Electric heating – Metal heating – Cutting or disintegrating
Patent
1979-08-13
1982-08-24
Shaw, C. C.
Electric heating
Metal heating
Cutting or disintegrating
219 69C, 219 69P, B23P 102
Patent
active
043462782
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to methods and apparatus for electrical discharge machining (EDM) otherwise known as spark erosion machining.
EDM machining according to known techniques has proceeded on the basis that once sparking has been initiated in the gap between the electrode of an EDM machine and a workpiece, the sparking inevitably degenerates into arcing unless the discharge is interrupted for example by removing the voltage applied across the gap for a time sufficient to allow de-ionization in the gap. Thus the voltage is applied as a series of pulses which are usually of predetermined duration or the pulse continues until the integrated gap current has reached a predetermined value.
The time taken for any machining operation is therefore considerably longer than theoretically necessary, since machining is not carried out in the intervals between voltage pulses.
An object of the present invention is to reduce the time required to carry out EDM machining operations.
According to a first aspect of the present invention there is provided an EDM machine comprising monitor means for providing a monitoring signal indicating the degree of sparking which occurs, during machining, in the gap between an electrode of the machine and a workpiece, characterized in that the machine comprises control means for repeatedly carrying out first and second operations, the first operation being the substantially continuous application of a voltage between the electrode and the workpiece to provide sparking in the gap, and the second operation, commencing when the monitoring signal indicates a low degree of sparking, being the removal of the said voltage for a time at least approaching that required for de-ionisation in the gap.
According to a second aspect of the present invention there is provided a method of electrical discharge machining comprising deriving a monitoring signal indicating the degree of sparking which occurs, during machining, in a gap between an electrode and a workpiece, characterized in that the method comprises repeatedly carrying out first and second operations, the first operation being the substantially continuous application of a voltage between the electrode and the workpiece to provide sparking in the gap, and the second operation, commencing when the monitoring signal indicates a low degree of sparking, being the removal of the said voltage for a time at least approaching that required for de-ionisation in the gap.
The present inventors have carried out photographic observations on electrical discharge in machining and have discovered that the discharge between the electrode and the workpiece during a voltage pulse is made up of a plurality of separate sparks during good machining and that machining deteriorates when a voltage pulse includes a period of arcing. Previously, it was thought that each pulse started with a spark which gave way to an arc. The inventors' discovery allows the continuous application of a voltage between the electrode and the workpiece, to be interrupted following the occurrence of arcing as indicated by monitoring.
Monitoring the discharge in the gap may be carried out by sensing energy emitted from the gap, for example by light radiation, or electromagnetic radiation (that is comprising the induction field and the radiation field) or as represented by the electrostatic or magnetic fields associated with voltages across, or currents in, the gap, provided, of course, that the signals derived by sensing in these ways distinguish between arcing and sparking. Where electromagnetic radiation is sensed an antenna in the vicinity of the gap may be used, and signals in the frequency ranges 16 to 24 MHz and 26 to 60 MHz, at least, have been found to provide a good distinction between arcing and sparking since the amplitude of the signals received is much higher during sparking than during arcing, and falls as sparking approaches arcing.
The differences in electromagnetic and light radiation from the gap are thought to be due to two mechanisms: firstly sparks are of comparati
REFERENCES:
patent: 3087044 (1963-04-01), Inoue
patent: 3705286 (1972-12-01), Kondo et al.
patent: 3729610 (1973-04-01), Kondo
Alfred M. Maillet, "The Repercussions in Engineering Caused by a New Principle of Electro-Discharge", published by Society of Manufacturing Engineers, 1971.
Bhattacharyya Sushantha K.
El-Menshawy Mohamed F.
National Research Development Corporation
Shaw C. C.
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