Methods and Apparatus for Electrical Discharge Machining

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 comparatively short duration compared with arcs so that the rate of change of current in the gap is higher during sparking: and secondly, when ionisation takes place in arcing and sparking the energy levels of electrons concerned change in different ways. With arcing, where a comparatively large amount of energy is supplied, more and greater energy level changes take place, re-combination takes place less frequently and with electrons taking up an intermediate state more often than is sparking. More energy level changes, as occur in sparking where less energy is supplied, mean greater magnitude and higher frequency electro-magnetic radiation, while more electrons taking up an intermediate energy level, as occur in arcing, mean more light emitted and light of different frequencies.

In addition while during arcing light is generated substantially continuously, during sparking generation is intermittent and corresponds with the sparks. Thus the intermittent nature of light emission during sparking can be used to generate a monitoring signal.

As another alternative in monitoring the discharge in the gap, a circuit may be connected directly or indirectly to the electrode and/or the workpiece to sense radio frequency signals in the voltage across the gap or the current in the gap. The amplitudes of signals in the ranges 5 to 10 MHz and 25 to 30 MHz have been found to be much higher during sparking than during arcing, and again these amplitudes fall as sparking approaches arcing.

Other ways of distinguishing between sparking and arcing may be used, for example the voltage across the gap may be monitored since there is a small reduction in this voltage when a change from sparking to arcing takes place.

Certain embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of some electrical circuits of an EDM machine according to the invention, and

FIG. 2 shows waveforms appearing in the circuit of FIG. 1.

In FIG. 1 an antenna 30 is located in the region of the gap 25 between an electrode 11 and a workpiece 10 of an EDM machine, or around a tank which contains dielectric liquid in which the electrode and workpiece are submerged. The antenna is coupled by way of an amplifier 40 to a detector 41 which may simply comprise a series diode and a shunt capacitor. The amplifier 40 has an output signal amplitude dependent on input signal amplitude and preferably has a linear gain characteristic. The amplifier is frequency selective and is tuned to accept signals at about 30 MHz.

The detector 41 is coupled to one input of a comparator circuit 52 which also receives an adjustable reference level derived from a potentiometer 53. The output of the comparator 52 is connected by way of an OR gate 71 to a drive circuit 18' for a group of transistors 18 which when conductive apply current and voltage from a d.c. power supply 17 to the gap 25. An optional rectangular waveform oscillator 19 is also shown in FIG. 1 but the function and operation of this oscillator is described later.

The output of the comparator 52 is also connected by way of an inverter 59 to trigger a pulse generator 70, which 10 milliseconds after receiving an input from the inverter 59 passes a pulse to the OR gate 71.

In operation when machining is to be carried out, there is no spark across the gap and therefore practically no radiation at 30 MHz is received by the antenna 30. Thus the output of the comparator is zero and after a 10 millisecond delay the generator 70 is triggered to provide a pulse, which reaches the drive circuit 18' by way of the OR gate 71 and causes the transistor group 18 to conduct. Thus if conditions are correct for sparking, r.f. radiation at a high level commences and a signal from the comparator 52 maintains a signal causing the transistors of the group 18 to continue conduction. Sparking continues in the gap until, for example, an arc occurs at a time 73 in FIG. 2 which is a waveform at the output of the OR gate 71. The signal from the comparator then ceases and the transistor group 18 immediately ceases to conduct. The waveform of FIG. 2 now falls to zero and a ten millisecond interval ensues for the arc ionisation channel to disperse, the start of this interval being triggered by the comparator 52 by way of the inverter 59 and ending when the pulse generator 70 applies a start pulse to the OR gate 71. This process is continuous as indicated in FIG. 2 where the level at the output of the OR gate 71 during sparking is designated `sparking`. A further arc occurring at a time 74 is also shown together with the subsequent cessation and re-appearance of sparking.

Clearly the advantage of the arrangement of FIG. 1 is that very high machining efficiency can be obtained since there are no interruptions in sparking until arcs occur and then the interruptions are just sufficiently long for ionisation channels to disperse. The waveform shown in FIG. 2 is by way of illustration and in fact, of course, sparking would occur for a much greater portion of any machining time. Provision is made for flushing the gap if arcing cycles of short period occur continually.

Coarse and fine machining used at the beginning and end of a machining operation are often controlled by using long machining pulses to start and short pulses at the end. Thus adjustment is not available in the arrangement of FIG. 1, since drive is applied at all times in which sparking occurs. However coarse and fine machining is achieved by adjustment of the power applied to the gap, for example by controlling the number of transistors in operation in the group 18.

The arrangement of FIG. 1 may include the oscillator 19 which has a rectangular output waveform. The function of this oscillator is to superimpose an oscillatory voltage, for example in the frequency range 50 to 100 KHz, on the gap voltage with the object of supporting the sparking condition. Since the oscillator 19 is connected in series with the group of transistors 18, it applies its output voltage to the gap 25 only when voltage from the supply 17 is also applied to the gap. In a typical arrangement the voltage applied across the gay by the d.c. supply is 85 volts and the superimposed voltage has an amplitude of 15 volts.

The arrangements of FIG. 1 can be further modified by replacing the d.c. supply 17 and the groups of transistors 18 and 19 by an SCR bridge (not shown) with an a.c. supply connected across two terminals and the electrode and workpiece connected across opposite terminals. The four SCRs of the bridge are then directly triggered by the output from the gate 71 by way of a drive circuit but of course only those correctly poled by the supply at that time conduct. The SCRs can be replaced by SCSs with the advantage that arcing can be made to cease directly it is detected by switching off those SCSs which are conducting.

It will be appreciated that while specific embodiments of the invention have been described the invention can be put into practice in many other ways, for instance by using different circuits to distinguish between sparking and arcing and to control the application of voltage across the gap.