Ram type (also called sinker type) EDM was the first type of Electrical Discharge Machining used in the manufacturing process. I was lucky enough to see the first EDM machine made for the purpose of eroding metal. Most historians give credit to a metallurgist couple from Russia for developing the process, and this may be true. But the first known use of the process was a tap disintegrator manufactured and sold in southern Europe in 1952. The machine still exists in running condition in the lobby of the company that produced and sold the product as a broken tap remover. EDM stayed in this form into the late 1950s and started its transformation into a versatile process of metal removal in early 1960s.
First, the technological description of the process.
EDM mainly consists of two major components: a machine tool and a power supply. The machine tool holds a shaped electrode and a workpiece, whereas the power supply provides energy for performing the machining. The power supply uses a transformer to convert AC supply to DC supply through a rectifier. Based on type of polarity—namely straight polarity and reverse polarity—either terminal of the power supply is connected to the electrode or workplace.
Both the electrode and workpiece are completely submerged under dielectric. The commonly used dielectrics are hydrocarbon oil, silicon-based oil and deionized water. A servo controller is used to maintain a constant gap between the electrode and workpiece. Depending on the voltage and the gap, an electrical field is established between the tool in the workpiece.
As the electrical field is established, free electrons on the tool are subject to electrostatic forces, and electrons having less bonding energy are plucked out from the tool and accelerated towards the workpiece through the dielectric medium. As they gain more velocity while moving towards the workpiece, collisions occur between the electrons and dielectric modules, resulting in the ionization known as dielectric breakdown. This cyclic process increases the concentration of electrons and ions at the gap, creating a channel known as “plasma.” The electrical resistance of plasma is much less; therefore, electrons with high energy move from the negative terminal to the positive terminal, and high-energy ions will flow from the positive to the negative terminal.
The kinetic energy of the electrons and ions on impact with the surface of the work and tool gets converted to thermal energy, which increases the temperature of the workpiece and tool, resulting in erosion of the material due to instant melting—resulting in vaporization of the respective material. The material is removed in the form of debris.
When the power supply is withdrawn, the plasma channel expands and collapses, generating pressure, which flushes the debris out from the machined surface. Simultaneous erosion of workpiece and tool increases gap between them, unless the electrode is lowered automatically by servo controller to maintain this gap and the process continues.
One of the main advantages of this process is that there is no direct contact between the tool and the workpiece, and thus there is no force in machining; even soft and delicate materials can be machined.
This is a common and well-understood description of the process. And although there are thousands of variations of workpiece to electrode combinations, they will all follow the same basic rules and laws of physics.