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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When operating a sinker EDM, don’t underestimate the performance impact of flushing and fluid flow. An inefficient back-burn punch setup, for example, can lose flushing during operations. This issue can lead not only to scrapped parts or rework, but it also extends your customer lead-times. To prevent such issues, it is well advised to consider applying flush pots. By using flush pots, manufacturers can steadily improve flushing and fluid flow in their shop operations.
In some applications where more precise control of flushing and fluid flow are needed for extended operations, a precision in-line needle value can be used. In this configuration, the machine-side flushing is set to maximum pressure, and the precision in-line value is used to regulate the amount of flushing required at the workpiece. This approach prevents the loss of pressure for flushing over time, as it is common practice to use low-pressure flushing (positive flow) when performing back-burn operations.
Below is a list of components and assembly for creating an in-line precision valve. These components can be purchased locally or from an industry supplier, such as McMaster-Carr. A precision analog pressure gauge can also be added to improve the repeatability of external flush settings. However, this configuration is optional and not a requirement. Once assembled, this setup is connected between the flush pot and the machine’s auxiliary flush port.
Today, we launch our “Back to Basics” series. This blog series will serve to provide 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 process utilizes short bursts or pulses of electrical energy to erode and machine conductive materials. This process can be thought of as machining with lightning bolts, called sparks. The number and power of each spark can be precisely controlled. By modifying the amount and power of the discharge spark energy, the material removal rate, attained surface finish and resulting accuracy can be predictably and repeatedly controlled.
While EDM is commonly thought of as a slower form of metal removal compared to conventional milling and some other processes, recent advancements in EDM technology have led to significant improvements in processing times and finish quality for even the most complex and involved part geometries. What has now become an essential process for die/mold shops, aerospace, automotive and other manufacturers humbly began with a failure.
In the early 1940s, two scientists in the former Soviet Union, B.R. Butinzky and N.I. Lazarenko, experimented with methods to prevent erosion of tungsten contacts caused by electrical sparking during welding. Although they didn’t find a better welding method, they discovered how to control metal erosion by immersing the electrodes in oil or water. From their research, Butinzky and Lazarenko built the first electrical discharging machine for processing metals that were difficult to machine with conventional milling, drilling or other mechanical methods such as tool steel and titanium.
Butinzky and Lazarenko drew on ideas developed by English physicist, Joseph Priestley, who wrote about the erosive effects of electricity on certain metals back in the 1770s. The Russians’ early work became known as spark machining because electrical discharges caused sparks that could be controlled to manufacture specific shapes.
How Machining with Electricity Works
In conventional machining, the material is removed by cutting tools that turn or grind against the workpiece with a mechanical force. In the EDM process, sparks of electricity create short bursts of high energy that instantly melt and vaporize the material without making contact. Due to the non-mechanical and non-contact machining process, EDM is referred to as a “non-traditional” type of manufacturing.
The key to EDM machining is the passage of electricity from a tool (electrode) to the workpiece, which must be composed of conductive material like steel or aluminum. The tool, which can either be a small diameter wire, hollow tube, or an electrode mechanically machined into a negative version of the workpiece’s final shape, is then placed and maintained in close proximity to the workpiece during the EDM spark erosion process.
Three EDM Methods
EDM technology has evolved into three distinct machining approaches:
Wire EDM: Wire EDM uses a small diameter copper or brass-alloy wire to cut parts much like a band saw. Traditional uses are to make punches, dies, and inserts from hard metals for die/mold tooling applications. Uses have since expanded to include part production uses over a wide array of industries.
Sinker EDM: Sinker EDM uses electrodes machined from a special graphite or copper material into the shape or contour feature needed on the final workpiece. Typically, uses include the production of small or complex cavities and forms for die/mold tooling, but have also found use in many production applications.
EDM Drilling: EDM drilling uses a small diameter hollow tube electrode made from copper or brass alloys to erode holes into the workpiece. This method is typically used to prepare start holes for the wire EDM process, but have also progressed to producing small hole features found in dedicated production applications such as turbine engine components and medical devices.
Stay Tuned for More!
If you’ve enjoyed this Back to Basics lesson, 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 to fascinating EDM advancements that we encounter each day.
To receive e-mail alerts on future updates, subscribe to our blog now!
Q: Does high-qualify surface finish (HQSF) technology produce the best possible surface finish on Makino EDMs, and what is added to the machine for this option?
