Welcome back, EDMers! This is the second segment of our Back to Basics series, providing informative insights on electrical discharging machining (EDM). This will serve as a refresher for experienced operators and as a primer for operators and shop owners just beginning to explore EDM technology and this unique machining process.
The EDM Spark
During machining, thousands of bluish-white sparks seem to appear as a result of the EDM process. However, EDM only really creates one spark at a time. Each spark is precisely generated and controlled by advanced adaptive computerized circuits. These circuits maintain and stabilize the electrical discharge power (sparks). Unlike conventional milling or turning that utilize fixed programmed speeds & feeds, EDM must function with a varying and dynamic feed rate, based upon the changing stability of the electrical discharge conditions.
The programming of an EDM machine does entail 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 distance that the spark energy can travel also increases, creating what is known and referred to as over-burn.
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 over-burn, which is defined as the amount of over-cut or over-sizing produced by the electrical discharge spark energy. This energy travels and emanates out from an electrode using a particular set of power setting values. This over-burn 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 earlier in this Back to Basics series, the electrode never comes into physical contact with the workpiece. During EDM machining, electricity jumps across the spark gap (distance between the electrode and the workpiece) to erode material from the workpiece. Before this can happen, a very complex series of events take place inside the machine’s generator (power supply), as well as between the electrode and workpiece.
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. Once this channel is created, it signals the generator to release the short duration, high-power discharge energy pulse (called ON-Time) that performs the work and erodes the workpiece material.
During ON-Time, the workpiece material is eroded by means of sublimation. In other words, a small amount of the workpiece material transformed directly into a gas. Since a dielectric fluid is also used, the sublimed material quickly cools and condenses 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, which is used to flush and remove condensed debris from the spark gap area.
One EDM discharge pulse contains (x1) ON-Time and (x1) OFF-Time in order to complete one full cycle. If the OFF-Time is not sufficient enough to evacuate the condensed debris, the re-ionization process of the spark gap is very unstable as a result of conductive debris floating in the gap area. If the spark gap ionization process is not controlled properly, the discharge energy is concentrated to a single point, rather than being dispersed evenly over the entire electrode surface. This is known as direct shorting or arcing of the discharge energy, which can result in damage to the workpiece.
The EDM discharge process is a delicate balancing game of tug-o-war, wherein the generator is continuously adapting and changing the ON and OFF time values to stabilize the process. For this reason, the EDM process is not operated with a set feed rate. Modernly advanced EDM generators automatically detect unstable conditions in the spark gap and will apply the first steps of adaptive power control by extending and increasing the OFF-Time. These discharge pulse cycles and ON/OFF time modifications are repeated thousands of times per second, which speaks to the machine’s generator high tuned electrical capabilities.
Rough & Finish Machining
During roughing EDM operations, power levels are higher in order to achieve greater material removal rates. The number of sparks is lower during roughing, but each spark has higher energy levels which produce larger amounts of over-burn. 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 over-burn amounts, but also produces a finer/smoother surface finish with higher accuracy. The finishing process is used after the roughing process, minimizing the amount of material that finishing operations must remove to achieve an efficient total overall cycle time, as in most cases the roughing process bares the largest amount of cycle time.
Stay Tuned for More!
If you’ve enjoyed this history lesson and return to the basics, be sure to stay tuned for future updates in this 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.
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