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“Copper
vs. Graphite”
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We have often heard discussions about the relative merits of different EDM Electrode Materials, and the longest running debate is about the two most commonly used materials - Copper vs. Graphite. It seems strange that EDM users in different parts of the world have gravitated toward different electrode materials to do exactly the same jobs, on the same or very similar EDM equipment. If we ask the EDM users which is the best material to use, Copper or Graphite, we usually get a definitive answer - and it's, generally, Copper if the user is from Europe or Asia, and Graphite if the user is from the United States. Of course, there are exceptions to that statement, which depend on job size, finish requirements, type of EDM equipment, and methods of making the electrodes. However, one thing is certain. As a new generation enters the EDM industry, they bring with them fresh new ideas, and are not burdened with opinions that were formed decades ago. It is with this thought in mind, that we re-examine the relative merits of Copper vs. Graphite. |
A User of Both Copper & Graphite
In order to make an educated comparison, we decided to consult with an EDM user that for many years has used both Copper & Graphite. The company is OAR Moldworks, of Providence, RI - the subject of the story "Always Searching for a Better Way". We spoke with Ozzie Rosenholm, the Company's President, and he explained that in his opinion, there is no one "best" electrode material. At OAR, they employ a few different materials, and that depends on several factors.
Ozzie first discussed the topic of graphite with us, since today, it is approximately 90% of the total electrode material used by his company. After years of using many different grades of graphite, from various manufacturers, they have settled on only two grades - both from the same manufacturer. They chose them based on their experience, and also for simplicity, to avoid the confusion that occurs when the original identifying marks are no longer on the graphite in their stock. This confusion could cause a standard grade of graphite to be used, when the job calls for a premium grade - thereby affecting the job quality. The two grades that OAR has decided upon are both from Poco Graphite, and they are Poco 200 for roughing and semi finishing, and Poco 3 for fine finishing.
Ease of Machining - A Big Advantage
Graphite has become a popular electrode material for several reasons. However, one of the most important reasons is its speed & ease of machining. OAR builds molds for many different industries where the electrodes must be accurate, and in many instances, are quite complex. The electrodes are designed by computer, and are machined on a high-speed Machining center built specifically for machining center built specifically for machining graphite electrodes. According to Ozzie, no other method can match the speed, accuracy, convenience and cleanliness of a Graphite Machining Center. He related that electrodes, which were formally produced by many different methods, including precision grinding, are now being produced on their Graphite Machining Center, with high precision and excellent surface finishes. OAR's machining center also has the ability to produce sharp internal corners as part of the program.
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The dust produced when machining graphite has, in the past, been somewhat of a negative factor in its use as an electrode material. Efficient vacuum systems have been able to control a very high percentage of the dust generated, and many super-clean shops use graphite exclusively for all their electrodes. It was common practice to isolate graphite machining to a specific location, where it could be contained and better controlled. However, Graphite Machining Centers usually have self contained vacuum systems, that are highly efficient, and completely control the dust - so much so that, the machines can be located anywhere in the shop, even in dust-sensitive areas. Graphite also lends itself to all the popular machining methods such as grinding, turning, milling, drilling and even Wire EDMing. However, it has distinct advantages when very small diameter flushing holes are required in the electrode. A special hole drilling machine produced by the Westhoff Company, of Fenton, MO, can quickly drill holes as small as .010" Diam., several inches deep in graphite. After EDMing with these electrodes, the small standing cores left by the flushing holes, can easily be mechanically removed or eliminated, by orbiting during the EDM cut. |
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Machining Strategy
Today, instead of segmenting a large complicated job into a number of different electrodes, OAR uses a high level CAD system to design the complete shape in a single graphite electrode. After machining, the electrode is inspected and used to EDM the entire configuration in a single set up. This strategy is quickly becoming popular, ever since Graphite Machining Centers have been introduced. In retrospect, it now seems counter-productive to take a large complicated shape and break it down into several smaller shapes, and then recompose the configuration in the EDM Machine - when today, it can be accomplished in a single set up.
Electrode Weight - an Important Factor
Since the trend seems to be in the direction of larger electrodes, with the entire configurations machined into them, the weight of the electrode becomes a very important factor. For instance, the piece of graphite that was used for the electrode on the cover of this issue was 12" x 7" x 2 3/3" (300mm x 180mm x 70mm), and it weighed 10.6 lbs. (4.8 kilos). Compare that to the same size piece of Copper, which weighs 74.6 lbs. (34 kilos). A weight difference of this magnitude effects many factors in the handling construction, and use of the electrode.
