CNC Wire Cut EDM Machining

EDM (electrical discharge machining) is a non traditional metal machining process that uses spark erosion to remove material from the workpiece. An electrical discharge (spark) is created between an electrode which is usually a wire, and the piece to be machined. There is usually a low conductive liquid medium between the two, such as deionized water, which is continually flushed as the work progresses.

The EDM wire electrode is moved close the workpiece, which is then gradually eroded by the electrical discharge. As the workpiece is eroded the electrode is moved further into the piece, along a computer controlled path.

Typically a finished piece will go through several passes, the initial pass is relatively quick, and removes the most amount of material. Subsequent passes will retrace the cut path at a lower speed and removing less material but as a result improving the surface quality and bringing the workpiece to specification. CNC wire EDM is performed AFTER heat treatment so that the finished dimensions are not affected.

The cutting wire is able to be moved in several dimensions, and through large angles. This allows tapers to be cut into the workpiece; this is particularly useful for the production of extrusion molds. Cutouts and be created by threading the wire through a previously drilled hole.

In addition to metals, other conductive materials such as graphite, diamond and metal alloys can be machined using wire cut EDM.

Wire cut EDM Process

Wire cut EDM is a specific kind of EDM process and is one of the most common.
As the wire is brought close the workpiece the electrical field increases as the distance squared. Eventually, the dielectric material will reach a point where electrical breakdown occurs and sparks will fly across the gap between the wire and the workpiece, thus causing both to be eroded by the current. The wire is used as it can be continuously spooled so that the electrode does not become too eroded.

The wire is usually made of a single strand of brass, and is held at the top and bottom by diamond guides, the guides can move in the x-y plane and the upper guide can move independently in three axes. This allows for tapers and other complex, intricate and delicate shapes to be machined. The movement of the guides and the workpiece are controlled via the CNC computer. The typical wire diameter is 0.25mm in diameter and comes in spools that are about 10 miles long, the cut is larger than the diameter of the wire, since the wire does not actually touch the work piece and “cutting” occurs due to the spark from the wire to the workpiece. The “over cut” is predictable and can be programmed into the CNC work steps. If the smallest diameter wire is used then the cut can be within plus or minus a micrometer.

Modern Wire EDM machines often include multiple heads to cut more than one piece and automatic wire threading.

Applications for Wire Cut EDM

Wire cut electrical discharge machining (WEDM), is used to cut hardened metal plates as thick as 300 mm that are typically difficult to machine in any other way. It is also used when the final piece needs to have low stress, Wire EDM has low cutting force for material removal and little change in the mechanical properties is usually seen.
Wire EDM is used in the manufacture of extrusion dies, blanking punches and tool fabrication.
Wire EDM is best used for relatively low production volumes of items that require tight manufacturing tolerances. Among the industries that make use of components manufactured by EDM include the semiconductor industry, medical and instrumentation.

Some of the advantages of EDM include machining of:

• Complex shapes that would otherwise be difficult to produce with conventional cutting tools
• Extremely hard material to very close tolerances
• Very small work pieces where conventional cutting tools may damage the part from excess cutting tool pressure.
• There is no direct contact between tool and work piece. Therefore delicate sections and weak materials can be machined without any distortion.
• A good surface finish can be obtained.
• Very fine holes can be easily drilled.