Electrical Discharge Machining
Electrical discharge machining (EDM) is extensively used in the fine machining of complex forms and shapes in mold and die cutting, and fine hole drilling in injector cups, etc. The process can be used to machine fully-hardened metal components such as tool steel, which would not be possible using ordinary machining processes. Some of the items that are made by using EDM include hardened steel dies, engine parts, and jet engine fan compressor blades. EDM is a thermal process which removes and re-deposits material on the object that is being machined. The area that is recast is usually much harder than the original surface with more resistance to abrasion and corrosion.
In the EDM process, there is an electrical discharge that is formed between the surface of the object and the finishing tool which can create temperatures of between 10,000 and 20,000 degrees centigrade. A dielectric field is formed that continuously removes and re-deposits material to the item's surface. Metal and elemental powders can also be introduced into this dielectric stream which will cause an alloy to be deposited.
EDM can also be used to cut, shape and form very hard metals into cutting tools that can be used to cut and machine items that are made from softer metals. EDM involves an electrical current through an electrode made of copper. The electrode is brought very close to the component to be machined. Then it is clamped on the worktable of the EDM machines. Due to the close contact, sparking occurs and material from both the component and the electrode are removed. Both the electrode and the component are immersed in a bath of kerosene, which acts as a coolant and removes heat as well as EDM grit. The process produces a fine layer of oxide that can be lapped using lapping paste. EDM can be used to machine both blind and through-hole components.
There are CNC EDM machines available that can burn small complex contours and shapes. This is not a high volume, material-removal process. It first uses rough machining to the shape desired and then uses EDM for finishing, in order to be more cost efficient.