EDM Machining Process
What is the EDM Process?
Any time an electrical arc jumps a gap to metal, some erosion occurs. Controlled EDM as a machining process though has only been around for about 70 years.
EDM stands for Electrical Discharge Machining. It is a precision process of creating high voltage sparks to vaporize metal.
In this process, a metal workpiece that is to be machined is clamped into machining bed. This material must be conductive and electrically grounded to complete the electrical circuit. An electrode is brought into very near proximity with the workpiece, though not actually in contact. Then a high electrical voltage is pulsed through the electrode. A spark jumps the small gap which essentially vaporizes a small bit of the metal workpiece at that point. This is all done within a bath of dielectric fluid.
A “dielectric” is a material that does not directly conduct electrons through itself, but allows and perhaps even facilitates an electric field through which electrons can jump. It is not electrically conductive, though it does not inhibit the flow of electrons either.
- An ideal insulator will not conduct any electrons and will block an electrostatic field.
- An ideal conductor will conduct electrons within itself without any resistance and will support an electrostatic field.
- An ideal dielectric will not directly conduct any electrons, though will support an electrostatic field encouraging electrons to pass through it without any resistance.
As an analogy, think of humans moving from place to place as electrons. If people are electrons, then vehicles could be thought of as conductors. People want to get from point A to point B and vehicles will carry them there. A motorcycle would conduct people from place to place, but it wouldn’t be very good at it since it can only take one or two people. A coach bus would be a better conductor since it could carry more people. A train even better as there are no rush hour congestion delays or stoplights slowing things down. In this scenario, roads could be thought of as dielectrics. The roads themselves don’t carry passengers from place to place, though cars can drive along them and pedestrians can walk along the sidewalks. Intersections with crosswalks that occasionally stop people from time to time interrupt the flow and would not be a very good dielectric. A more ideal dielectric in this case might be a dedicated jogging path with bridges and tunnels so that stopping is not necessary. The best one takes people directly on the shortest straight path to their destination.
With an EDM process, the dielectric fluid encourages a strong localized electrostatic field forming the shortest path between the electrode and the workpiece. In this way, the spark jumps the gap in the most direct and predictable way. Sparks aren’t randomly jumping anywhere between the electrode and workpiece. It’s focused where the gap is smallest. This moving fluid is also a great way to flush away the eroded particles of material from the work piece and disperse the heat generated by each spark. All these factors aid in providing the high precision possible with EDM.
Common Applications for EDM
Given the EDM’s complete disregard for the hardness of a metal, it’s most commonly used for creating hardened alloy tooling needed for other manufacturing processes – alloys that are very difficult to machine with traditional cutting and milling methods. The vast majority of injection mold tooling, extrusion and stamping dies need these alloys to handle the high forces in their processes and are created using EDM. Also, since the process does not use any mechanical force whatsoever, EDM can drill holes on an angle through curved surfaces without any deviation. In some cases, electrodes can be rotated or moved along curved paths through workpieces in ways not possible with other processes like CNC Machining.
Types of EDM Machining
Time to call a spade a spade… “EDM Machining” is really like saying “electrical discharge machining machining”. It’s redundant and grammatically wrong. The same as “LCD display” or “ATM machine”. The thing is, it also rolls off the tongue nicely and the world is already riddled with the phrase “EDM Machining” all over the place. Sorry Mrs. Heinrich – totally cool with the rest of your grade school grammar lessons though.
Wire EDM uses a straight tensioned wire as the electrode, which is slowly drawn through the workpiece to cut a ribbon-like path. The cut width (kerf) is very narrow; only a slight bit wider than the wire electrode itself. It offers the finest kerf process available for cutting metals. It’s quite like a cheese cutter.
This version of EDM is the one commonly used for extrusion dies. It is a great option for cutting profiles through hard alloys and can readily cut a varying draft angle in a single pass.
Small Hole EDM
Where wire EDM uses the side of a narrow wire drawn through a block of metal, Small Hole (Drilling) EDM plunges the end of a narrow wire into the workpiece. Since EDM does not use mechanical force, a thin, fragile electrode can be moved into a hard alloy without concern for it bending or breaking.
Traditional drilling operations struggle to pierce a hole through hard surfaces at an angle. Drill bits work well when they are perpendicular to the surface and the sharp point at the tip can bore into the material. The contact surfaces and forces at the tip are irregular when the angle is reduced though. Drill bits bow to one side and tend to walk down the surface, so the positioning is not reliable. This also causes a high amount of fatigue stress and usually breaks small drill bits. Small hole EDM has none of these issues.
Small Hole EDM can be used to put a deep hole at a very shallow angle through a hardened ball bearing with the ball simply sitting on a stand to prevent it from rolling away. It almost doesn’t need to be clamped in place.
The electrode isn’t exactly a wire – the point at the end of a two dimensional line. It’s more like an injection needle; a small hollow tube through which dielectric fluid is pumped. The fluid helps to cool and irrigate the hole and carry away any particles. The tube is also continually and slowly rotated to further aid in passing debris away from the tip.
As you might imagine, Small Hole EDM is usually the best way to pierce through the middle of a block of hard metal before Wire EDM can take over and work sideways.
Ram (or Sinker) EDM is arguably the most impressive application of this technology. Wire and Small Hole EDM processes remove material along a simple line or point. Ram EDM removes material with a complex three dimensional shape. The electrode is a negative image of the finished part. It is slowly moved into the workpiece to erode any material that it (almost) touches until it reaches its full depth.
