EDM 101: What is EDM & When do you need it.

 

ã Mercatech, Inc., 2004

 

What is EDM?

EDM stands for electrical discharge machining, the applications best suited for this metal removal process are those characterized by extremely exacting tolerances and situations that would be extremely difficult or impossible to handle with any other method of machining.

An Overview Of EDM

The origin of electrical discharge machining goes back to 1770, when English scientist Joseph Priestly discovered the erosive effect of electrical discharges. In 1943, Soviet scientists B. Lazarenko and N. Lazarenko had the idea of exploiting the destructive effect of an electrical discharge and developing a controlled process for machining materials that are conductors of electricity.

With that idea, the EDM process was born. The Lazarenkos perfected the electrical discharge process, which consisted of a succession of discharges made to take place between two conductors separated from each other by a film of non-conducting liquid, called a dielectric. The Lazarenkos achieved a form of immortality with this circuit, which today bears their name. Today, many EDMs use an advanced version of the Lazarenko circuit.

 

How It Works

During the EDM process, a series of non-stationary, timed electrical pulses remove material from a workpiece. The electrode and the workpiece are held by the machine tool, which also contains the dielectric. A power supply controls the timing and intensity of the electrical charges and the movement of the electrode in relation to the workpiece.

At the spot where the electric field is strongest, a discharge is initiated. Under the effect of this field, electrons and positive free ions are accelerated to high velocities and rapidly form an ionized channel that conducts electricity. At this stage current can flow and the spark forms between the electrode and workpiece, causing a great number of collisions between the particles. During this process a bubble of gas develops and its pressure rises very steadily until a plasma zone is formed. The plasma zone quickly reaches very high temperatures, in the region of 8,000 to 12,000' Centigrade, due to the effect of the ever-increasing number of collisions. This causes instantaneous local melting of a certain amount of the material at the surface of the two conductors. When the current is cut off, the sudden reduction in temperature causes the bubble to implode, which projects the melted material away from the workpiece, leaving a tiny crater. The eroded material then resolidifies in the dielectric in the form of small spheres and is removed by the dielectric. All this without the electrode ever touching the workpiece! Making EDM a no-contact machining process allowing you to achieve tighter tolerances and better finishes in a wide range of materials that are otherwise difficult or impossible to machine with traditional processes.

 

 

Growth of EDM

EDM has rapidly earned its place alongside milling and grinding equipment as a proactive, mainstream technology. EDM is best known for its ability to machine complex shapes in very hard metals. The most common use of EDM is machining dies, tools and molds made of hardened steel, tungsten carbide, high-speed steel and other workpiece materials that are difficult to machine by "traditional" methods.

The process has also solved a number of problems related to the machining of "exotic" materials such as Hastelloy, Nitralloy, Waspaloy and Nimonic, which are used on a large scale in the aeronautical and aerospace industries.

With the reduction in electrode wear and increased sophistication of EDM controls in rams, new EDM processes use simple-shaped electrodes to 3D mill complex shapes. EDM also is being used for polishing small, intricate surfaces.

Since EDM does not involve workpiece/tool forces like a mill or grinder, it is possible to EDM shapes that would break conventional cutting tools or be broken by them.

Different Types of EDM

RAM EDM

RAM EDM, also known as plunge EDM or standard EDM, is the oldest form of EDM machining. It generally consists of an electrode usually made out of graphite that is plunged into a workpiece in order to create a blind-shaped cavity. It can also be used to generate through holes and geometry but these are not the primary uses of the process.

 

Drill EDM

Drill EDM uses rotating concentric electrodes to drill through a workpiece and basically performs the same functions as a drill press, except that the material hardness is not a factor and the accuracy of the finished hole is far superior than what any drill press can produce. It is best used to drill start holes for the wire EDM in already hardened material as well as accurate very small holes for industries like aerospace and medical equipment. For reference, our electrode diameters range from .006” to .250”.

