What are the precautions for sinker EDM?
The successful operation of sinker EDM depends on the understanding of the relationship between each spark, electrode and workpiece.
Most cameras cannot keep up with the productivity of 30,000 sparks per second. Photo: Adam Wysuph of MC Machinery.
The frame rate per second (fps) of movies and television is usually standardized to 24 and 30 fps, respectively. Since the moving speed of each frame is so fast that humans cannot perceive the visual isolation, we treat the collection of single frames as a continuous moving image.
The sinker electrical discharge machining (EDM) equipment shown in the figure above has a similar illusion. Of course, the EDM process involves metal cutting in a dielectric liquid by electrical discharge or sparks between electrodes or workpieces. The sparks produced by the sinking EDM process-500 to 30,000 sparks per second, seem to occur at the same time. In fact, these rapid sparks are produced once at a time. Obviously, the EDM process is not too normal for us to perceive.
Although our opinions betray us, it is still important to understand this fact to properly control the EDM process.
Every EDM operation starts with a central question: Which waveform will best produce the desired result? The answer of course depends on the workpiece material. The decisive factor is the electrical conductivity of different metals and their ability to absorb heat. Photo: Adam Wysuph of MC Machinery
Understanding and optimizing sinker EDM was the subject of a technical presentation made by Pat Crownhart at the MC Mechanical Systems Open House last fall. Mr. Crownhart is the company's submersible EDM product manager. He has deep expertise in electromechanical functions in the EDM process. Since Mr. Crownhart’s speech has left most of the audience in trouble after only a few slides, I recently got in touch with him to better understand the core principles of submersible EDM technology.
Each EDM spark can be treated as a shovel of material removed from the part. The electrical conductivity of different metals and their ability to absorb heat are key determinants of the type of "shovel" that should be used. Picture: Pat Crownhart, MC Machinery Company
Choose the right shovel
Mr. Crownhart said: "Think about all the EDM sparks are discharged from the shovels that are removed from the parts." The size and shape of the shovels (each shovel is designed to perform a specific task) can be manipulated through how the electrons travel between the electrode and the workpiece. To control the gap between them.
Mr. Crownhart said that the most common shovel shape is the square wave waveform (called transistor pulse or TP waveform on the EDM machine produced by Mitsubishi Electric, which owns MC Machinery Systems) 90% of the sinking EDM that can be used is mainly due to its large The versatility of this kind of material. The square wave generates a lot of heat by rapidly increasing the amperage. Since EDM devices in the United States usually have graphite electrodes that can withstand high temperatures, most of these devices use TP circuits for daily operations.
If the current waveform represents the shape of an EDM shovel, the voltage can be compared to its handle. How long can the shovel last? Voltage will cause sparks, but if it takes too long due to electrode contamination or other reasons, it will reduce the efficiency of the EDM process. Every time a spark is generated, the machine will make a small amount of position adjustment of the servo. Changing the voltage parameter (that is, changing the gap length between the electrode and the workpiece) will affect the operation, no matter how good it is. Kronhardt said: "When you change the gap, you change the distance the spark travels." "You can safely change the voltage to a certain extent, but because the adjustment is so fast, you need to understand the effect of the voltage or damage The risk of parts."
Every time a spark is generated, modern sinker EDM equipment will make small adjustments to the server. Changing the voltage parameter (or the length of the gap between the electrode and the workpiece) will affect the operation. Picture: Pat Crownhart, MC Machinery Company
Polarity (for us, we describe it as the direction of electron propagation) is another central variable in sinker EDM operation. Before adaptive control was available and developed on machines such as Mitsubishi Electric's new SV series, many older machines adopted negative polarity. If there is no adaptive control, if the DC power cannot be turned off but produces long sparks, the workpiece may be damaged. Negative polarity will result in a faster burning rate (it is still the best choice for titanium and some foreign metals), but damage caused by electrical shorts usually affects the electrode rather than the workpiece. When electrons flow from the part to the electrode, a positive polarity spark usually reduces wear on the electrode.
Spark is the enemy
Although it may be counterintuitive, sparks are the enemy of the EDM process. Crockhart said that sparks represent heat. "If you just heat it, you are actually using a welding machine." The higher the amperage, the longer the on-time, the greater the heat generated by each discharge. Although a faster burning rate will generate more heat, too much heat will cause particles to grow on the electrode. Conversely, this contamination or chipping will cause the machine to continuously spark at the same location, resulting in poor machining quality or damage to the workpiece.
Mr. Crownhart said: "This is an old saying that EDM often says." "This is because in the past when electronic devices were not so advanced, it was very important to decompose contaminants and bring them into the pressurized fluid." Modern power supplies would be inconsistent. The spark reacts and adds extra time for cooling, but for sedimentary EDM technology, pollution is still a major consideration. Mr. Crownhart said: "You can only process the machine as fast as you can move contaminants."
In summary, these may be the basic points for experienced EDM users. But Mr. Crownhart said that he has met many users who run EDM based purely on past working principles without having to understand why certain settings work. He said: "Maybe more predictive than "it worked for me in the past, so now I will try"." "With this mindset, you can modify the settings until you can use them. The more you know about the process, The sooner you can get the most optimized settings for your work."
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