, all bores may be obtained between these extreme dimensions and, if desired, continuous transitions intermediate of the said values.

It has so far been difficult to ensure accuracy in cylindrical bores by means of spark erosion owing to the fact that a certain amount of burning off occurred at the electrode, which reduced the eifective cross-section of the latter. The bores produced having accordingly been slightly conic-a1, which limited the application of spark erosion to certain problems. This disadvantage may be circumvented by an advantageous development of the method according to this invention; i.e. by continuously altering the suppression of the half cycle relatively to the progress of the electrode in the work in such a manner that the alteration of the bore cross-section caused by the burning off of the electrode is compensated by an extension of the spark gap with the result that perfectly cylindrical bores are obtained. Readjustment of the resistance 6 is preferably effected automatically with the advance of the electrode in the work, but it may be made dependent on other factors, such as the current intensity in the spark gap.

A further application of the method is in the production of workpieces having the shape shown in Fig. 7b. It has been found that projecting edges (A) may best be obtained, when the work is treated by means of spark erosion, if the workpiece is the positive pole and the electrode the negative pole of the spark gap. In the case of recessed edges (B) in Fig. 7a it is most advantageous to connect the work with the negative pole and the electrode with the positive pole.

In the manufacture of bores having e.g. the cross-section shown. in Fig. 7b, an electrode having the cross-section'shownin'Fig. 7a'is" first used; the projecting edges (A) are not provided'on the electrode. With this electrode only the edges (B) areobtained. As" stated above, the electrode is the positive pole and'the work the negative pole in this stage of production. When this first elec trode' has been passed through the work, thesecond electrode, which is shown in'cross-section in Fig. 7b, is applied. In the second stage,'the edges' (B) are not treated While the projecting edges (A) are produced, theconneo tions being reversed in this second operation.

, As'it is "diflicult accurately to centre the second electrode with the bore obtained in thefirst stage, it ispretfer'red to'use a single electrode of which the lower portion, which is first passed into theworkfhas the crosssection of the electrode shown in Fig. 7a,. while its upper portion possesses the supplementary crossasection according to Fig. 7b.

Various changes and modifications may be made without departing from the spirit and scope of the present invention'and it is intended that such obvious changes and modifications be embraced by the annexed claims.

Having thus described the invention, what is claimed as new'and desired tobe secured by Letters Patent, is:

1. A device for detaching material from a metallic bodylby means of spark erosion; comprisinga voltage generator for generating an alternating voltage having a positive and a negative wave portion, a metallic electrode forming one pole of a spark gap, a metallic body arranged in'said device and forming a second pole of said spark gap, rectifying means, variable resistor means, respective means connecting said spark gap with said voltage generator and connecting said rectifying means and said variable resistor means in parallel with said spark gap to suppress at least part of one of said wave portions, and a selectively operable reversing switch arranged between said rectifying means and said spark gap to thereby suppress at least a part of a selected one of said wave portions of said alternating voltage.

2. A device for detaching material from a metallic body by means of spark erosion; comprising a voltage generator for generating an alternating voltage having a positive and a negative Wave portion, a metallic electrode forming one pole of a spark gap, a metallic body arranged in said device and forming a second pole of said spark gap, rectifying means having a rectifier and an adjustable resistor arranged in series connection, connecting means connecting said spark gap with said voltage generator and connecting said rectifying means in parallel with said spark gap, respectively, and a two-position reversing switch arranged between said rectifying means and said spark gap, whereby said rectifying means suppress at least a part of said positive wave portion in one position of said reversing switch and said negative wave portion in the other position of said reversing switch to therebyinfluence the sparking process.

3. A device according to claim 2, in which said connecting means includes a second reversing switch arranged between said generator means and said spark gap.

.4. A device according to claim 2, in which said generator means include an oscillatory circuit.

5. A device according. to claim 2, in which said generator means include -a pulse generator.

References Cited in the file of this patent UNITED STATES PATENTS 2,273,819 Cooke et al. Feb. 24, 194-2

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