Wire EDM

Wire EDM uses a traveling wire electrode (usually .010” diameter or smaller) that goes through the workpiece. The wire, in this case, is controlled by computer following the assigned geometry for the part to be produced. At Mercatech, Inc., with the help of our fully programmable 4 axis EDM equipment, we can produce larger components faster and with greater precision, as well as a higher degree of economy and flexibility for all your EDM machining needs. Our non-traditional integrated approach, as well as technological advantages, can provide you with the best and most profitable solutions. By helping you make the right decisions up front... before machining... your work will be done faster and more cost-effectively than you ever thought possible.

 

                            

When Should You EDM?

This question can be answered either by looking at geometries produced or materials machined. Hereafter, are two summary tables to help you determine the benefits of EDM for your applications.

Table I - When To EDM By Geometry

When

Why

Very thin walls

No contact, no force, no deformation

Internal radii equal to or less than 1/32 inch parallel to tool axis

Radius is as small as the spark gap. Generally, tool is not rotated.

High ratios of cavity depth to width, for example, slots and ribs

No force means very think, long elecrodes can be used.

Non-round cavitites/openings

Electrodes don't have to rotate.

Intermittent cuts

No contact, no force

Very small parts
(fit in a 0.25-inch cube)

Easy to fixture since no force or vibration is involved

Recessed cuts

Cutting tools couldn't reach cutting area or generate desired shape.

Requires special/unique cutting tools

Electrodes often less costly than special cutting tools. Electrode is easy to machine, unlike carbide. Wire is available standard.

Accuracies that are difficult to hold, maintain after heat treating (stress relieving, and so on)

Can EDM conductive materials of any hardness

Different geometry at top and bottom

Wire EDM cuts ruled surfaces with a simpler program and machine than milling.

Complex shapes

Easier to program because you are using a tool of constant dimension instead of a variety of different diameter milling cutters.

Requires multiple component assemblies

Use taper or recess or depth: diameter capability to make it one piece.

Angled cuts

Ability to 3D orbit in space.

 

Table II - When To EDM By Material

When

Why

Hardness above Rc 38:hardened steel, Stellite, tungsten carbide

EDM vaporizes material rather than cutting it.

Toughness: Inconel, Monel, Hastelloy, Nitralloy, Waspaloy, Nimoric, Udimet

EDM is non-contact, therefore no adhesion of workpiece to tool.

Tends to leave tough burrs when machined conventionally

Vaporized material is flushed away leaving no burr.

Frail/fragile (can't take stress of machining)

No contact, no force

Expensive material

Lower chip/workpiece mass ratio. Slugs from wire EDM may be reusable whereas chips from conventional machining are recyclable at best.

Certain explosive or flammable materials

EDM takes place under water.

Material with hazardous dust particles

Particles are flushed away to the filter. Reduced risk of fumes.

Note: Workpiece material must be electrically conductive or semi-conductive with no non-conductive cutting zones.

Tables by Charmilles Technologies

 

EDM for tooling applications

When a part requires special/unique conventional cutting tools. Electrodes are easy to machine, unlike carbide. Equally important, the wire used by a wire EDM is available as a standard, off-the-shelf component. EDM is a low cost tooling option when you need short run stamping (under 5,000 pieces) and low volume broaching. With EDM, there's no need to make a die set. That's why EDM is used to make sewing machine components and prototypes. Instead of using expensive broaches, EDM is a very attractive form of low-cost tooling. This is a reason companies use EDM to produce splines and gear teeth along with all their metal stamping & mold making needs.

 

Limitations Of EDM

Clearly, the benefits of EDM are considerable, and it is often appropriate to EDM instead of using conventional manufacturing processes. But not always. What are some of the restrictions of EDM?

Sample Applications

Tooling: Speaker Die for Car Stereo System

Machining: Side Burn in .025” wall thickness

 

Finally….

If in doubt about any aspect of the EDM process or workpiece preparation, give us a call!

Our many years of experience in the field will be used to answer even your toughest questions.

 

For all your EDM need and questions. Call us at  972-247-1821