EDM Method and apparatus utilizing successively displaced magnetic field

#The present invention relates generally to electrical discharge machining (EDM) having a tool electrode juxtaposed with a workpiece electrode to form therebetween a minute machining gap in the presence of a machining medium and a succession of electrical pulses applied between the tool electrode and the workpiece electrode to produce successive electrical discharges across the gap and thereby electroerosively remove material from the workpiece electrode; as the material removal proceeds the workpiece electrode and the tool electrode are advanced toward one another to form a cavity in the workpiece electrode. More particularly, the invention relates to an improved EDM method and apparatus in which the interaction of the high-amperage electrical discharge and an externally applied magnetic field are utilized and, moreover, applied in a novel manner in the EDM system constituted by the juxtaposed effective surfaces of the machining tool electrode and the machined workpiece electrode in the gap.

#The proposal to use an externally applied magnetic field auxiliarily in the EDM process dates back to earlier stages in the EDM history. When a magnetic field is applied externally to the region of an EDM gap, the ensuing magnetic flux interacts with the high-amperage electrical discharge current flow and causes on media generated in the gap certain dynamic actions which have not yet been fully explained. It has been recognized that these interactions facilitate the production of electrical discharges and allow the effective machining gap spacing to be advantageously enlarged so that an increase in the discharge repetition rate and an enhancement in the removal of machining chips and other products from the gap region may result to promote the stabilized EDM action and thus generally improve the EDM process. For the prior art in these particular techniques in which the magnetic field is externally applied in the EDM gap, reference should be made to my earlier contributions disclosed in Japanese Patents published under publication No. 29-6942 on Oct. 25, 1954, No. 30-833 on Feb. 11, 1955, No. 30-2943 on Apr. 28, 1955, No. 39-13297 on July 11, 1964, No. 46-11400 on Mar. 23, 1971, No. 46-12520 on Mar. 31, 1971 and No. 54-9759 on Apr. 26, 1979 as well as Japanese Utility Model Registration published under publication No. 31-5790 on Apr. 19, 1956.

#In one prior-art proposal, a magnetic-field generating means is provided, e.g. one or more coils energized by an external source of direct-current or alternating-current nature and is positioned so as to develop a magnetic flux which is constantly fixed relative to the electrode system with the resulting magnetic lines of force fixedly traversing the confronting surfaces of tool and workpiece electrodes across which machining actions are in progress. The coil may be wound on the tool electrode or workpiece or both, or alternatively an iron core on which an energizable coil is wound may be located in contact with or in the proximity of the tool or workpiece electrode or both to establish the desired stationary magnetic field.

#These and other objects, features and advantages of the present invention will become more readily apparent from the following description of certain embodiments thereof, reference being made to the accompanying drawing in which:

#FIG. 1 is a schematic view diagrammatically illustrating an embodiment of the present invention in which two or more electromagnetic coil means are fixedly arranged at preselected spaced-apart locations about the EDM gap zone and energized in sequence to produced successively displaced magnetic fluxes;

#FIG. 2 is a schematic view illustrating a modification of the embodiment of FIG. 1 in which the energizing current for each said coil means is varied;

#Referring first to FIG. 1, a typical EDM system comprises a tool electrode 1 juxtaposed with a workpiece electrode 2 to form a machining gap G therebetween, the gap being filled with an EDM liquid, e.g. Kerosene, transformer oil or distilled water supplied continuously or in pulses from a nozzle or any other supply unit (not shown). An EDM power supply 3 is connected between the tool electrode 1 and the workpiece electrode 2 to apply a series of machining pulses between them to produce across the gap G a succession of electrical discharges which electroerosively remove material from the workpiece electrode 2. An electrode feed servo unit 4 is shown associated with the tool electrode 1 to effect the relative advance movement of the tool 1 towards the workpiece 2 required to maintain the size of the machining gap G substantially constant as the material removal from the workpiece 2 proceeds. The servo unit 4 is also responsive to a gap short-circuiting condition or excessively low resistance or impedance condition which may occur in the machining gap G from time to time. In accordance with the customary EDM practice, the system may also be provided with an electrode reciprocating unit (not shown) associated with the movable electrode 1 to periodically retract the electrode 1 away from the workpiece 2, thereby promoting gap flushing or chip removal in the region of the machining gap G.

#In accordance with the present invention, a plurality of magnetic-field generators, shown by two coils 5 and 6, are provided adjacent the region of the machining gap G to successively produce magnetic fields oriented in different directions. The coils 5 and 6, here arranged in a region which surrounds the workpiece electrode 2 being machined by the tool electrode 1, are connected in parallel to a common power supply 7 via switches 8 and 9, respectively, which are controlled by a switching timer 10. The switching timer 10 maybe any known timer, such as an astable or bistable multivibrator device, which alternately provides a first and a second output pulse which are applied to the switches 8 and 9, respectively. When the switch 8 is energized with the first output pulse of the timer 10, the coil 5 is energized by the power supply 7 to produce a magnetic field oriented in a first direction, here in the vertical or machining-feed direction in which the tool electrode 1 is advanced by the servo-feed unit 4 to penetrate into the workpiece electrode 1. When the switch 9 is energized with the second output pulse of the timer 10, the coil 6 is energized by the power supply 7 to produce a magnetic field oriented in a second direction, here in a horizontal direction orthogonal to the machining-feed direction. The resulting magnetic fluxes oriented in the mutually orthogonal directions are applied successively to the region of the machining gap G.

#Magnetic fields successively generated and applied to the gap G region selectively in the varying directions should each be of a field strength sufficient to produce a magnetic flux density ranging generally between 100 and 1000 Gauss and, preferably, not less than 200 Gauss. As has been recognized, each resulting magnetic flux in succession, when so adjusted, as such a strong interaction with the discharge current in the gap G applied between the tool electrode 1 and the workpiece electrode 2 in the form of successive time-spaced pulses as to lead the machining action to favorable results. Thus, a favorable gap condition is created which facilitates gap breakdown and sparkover by an individual discrete machining pulse so that successive pulses have a uniform discharge characteristic while permitting the effective sparkover distance of the machining gap G to be increased, phenomena attributable to the magnetic tendency of magnetically susceptible particles present in the machining gap G and the electromagnetic force acting on the discharge current.