Abrading Graphite
A process known as Abrading is an excellent method of quickly producing duplicate graphite electrodes from a master, called a "stone". The stone is formed by mixing silicon carbide and epoxy, and then pouring it around a model of the electrode shape desired. After curing, the model is removed, and the stone is mounted on the platen of a special abrading machine. The table of the machine where the graphite blank is mounted, orbits slightly in an X & Y motion, while the platen with the stone attached, cycles up and down under hydraulic power. This causes the stone to abrade the shape faithfully into the graphite blank . This process has been used for many years. However, during that time, various refinements have been implemented, and using today's equipment and technology, accurate electrodes with fine detail are routinely produced by the abrading process.
Copper as an Electrode Material
In Europe and Asia, copper has been a popular electrode material, and in our opinion, this is due to several factors. First, the early EDM Machines that were predominant in those areas, used R-C (Resistor Condenser) power supplies. Metallic electrodes, such as Copper, Brass, Copper Tungsten, etc. were the only electrode materials that would perform effectively with an R-C EDM. When the next generation of machines were developed, Copper was already firmly entrenched as the most popular electrode material.
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Another reason, was that the type of Graphite ideally suited for use as an electrode material, was not readily available. Because of this, the Graphite that was available, was used only for large roughing electrodes, and the results were very rough surface finishes, along with high electrode wear in most cases. In the U.S., Graphite enjoys an immense popularity, because of the early tube-type EDMs, that performed effectively with Graphite electrodes. This factor was combined with military rocket research, that spawned an entirely new class of dense, fine-grained graphite. The use of metallic electrodes quickly diminished in the U.S., in favor of the more easily machinable, fine grained graphites. |
The Advantages of Copper
We asked Ozzie Rosenholm to explain the advantages of Copper as an electrode material, and how he uses it. He explained that Copper, machines very efficiently on the Wire EDM, and OAR produces many Copper electrodes via that method. They have also Wire-cut Graphite effectively, however, they prefer Copper, because it cuts faster with fewer flushing problems, using plain brass wire. Thin, odd shaped, rib style electrodes, that are problematic to form grind, can easily be Wire-cut in Copper. Over the years, OAR has adapted special tooling and designed fixtures to Wire-cut electrode shapes that are not normally produced on a Wire EDM. This has saved many hours of tedious work on the grinders, and keeps one of their Wire EDM's constantly busy.
Another advantage of Copper is its ability to be coined. OAR has utilized this advantage for many years to produce engraving electrodes. A Graphite electrode is first produced, and then used to EDM a coining die.
After polishing the die, Copper blanks are pressed into it, thus producing multiple duplicate electrodes. A clever method of mounting the coined electrodes, by using the coining die as a locating fixture, ensures, accuracy when EDMing with them. This system is not limited to engraving electrodes, but works well with any shape that can be coined into copper.
Machining copper can sometimes be difficult, since it has a tendency to drag on the cutter, saw blade, and grinding wheel, especially in the hands of an inexperienced machinist. There is however, a highly machinable Copper that is very popular as an electrode material - Tellurium Copper. The addition of 2% Tellurium to the Copper, greatly enhances its machinability without changing its EDMing capabilities, to any great degree. As a comparison, if Brass is rated at being 100% machinable, then Tellurium Copper's rate would be 90%.
On certain restrictive jobs for the medical industry, OAR takes advantage of the fact that Copper can be highly polished. The Copper electrodes are first Wire cut, and then they are highly polished. After a close inspection, they are used to EDM mold cavities to the finest EDM finish possible. Due to the very close tolerances on the plastic parts produced, the cavities cannot be polished after EDMing, and must be used just as they come off the EDM Machine. This is done because of strict regulations that demand absolute repetitiveness to the initial item that was cleared by the agencies.
EDMing with Copper vs. Graphite
Any EDM job that can be done with Copper can also be done with Graphite. This is evidenced by the fact that every day, similar jobs are being accomplished all over the world. The difference is the efficiency of the electrode manufacturing operation, and also in the EDMing process. The end result might be identical, however, the cost to accomplish it could be vastly different, and must be evaluated. The reader should understand that new equipment and methods of using it are being developed every day. To be successful, shops must stay abreast of these developments, and must sometimes discard former methods that have served them well for many years, and adopt new technology. Only they can decide - and either gain or lose by their decision. We hop that this article has helped you to make an educated decision on when to use either Copper or Graphite.