The present invention relates to a method and apparatus for machining electrically resistant materials such as Mn--Zn ferrite by means of electric discharge in a dielectric fluid.
Workpieces made of metallic materials such as stainless steel, iron and aluminum have conventionally been worked into small shapes by performing electric discharge machining with a dielectric fluid placed between a discharge machining electrode (made of tungsten or a sintered hard alloy and used as negative electrode) and the workpiece (positive electrode). However, this conventional method has presented considerable difficulty in machining workpieces that are made of electrically resistant materials such as Mn--Zn ferrite. Non-conductive materials such as ceramics are worked electrically by a method generally known as "electrolytic discharge machining" and that is described in a prior art reference such as H. Tsuchiya, "Kikai Gijutsu (Machine Technology)", 32-12 (1984), p. 77. However, the electrolytic discharge machining method presents difficulty in handling since it customarily uses NaOH and other strong alkali fluids as electrolyte. With a view to solving this problem, a method that adopts arc discharge in a neutral salt electrolyte such as NaNO.sub.3 or NaCl has been proposed in Japanese Patent Unexamined Publication No. Sho. 63-229225. This method, however, is hardly suitable for practical applications since the consumption of the tool electrode is rapid and the precision of machining is low.
A Mn--Zn ferrite as a hard and brittle material can be worked by a cutting method but the applicability of this method is limited to machining the workpiece to simple shapes such as flat plate and a round bar. Ultrasonic machining can be applied using grain particles and this approach takes advantage of the brittle nature of the Mn--Zn ferrite. However, cracking tends to occur upon contact with the working tool and it has been difficult to machine the workpiece to a solid shape having a plate thickness of less than 0.5 mm. Further, the Mn--Zn ferrite has an intrinsic resistance of about 5 to 10 .OMEGA.cm and can hardly be worked by the conventional methods of electric discharge machining. Even if discharge machining proceeds to some extent, cracks will readily develop on account of thermal shock and commercially satisfactory working can hardly be accomplished.