A: For over 20 years, Makino has offered HQSF technology as an option on sinker EDM products. The benefits of this technology are still a valuable and important function for fine-finish applications. When initially released, HQSF was designed with a focus on the improvement of surface finish and finish consistency to eliminate post-machine polishing, but the technology also yields additional benefits.
HQSF technology uses a special proprietary powder (µSc) designed to be neutrally buoyant so that it remains suspended when added to the dielectric oil fluid. During machining, the powder additive evenly distributes the discharge spark energy over the entire electrode contour. This design provides a more controlled and stable EDM process by minimizing any local concentrations of power that can create shortages resulting in “hot spot,” or pin-hole, discharge.
HQSF does not necessarily improve the “best possible surface finish” capability of a machine, as seen with the use of copper electrodes. However, the use of HQSF with copper electrodes does dramatically improve the total cycle time.
When using graphite electrodes, HQSF dramatically impacts and improves the best achievable surface finish. Performing standard EDM processing using graphite electrodes yields a best surface finish of 0.7µRa (27µinRa). Using HQSF with graphite electrode can produce a best surface finish of 0.4µRa (16µinRa), which is a significant improvement of almost 50 percent. Additional cycle time, however, is required to achieve this finer finish with the HQSF special generator settings.
During HQSF machining, the machine filters are bypassed, and the machine tracks the machining hours of the HQSF powder. Once the powder is depleted, the filters are reactivated to collect the material debris from the fluid. Early machines with HQSF technology are equipped with magnetic separators used to filter large metal contaminants from fluids. The magnetic separator system is still an available option, but this system is recommended only for large EDM applications that use 120-amp or higher power settings.
The Makino EDAF-Series machines offer HQSF technology as a retrofit option (except for machines equipped with the fine-hole EDM drilling options), but the larger EDNC-Series machines offer HQSF only as a factory-ordered option.
HQSF Technology Benefits:
Faster roughing speeds
Faster total cycle time to fine surface finishes using copper electrodes
Faster total cycle time to standard surface finishes using graphite electrodes
Up to 50 percent finer surface finish with additional cycle time using graphite electrodes
Improved metallurgical quality (less recast, HAZ and micro-cracking) that can extend the service life of the part/tooling component
Q: Is it acceptable to utilize the same electrode reduction undersize amount for all of the electrodes?
A: The real answer is yes and no, as a proper answer requires a detailed look at the overall EDM operation. The practice of using a standard electrode reduction amount is very common. However, depending on specific applications, it may or may not yield the best machining speed or accuracy results.
Benefit of Using a Standard Electrode Undersize:
In many shops, the electrodes are machined by milling, and different mill programmers typically have varying levels of EDM knowledge. Using a common or standard electrode reduction amount for all electrodes can help streamline and simplify the process of electrode programming, regardless of the operator.
Applying the same electrode reduction amount to all electrodes prevents confusion in EDM setup and tracking. If all electrodes have the same reduction amount, there is no need to mark and/or measure every electrode during EDM setup.
Applying the same electrode reduction to all electrodes lends greater EDM process flexibility, as the electrodes can be shuffled around and reused as roughers and finishers when needing to produce multiple cavities of the same contour (for example: cavity #1 finish electrode becomes Cavity #2 roughing electrode).
Downside of Using a Standard Electrode Undersize:
Using a standard electrode reduction amount can waste valuable shop machining time. With too small of an electrode reduction amount, the EDM operator is often handicapped into using a lower power discharge setting in order to maintain proper size, which results in slower machining speeds and increased electrode wear.
Applying the same electrode reduction amounts between the roughing and finishing electrodes can deteriorate form accuracy, as the finishing electrode is machined further away from the final net shape of the desired cavity and often requires greater optimization. This method can also lead to declines in machining speeds (slower roughing and more required orbiting on finishing).
Intelligent Review on Electrode Preparation:
It is paramount that the EDM operator and electrode milling programmer communicate to discuss plans for electrode reduction on each job.
Through proper review and planning, shops can significantly improve efficiency and apply optimal electrode reduction amounts that produce the fastest machining speeds (roughing speeds often see the largest improvements).
The use of different electrode reduction amounts can be strategically deployed on critical part details that require tighter tolerances.
It is important to establish a reliable identification system when manufacturing electrodes with different reduction amounts, such as a marking or color code system on the electrode holder for roughing and finishing.
For more open tolerance details, a standard electrode reduction size can be established.