In the EDM process, electric energy is supplied in the form of discrete electrical pulses across the machining gap filled with a machining fluid or liquid dielectric (e.g., kerosene, transformer oil, distilled water or weekly conductive water) to effect a succession of electrical discharges between the tool electrode and the workpiece to remove material from the latter. As material removal proceeds, the tool electrode is advanced relatively towards the workpiece by servo feed means adapted to maintain the machining gap spacing substantially constant and thereby to allow material-removal discharges to be successively created. The contamination of the machining gap region with chips, tar and gases produced by machining discharges may be clarified by continuously or intermittently flushing the gap with a fresh machining fluid and/or intermittently or cyclically retracting the tool electrode away from the workpiece to allow the fresh machining medium to be pumped into the machining gap and the machining contaminants to be carried away from the latter.
Parameters of individual and successive electrical discharges, i.e. pulse on-time .tau.on, peak current Ip and off-time .tau.off are, for a given combination of electrode materials, determinative of machining results, e.g. removal rate, surface roughness and relative electrode wear and, therefore, are individually or in combination adjusted to establish a particular machining condition suitable to achieve desired machining results.
As proposed by the present inventor in Japanese Patent Specification No. 39-20494 published Sept. 19, 1964 and No. 44-8317 published Apr. 18, 1969, there is known an improved EDM pulse supply technique in which a succession of pulse trains individually consisting of discrete elementary pulses of short on-time (.tau.on) and off-time (.tau.off) occurring at a high frequency, say, 10 kHz to 100 kHz, are applied across the machining gap, the pulse trains occurring at a low frequency, say, 100 Hz and having a longer duration (Ton) with the successive trains being separated by a longer cut-off time interval (Toff). Elementary pulses in each train may be modified as to their triggering or peak voltage as described in the aforementioned Japanese Patent Specification No. 44-8317. Other have also proposed various circuits generally of this or similar genre, which are described, for example, in U.S. Pat. Nos. 3,056,065 and 3,943,321.
Fine-surface and precision machining results are obtained with a train of elementary pulses of a duration .tau.on set to be short and preferably at a minimum which, when repeated at a high frequency and with a pulse interval .tau.off set to be at most equal in length to the pulse duration .tau.on, permits an increased removal rate to be obtained. The shortness of the pulse interval .tau.off may, however, cause the production of machining gap chips and other products to bring about a continuous arc discharge with ease within a short time period. This possibility can advantageously be eliminated or alleviated by the presence of cut-off time periods Toff which separate from one another the successive trains Ton of elementary pulses (.tau.on, .tau.off). Thus, the cyclic interruption of elementary pulses allows accumulated machining products to be carried away substantially within each interruption period from the machining gap and may thereby serve to maintain the latter from continued contamination. By establishing the cut-off interval Toff at a level sufficient to allow clarification of the contaminants resulting from the machining action of the previous train of elementary pulses, the gap can be ready to accept the next train of elementary pulses to continue stabilized machining discharges.
On the other hand, in the interest of increasing the removal rate, it is desirable to have the electrode-feed servo system operate so as to minimize production of non-striking pulses or pulses which do not cause an electrical discharge. In order to facilitate production of discharges by application of successive trains of elementary pulses, the gap can be reduced but this can also facilitate production of a continuous arc or result in a difficulty in gap flushing or decomtamination.
When a continuous arcing, short-circuiting or excessive gap contamination condition arises during a given course the machining operation, it has been recognized to be desirable to stretch or increase either of the off-time .tau.off of elementary pulses in each train or the cut-off time period Toff between successive trains. Thus, the pulsing power supply may include a timer circuit having a time constant determining the time .tau.off or Toff varied by a control signal and a sensing circuit may be connected between the machining gap and the timer circuit to respond to a change in the gap condition for providing the control signal which is acted upon the timer circuit for increasing the pulse interval .tau.off or Toff. When both .tau.off and Toff are arranged to be variable, it has commonly been believed that they should proportionally be stretched or increased in response to a signal indicating the continuous arcing, short-circuiting or excessive gap contamination condition. These prior recognitions or belief simply derive by way of analogy from the generally accepted practice in an earlier simpler pulsing system using a uniform train of machining pulses whereby, when an abnormal gap condition develops, the pulse interval (.tau.off) can selectively be controlled with or without a simultaneous change in the pulse duration (.tau.on). This latter basic control concept is described in U.S. Pat. No. 3,539,755 issued Nov. 10, 1970 and U.S. Pat. No. Re. 29,589 issued Mar. 21, 1978.
The stretching of the .tau.off or Toff period of both continues for a preselected time interval determined in the timer circuit by the control signal or until after the gap sensor indicates recovery of a normal gap condition. When the gap condition fails to be resumed within the time interval, the control signal is again developed and the stretching control is reestablished or continues.