Breaking Down Dielectric Breakdown
Liquid dielectric used in electrical discharge machining serves a few purposes: to flush eroded material away from the workpiece, to cool the electrode and workpiece, and to minimize spark gap size. The mechanisms by which liquid dielectric flushes and cools are fairly obvious. How the spark gap is minimized by the dielectric is a bit more complex and requires an understanding of the physics of electricity and magnetism.
Let’s start with the basic mechanism of EDM: the spark. A spark is created when the attractive force between separated charges grows strong enough to create a conductive channel in an otherwise insulating material. The spark generated when you touch a doorknob after shuffling around on a carpet is a channel of plasma; normally insulating gas particles are separated from their valence electrons (ionization), which momentarily turns that channel of air into a conductor, by which the separated charges evenly redistribute themselves, attaining electrostatic equilibrium.
A certain force between separated charges must be achieved in order to create such a conductive channel, and the agent through which these forces are exerted is called the electric field. When a spark is generated between two objects, charge separation is lost, and the electric field disappears. So the process that allows for the creation of a spark can be described as breakdown of the electric field, and the voltage required to achieve this breakdown is called the breakdown voltage.
With the simplification of the workpiece and EDM electrode as spheres or points of charge, the voltage between electrodes can be seen as the product of the distance between electrode and workpiece and the electric field strength. Since the strength of the electric field exhibits an inverse square relationship with distance, the product of the electric field with the distance between electrode and workpiece results in an inverse relationship between voltage and distance. The farther away the electrode is from the workpiece, the lower the voltage. Because of the relationship between the electric field strength and voltage, a very strong electric field can achieve breakdown voltages at relatively long distances from the workpiece. In EDM, this means a large spark gap, and a large spark gap introduces randomness and inaccuracy in drilling.
To reduce the spark gap, the electric field can be weakened, and without lowering the voltage, this can only be achieved with the use of dielectrics. By placing a dielectric in between two conductors, the electric field between those conductors is indeed weakened. But dielectrics are insulators with no net charge, and neither of these properties influence electric field strength. Reconciling these properties with the undeniable effects of dielectrics requires a discussion of polarization.
Another situation in which the influence of electric forces on neutral conductors is observed is when a balloon is rubbed against hair, and then stuck onto a neutral insulator, such as a wall. The molecules in wooden paneling exhibit an uneven charge distribution, and an excess negative charge on a balloon creates an electric field that spins the positive ends of these molecules towards the balloon. Since the electric force is inversely proportional to the square of the distance, even the very small distance separating the positive ends of the molecules in the wood from the negative ends creates a slight net positive electric field, the induced electric field. The negative electric field from the balloon interacting with this positive field results in an attractive force.
A dielectric placed between two conductors is similarly polarized. As seen in the balloon example, this induced electric field points in a direction opposite that of the field that induced it (negative balloon inducing a positive wall). These opposing electric fields create a net electric field weaker than the original field, thereby allowing for a very small spark gap.
Without dielectrics, spark gaps would be so large that EDM would not be viable as a machining method. Their ability to dampen electric fields, flush material, and cool the workpiece and electrode make dielectric fluids an essential part of any EDM operation, and while often overlooked, they should be appreciated for their unique properties.