Ram EDM electrodes need to be made for each new part design, while wire and small hole EDM processes reuse generic electrodes for a variety of tasks. Ram EDM electrodes are usually made by multi-axis CNC milling.
Why would you use CNC milling to create a part that just makes another part with the same shape!?
While this may seem counterintuitive, remember that EDM does not use mechanical force. An electrode can be made by CNC from an easily machinable material like brass, and then be used in EDM to make things of hard metals very difficult to machine like tungsten carbide or Inconel.
Development EDM Process
One doesn’t usually set out to design parts specifically with EDM in mind. We already mentioned EDM’s very common application for tooling for injection molding and the like. Injection mold tooling only works if you can get the plastic part out of the tool when it’s finished. Thankfully, the design restrictions for injection molding are even more strict than for EDM to create the tool. An EDM electrode can move in and out of a mold tool block with absolutely no draft. Straight perpendicular walls are not a problem, because the electrode technically doesn’t touch them. The vaporizing sparks jump the gap, so a little buffer of space the length of that jump is always maintained.
The same goes for extrusion tooling. The natural requirements for material to flow and extrude properly are conducive to EDM. Undercuts and blind internal cavities are undesirable features, so tooling designers avoid them. This means that EDM works well.
Perhaps a unique variation in Ram EDM is where the electrode is rotated or moved in a horizontal plane in combination with the vertical movement into the workpiece. Rotating can produce flutes, screw threads, rifled grooves, or vanes of an impeller. These features are not typically compatible with molding processes. EDM applied in this manner can create parts of shapes and materials not possible by other methods. Even though some casting methods can yield the shape, they cannot produce some of the alloys and tempering that EDM can work with.
EDM is almost more about electrode design than part design. As long as the workpiece material is electrically conductive, EDM can handle it. The shape, surface finish, speed of machining and quantity of parts made is all about the electrode material selection and design.
The most common electrode materials used in EDM are copper, brass, graphite, and copper tungsten.
Copper is likely first to come to mind when you think of conductive metal. The overwhelming majority of all electrical wires on the planet are made of copper. It is a great conductor and is fairly easy to machine with a great surface finish owing to it’s lack of porosity. Brass is an alloy of copper and tin. It’s easier to machine and retains a lot of the same properties.
Graphite may be a surprising one you had not thought of. It is another great conductor that is even easier to machine than copper. One of the greatest advantages of graphite is in tool wear. The EDM process vaporizes the metal workpiece at small localized areas. Though the full part does not heat up significantly, the particles coming off the workpiece can be in excess of 2,700ºF. That’s about 700ºF above the melting point of copper. These hot particles and gasses can erode the copper electrode, but don’t get anywhere near a damaging temperature for graphite.
In fact, the particles can tend to lodge into the tiny pores of a graphite electrode and effectively plate it rather than erode it away. This gives graphite a solid edge over copper. There are many different grades of graphite and they are usually more costly than copper in raw materials, but the longer life of the electrode and easier/shorter machining time to create it give graphite electrodes an offsetting advantage.
If the goal is to produce an injection mold tool, electrode life isn’t a big factor. It’s one and done. You make one tool. That one tool makes a bajillion parts. If the EDM process is to result in individually useful parts though, you don’t want to have to remake electrodes all the time. Graphite is a better material than copper in these situations. Better yet though, is copper tungsten.
Tungsten alloys are very hard and tungsten has a high melting temperature of 6,170ºF – right up there with graphite. Tungsten and copper together provide a great blend of conductivity and wear resistance. Due to tungsten’s hardness and high melting point, it is normally worked into electrodes by metal sintering. For pre-formed generic shapes like the ones used for small hole drilling and standardized slotting, this works well. For high volume production parts of the Ram EDM type, it usually makes more sense to use graphite. Otherwise, you’re using CNC machining for a mold, then powdered metal sintering to make the electrode, to then use in an EDM process. The chain gets a bit long.
A wide variety of machine designs and manufacturers exist for EDM equipment, though here are a few basic diagrams to point out the important functional blocks.
One thing not yet noted is the need for auxiliary flushing jets of dielectric fluid. These are needed mainly for removal of debris, but also to aid in cooling. You will always find these jets with Wire EDM and commonly with Ram EDM.
The goal for prototyping is to get a sample part in as close to the final material and specifications as possible at a low cost and fast. EDM doesn’t generally fit with this description. It is a relatively slow process that you wouldn’t normally use if you weren’t already planning for EDM to be the production method. At that point, it wouldn’t really be considered a prototype method anyway.
One notable exception though is aluminum extrusions. The time and expense for creating an aluminum extrusion tool can be significant – especially for larger dies. Rather than investing in and waiting for extrusion dies to be made before an extrusion design can be tested, Wire EDM can make a sample section of an extrusion about a foot long or more without special tooling. The dimensions will be even more accurate than the extrusion process, though the surface finish will be rougher. Parts can be sanded and coated if necessary, then fit-checked to mating assemblies.
Hopefully with this high level summary, you’ll have an idea of what EDM is, how it works, and how it can be best utilized. More applications for EDM tend to fit into the production phase of a project. Perhaps you were surprised to learn that EDM has a hand in the vast majority of plastic injection molding. Perhaps you’re able to take advantage of EDM as a way to prototype your new aluminum extrusion design. Perhaps you’ve needed to drill a 1/16” hole through a ball bearing. Whatever the need, watch for technology expansions in EDM to improve speed, reduce costs, and expand in capabilities further. As a machining process, it’s a relatively young one with a lot of room to grow.
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