BEYOND THE CNC: WHAT IS ELECTRICAL DISCHARGE MACHINING?

Many of today’s turbine blades, stents and optical mirror finishing parts are being made using EDM processes. Here’s what you need to know about zapping your materials free from their raw form.

A rising number of CNC turning and milling shops are using EDM to create part features that are otherwise impossible to machine. These components are primarily found in the aerospace and medical industries. Today’s automotive industry is generally not using EDM except in some tooling applications.

EDM does mean adoption of new machining practices. It’s not “traditional” CNC machinery. There are graphite and copper electrodes to understand and manage, a need for the proper work holding and application-specific software.

None of it is particularly difficult to do, but like any new technology, it requires some initial investment as well as an open mind. The good news for job shops is that EDM opens doors that might otherwise remain closed.

What Is Electrical Discharge Machining and How Does It Work?

Electrical discharge machining uses electrical energy to “erode” workpiece material. It generates a series of high-frequency sparks between an electrically conductive workpiece and an electrode made of copper, brass, graphite or tungsten, and alloys from each of them.

As the spark jumps from the electrode to the workpiece, it carries away microscopic bits of material that are then flushed away by a dielectric fluid. The fluid is generally deionized water or a specialized non-conductive oil. Extremely fine finishes are possible, as well as accuracies best measured in microns.

There are three distinct types of EDM: Wire EDM, also called “WEDM,” EDM hole drills, and conventional, or “sinker,” EDM. They all rely on the same principle of high-energy erosion and are all available with up to five axes of motion—but you can get more axes with a rotary attachment.

All three are able to machine everything from aluminum to carbide, provided the material is conductive. One key difference, however, is the electrode, which can be thought of as a cutting tool.

What Is Wire EDM?

WEDM uses a spool of copper or brass alloy wire that is continuously fed through the workpiece. Think of it as a highly accurate bandsaw.

Typical wire diameters range from .004 inches to .012 inches, although wires down to .0006 inches are available. WEDM is widely used in the tool and die industry, and for cutting small slots and similarly difficult features in medical components.

What Is EDM Hole Drilling?

EDM hole drilling, or “hole poppers,” relies on a tubular electrode with dielectric fluid flowing through its center and back out similar to a coolant-fed drill bit.EDM hole drills are the primary method for producing diffuser holes in jet engine blade and vane details.

What Is Sinker EDM?

The electrodes used with sinker EDM (typically graphite or copper) are often machined in a mirror or reverse image of the workpiece shape, although round, square and other shapes are also used. As with EDM hole drilling, the electrode can be made to rotate, orbit or plunge into and out of the workpiece, increasing metal removal rates while generating a variety of unique geometries.

When Is EDM Needed?

Where can you use EDM in your shop? As workpiece materials become harder to machine by conventional means, the EDM process becomes more attractive and productive. Examples include the increase in length-to-diameter ratios and anywhere extremely small cutting tools are needed.

EDM offers some advantages over milling and turning. Because EDM is a noncontact process, it’s possible to machine extremely small, thin-walled, honeycombed or tall workpieces without worry of deflection.

Aerospace manufacturers use EDM to produce the shaped holes in turbine blades. Medical companies use it to slice up stents, cut clips and machine the teeth on tiny alligator clips. Gear makers use it to produce one-off sprockets. Optical component manufacturers make mirror finishes with it.

What Kind of Tooling Do You Use with EDM?

Despite its extensive capabilities, EDM is no more difficult to operate than any other CNC machine. EDM machines have the same X, Y and Z axes. Many EDM systems have conversational controls similar to lathes and maching centers,  while others support the use of G-code, canned cycles and offline programming software.

The vises and clamps used to grip workpieces are also similar to what’s used everywhere else in the shop, but they are usually made of 400-series stainless steel rather than tool steel and cast iron.

EDM machining processes are significantly different, though, with emphasis placed on flushing, orbiting strategies, slug retention and the number of skim passes. But it’s nothing that any skilled machinist can’t wrap his or her brain around quickly.

 

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Post time: Jun-24-2020