What is electrical discharge machining? Good question, and the answer tends to leave the layman rather confused. No surprise, really, because EDM is very different from conventional machining, and is still somewhat shrouded in mystery. We shall attempt to explain just a bit, read on…
Just What Is Electrical Discharge Machining?
The plastic injection mold making and die making industries rely heavily on EDM machining, both with the sinker EDM and wire cut EDM machine. The electrical discharge machine is a totally amazing and somewhat mysterious tool, without which, our modern way of life would be much different.
As far as I can gather, two Russian scientists were doing experiments during WWII and came upon the EDM machining process as a result of their experiments. Around the same the time, several Americans were trying to develop a tap burner to remove broken taps and drills from aluminum casting.
Eventually the Swiss became involved and developed high performance quality machine tools that had a dramatic impact on manufacturing, especially in the mold making industry.
It is no wonder that the Swiss companies, Charmilles, Agie and Erowa are all world leaders in the EDM discharge business.
Strange how things get invented and developed sometimes! I have heard that the Russians were trying to manufacture hollow ball bearings when they developed the process, but it does sound like either an urban myth or some weird thing that the Soviets would try to do.
Global EDM Developments
As time passed, both the USA and Japan became heavily involved in electrical discharge machining, and soon both countries had many excellent companies producing high quality products.
The Americans came out with Eltee Pulsitron, which was the most popular machine for years, and many are still in operation today. Eventually CNC machining took over, and the company tried making a basic CNC EDM, but it never caught on. One company I worked for had two of these machines, and they just never really worked well.
The Japanese companies, Sodick, Makino, and Mitsubishi became leaders in the development of the CNC EDM. All 3 of these companies have very high quality machine tools with excellent reputations.
The Swiss are still huge players in this high precision machining arena: Charmilles and Agie are very popular all over the world. Both their wire cut EDM and sinker EDM machines have been in use for many, years and maintain high levels of accuracy.
There are numerous other excellent brands on the market as well, with major players coming from Asia, Germany, Taiwan and China. You get what you pay for, and most of these machines are capable of doing excellent work.
Any tool and die or mold making shop will necessarily have a lot of support for the EDM department. A well equipped EDM department will have a toolmakers microscope, optical comparator, and CMM (coordinate measuring machine) for inspection.
The granite surface plate is the foundation of any inspection area. There will also be a surface gage, sine plate, gage block set, pin gage set, dial indicator, various drop indicators, precision vise, tooling ball set, and various electrode holders.
It is tempting to just “burn to the numbers” and hope that everything will turn out perfect, but that is almost never the case. EDM is quite scientific, yet a high degree of skill and experience is required in order to achieve the desired dimensions and surface finish.
The use of roll dimensions is very common as an accurate method of inspecting shut-off surfaces and molding dimensions. Through the use of gage blocks, gage pins, sine plates, height gages and test indicators it is possible to attain accuracies as fine as .0002 in.
The sinker EDM requires some type of tooling to mount the graphite or copper electrode, so it can do it’s magical work on the injection mold or stamping/forming die. Years ago this was done with stick electrodes mounted in angle blocks, vises and homemade fixtures.
Today almost all shops use high end, highly engineered systems, such as System 3R and Erowa. By combining standardized tooling with robotics the CNC EDM can literally run for days, completely unattended. This is easier said than done, however.
Poco graphite is very popular as an EDM electrode material, but every year a new brand comes out to try and outperform this standby. Graphite has many advantages over it’s rival, tellurium copper, but in many cases the copper is the material of choice anyway. Occasionally tungsten carbide is used as well.
EDM Factory Automation Systems
Automatic tool changers have made life much easier in the EDM department. This allows the machines to run continuously, sometimes for weeks on end! Now, with a pallet changer system integrated, you can even change the workpiece without human intervention. This requires a great deal of up-front planning, but pays off in the long run.
Some companies have heavily invested in the cell manufacturing concept. This means that the electrodes are placed by a robot into the high speed machining center, retrieved by the robot, moved to the CMM for inspection, and eventually loaded into the EDM for burning.
The workpiece follows a similar path, both for EDMing and inspection. Palletized magnetic chucks are stored with the electrodes in an organized manner, awaiting the robotic arm. Once this system is put in place the results can be dramatic, but, once again, this is easier said than done.
In the USA, the skilled workforce is aging and very few companies are interested or able to train a new generation of EDM toolmakers. A mechanically inclined, diligent person can become an EDM operator in a few years, provided he has good training.
A skilled operator will always have a secure EDM job, if he can get the training in the first place! This is the problem: how can you get the job without any training, but how can you get any training without the job? It’s the old conundrum all over again. Some schools offer courses in machining, including EDM, but these are few and far between. Because of budget cuts and the offshoring of American manufacturing, many excellent programs have ceased to exist.
Manufacturing EDM Electrodes
As much as possible, the electrodes are made using high speed milling, with coated carbide cutters. The carbide tooling is usually coated with a special material to give it high wear properties, due to the fact that graphite is highly abrasive.
It is possible to use conventional vertical milling machines, if they are equipped with a high speed spindle, and many companies rely on the old standby machining centers to manufacture electrodes.
Haas, Makino, Sodick and DMG are all key players in high speed milling. It is advisable to stick with one company for the high speed machining center, EDM and WEDM, if possible. This makes integration much easier, rather than attempting to combine completely different technologies.
Injection Mold repair
In the event that something goes wrong in the EDM process, it is usually possible to repair the mold or die by using a laser or micro welder. This process has saved the day, many, many times! Both the laser and micro tig welders are essential for life in an EDM machining environment.
The EDM machine plays a major role in the maintenance of any type of tooling, such as injection molds, stamping dies or extrusion dies. All tooling eventually wears out, and often it is the EDM and welder who get the job done.