The electrical discharge machining art has advanced from the early stages in which relaxation oscillators were used to provide machining power pulses to the gap for eroding portions of an electrically conductive workpiece in the pattern of a tool electrode. Independently timed and controlled pulse generators such as multivibrators are now almost universally used and in these generators the electronic switches employed are generally in the form of solid state switches, particularly transistors. In the electrical discharge machining process, it is necessary as the workpiece material is removed, that a predetermined gap be maintained between the tool electrode and the workpiece through an automatic servo feed system which provides a continuous advance into and toward the workpiece as the material removal is carried out. During the electrical discharge machining process, a fluid coolant, generally a liquid, is circulated and recirculated through the machining gap to flush the workpiece particles from the gap. The coolant is usually furnished under pressure by a pump through one or more openings provided in the electrode. One necessary and defining characteristic of electrical discharge machining is that the coolant be a dielectric fluid such as kerosene, transformer oil, distilled water, or the like. The dielectric fluid, as broken down in minute, localized areas by the action of the machining power pulses passing between the closely opposed surfaces of the tool electrode and workpiece. For control of the servo feed system, there is generally utilized an electrical signal from the machining gap in order to control the rate and the direction of servo feed. In most cases, this gap signal is compared to an adjustable reference voltage so that the operator can select the rate of servo feed desired for the particular operating conditions at hand.
In order to control machining rate and overcut as well as finish, it is necessary that the operator and his equipment have the ability to precisely control the magnitude of current being passed to the gap in the form of machining power pulses. In some prior art systems, the level of current was controlled by adjustment in the multivibrator of on-off time so that the actual machining power magnitude was controlled by the duty factor or ratio of on-time to off-time of the pulses. In other arrangements, there were provided relay switches which, in accordance with operator selection, could couple any of a number of output switches to the gap and in this manner increase or decrease the magnitude of the machining power being furnished to the gap. In still other prior art arrangements, a bank of resistors were included connectible in series with the gap. Different magnitude resistors or combinations of resistors were switched in either manually or by relays thus to control the current magnitude of machining power pulses.
The known prior art arrangements just discussed for controlling machining current were subject to many disadvantages, particularly by reason of the difficulty in maintaining uniform steps of current over a broad range. Also, those systems using mechanical or electromechanical switching elements were both complex to build, difficult to service, and expensive in their original cost. By way of summary, it will be seen that prior art arrangements for controlling magnitude of machining current were subject to a number of disadvantages.
It will be understood hereinafter in the specification that when I refer to "electronic switch", I mean any electronic control device having three or more electrodes comprising at least two principal or power conducting electrodes acting to control current flow in the power circuit, the conductivity between the principal electrodes being controlled by a control electrode within the switch whereby the conductivity of the power circuit is controlled statically or electrically without movement of mechanical elements within the switch. Included within the definition are transistors in which turn-on is accomplished by a control voltage applied to the transistor control electrode and in which turn-off is accomplished automatically in response to removal of that control voltage. Also included in the definition are electronic devices of the gate type in which turn-on is accomplished by a control voltage applied to the control electrode, which control voltage may be then removed and in which turn-off is accomplished by application of a subsequent control voltage to the control electrode. An additional class of electronic switches called "electronic trigger devices", falls within this definition and includes ignitrons, thyratrons, semiconductor control rectifiers, and the like. By "electronic trigger device", I mean any electronic switch of the type which is triggered on at its control electrode by a pulse and is then turned off by a reverse voltage applied for a sufficient time across its principal electrodes.