Originally published at: https://edmproud.com/back-to-basics-2-6-the-edm-machining-spark/
When you see an EDM in operation, it can appear that there are hundreds or even thousands of bluish-white sparks simultaneously discharging. But in fact, an EDM creates only one spark at a time. Each spark is precisely generated and controlled by advanced adaptive computerized circuits that maintain and stabilize the electrical discharge power (sparks). Unlike conventional milling or turning that utilize fixed programmed speeds and feeds, EDM by necessity functions with a varying and dynamic feed rate based upon the changing stability of the electrical discharge conditions.
Programming of an EDM machine involves setting the initial target values of the electrical discharge energy. This is typically determined by the actual square contact area of the electrode (Sinker EDM) or by the thickness of the section to be machined (Wire EDM). As the power levels are increased, the amount or distance that the spark energy can travel also increases, creating what is known as and referred to as overburn.
It is critical to understand that the final feature produced by EDM will alwaysbe larger than the electrode used to machine it. This is the result of overburn, which is defined as the amount of the overcut or oversizing produced by the electrical discharge spark energy that travels and emanates out from an electrode using a set of power setting values. This overburn amount is also commonly referred to as the spark gap distance and must be calculated and accounted for during the preparation and programming of the EDM process.
As stated in an earlier post, the electrode never comes into physical contact with the workpiece. During EDM machining, electricity jumps across the gap (spark gap) between the electrode and the workpiece in order to erode and remove material from the workpiece. Before this can happen, there is a very complex series of events that takes place inside the machine’s generator (power supply) and between the electrode and workpiece prior to the erosion process.
The machine generator establishes and controls a very precise voltage between the electrode and the workpiece. Once the voltage builds and achieves a specific level, it ionizes the spark gap creating a conductive channel that signals the generator to release the short-duration, high-power discharge energy pulse. This “on-time” refers to the duration that the spark is performing the work and eroding the workpiece material.
During the high-power discharge process (on-time), the workpiece material is eroded by means of sublimation. Put simply, a small amount of the workpiece material is vaporized and transformed directly into a gas (similarly to how dry ice turns directly into a gas and skips the liquid melting stage of matter). Since dielectric fluid is also used (dielectric fluid will be covered in a future lesson), the vaporized material quickly cools and condenses back into small metal particles that must be removed from the spark gap before the discharge erosion process can start over. All EDM generators utilize a delay time where all power is turned off. This is called “off-time.” This off-time is used to flush and remove the condensed debris from the spark gap area.
One EDM discharge pulse contains one on-time and one off-time element to complete one full cycle. If the off-time element is not sufficient enough to evacuate the condensed debris, the re-ionization process of the spark gap will be very unstable as a result of conductive debris floating in the gap area. If the spark gap ionization process is not controlled properly, a direct shorting or arcing of the discharge energy can occur (the discharge energy is concentrated to a single point rather than being dispersed evenly over the entire electrode surface), which can result in damage to the workpiece.
The EDM discharge process is a delicate balance wherein the generator is continuously adapting and changing the on- and off-time values to stabilize the process, which is why the EDM process is not operated with a set feed rate. Modern advanced EDM generators will automatically detect unstable conditions in the spark gap and apply the first steps of adaptive power control by extending and increasing the amount of off-time. These discharge pulse cycles and on/off-time modifications are repeated several thousand times per second, so the machine’s generator is a highly tuned electrical instrument.
Rough & Finish Machining
During roughing EDM operations, the power levels are higher to achieve greater material removal rates. There are a lower number of sparks during roughing, but each spark has higher energy that produces a larger amount of overburn. The roughing process achieves a rougher surface finish and lower accuracy levels.
In finishing EDM operations, power levels are reduced, and the amount or frequency of sparks is increased. This achieves lower material removal rates with smaller overburn amounts, but also produces a finer/smoother surface finish with higher accuracy. The finishing process is used after the roughing process, which minimizes the amount of material that finishing operations must remove to achieve an efficient total overall cycle time, as in most cases the roughing process carries the largest amount of cycle time.
Stay Tuned for More!
If you’ve enjoyed this lesson and return to the basics, be sure to stay tuned for future lessons in this 6-part series as we reflect on our industry and celebrate the building blocks that have led us to the fascinating EDM advancements that we encounter each day.