Techspex

MAY 2018

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T H E R E M A R K A B L E A B I L I T I E S O F W I R E E D M 2 How Wire EDM Works A typical system consists of a CNC unit, a power supply with anti-electrolysis circuitry, a mechanism for automatic wire threading, a tank to contain the dielectric fluid, a chiller to keep the fluid at a steady temperature and a filtration system to keep the dielectric clean. Wire EDM uses an electrically charged strand of wire fed from a reel and moved through upper and lower guides. The wire is energized as it passes across a power contact at the top and bottom. The distance between the guides can be adjusted to accommodate the thickness of the workpiece. A tightly controlled sequence of electrical discharges between the wire and the workpiece creates hundreds of thousands of sparks per second to remove the workpiece material. The heat of each electrical spark, estimated at around 15,000 to 21,000°F, melts away a microscopic bit of the workpiece material. Although the volume of metal removed during this short period of intense heat, lasting 1 millionth of a second or less, it is quite small, the frequency and multitude of sparks is an effective method to cut a narrow slot through very hard materials. The action of the spark may also remove material from the wire at the same time. Submerging the wire and part in deionized water allows cutting debris to be flushed away. Splash flushing can be used when the part cannot be fully submerged but a top and bottom nozzle must be constantly directed at the wire to help wash away debris. When machining submerged, cutting occurs in a more easily controlled environment and the flushing nozzles need to be adjusted only once. A CNC unit moves the machine in an X-Y plane and can position the upper guide independently in the U-V axis, giving the machine the ability to move all four axes (X, Y, U, V) simultaneously to cut tapers or shapes that transition from top to bottom. A programmable Z axis enables workpieces with different thickness to be machined automatically. Types and Sizes of EDM Wire The wire used in the EDM process may consist of brass, zinc-coated brass, brass-coated copper, tungsten, molybdenum or brass with a steel core, to name a few. Each type has its own purpose, benefit and cost. EDM wire ranges in diameter from 0.0008 to 0.013 inch. A wire with a diameter of 0.0008 inch will produce a 0.00044-inch radius in a corner, a feature nearly impossible to manufacture any other way. Such precision may be required for openings in dies to produce electrical components, for example. However, wire this fine is rarely used, because of the unusual challenges it creates. Typically, the smaller the wire, the lower the power settings and the slower the cut. Most shops do not want to pay for this lengthy machine time unless the application absolutely requires it. Plain brass wire 0.010 inch in diameter is used in more than 80 percent of EDM work. Three types of brass wire exist: hard, half-hard and soft. Soft brass is typically used for cutting tapers because a low tensile strength is needed. This type can bend while cutting at an angle without breaking. Hard brass is best for both roughing and skimming when higher tensile strength is desired. Hard brass withstands aggressive flushing and enables a high voltage to be applied to the wire without breaking. A faster cutting rate is the result. Brass wire is available with a zinc coating. Because zinc melts at a lower temperature than brass, the zinc absorbs heat as it boils away. Less heat enters the wire, so it retains its strength. Whereas brass wire is very smooth on the outside, zinc-coated brass has a rougher outside finish. This rougher finish

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