EDM cuts cleaner, smoother

Coordinates for the shape to the cut are entered on a keypad of the EDM (in this case a cutting tool as shown on the monitor). The actual cutting is done by an electric arc in the water-tight chamber on the left.

The Charmilles Technologies Robofil 290 Wire Electrical Discharge Machine, or EDM for short, uses an electrical spark delivered by a fine wire to cut through any material that conducts electricity. Mike Harper, design engineer and the technical support for the EDM, says that this type of cutting procedure works cleaner and smoother than any conventional rotary cutting tool.

Looking at the electrical discharge machine in action, one might think that the material is being cut by the wire in somewhat the same way a band saw operates. However, the 0.01-inch diameter wire is actually one of two electrodes, the second being the soon-to-be-cut material, itself. The EDM can carve anything that conducts electric current, including hardened steel and silicon carbide, a ceramic. A spool of half-hard brass wire feeds continuously downward, maintaining a spark gap as small as 0.0002 of an inch, depending on the type and thickness of the material being cut.

Cutting is achieved by spark implosion. As the spark discharges, the workpiece melts locally and disintegrates, throwing off small particles. This waste material is then flushed away by a powerful jet of deionized water, and the wire advances to repeat the procedure until the piece is sliced to the desired shape.

Virtually any shape and dimension can be programmed into the computer that guides the wire. For instance, Harper and Steve Lee, senior lead machinist, fabricated an artificial elbow joint for a dog to specifications from the ISU College of Veterinary Medicine. The largest workpiece the machine can accommodate in centimeters is 80 cm long, 50 cm wide and 25 cm high, or roughly the size of a half-filled duffel bag.

Since the whole process is computer-controlled, it is highly accurate and allows for creation of precise molds and dies.

"The capabilities of the EDM are amazing," Harper sums up, after expounding the machine's abilities. "You can actually cut an hourglass shape through a part," by feeding the wire through a starter hole, then rotating it in an offset from the center point. The five axes of translation allow the upper head to offset the lower head, enabling an angle cut, such as a taper or an hourglass.

by Courty Arney, student intern