Techspex

MAY 2018

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3 techspex.com | The Machine Tool Search Engine techspex.com | The Machine Tool Search Engine improves flushing, resulting in increased- speed. Typically, coated wire delivers a 10 to 15-percent improvement in speed. Wire with a copper core and a defused zinc outer layer is often referred to as high-speed wire, or stratified wire. The high electrical conductivity of copper enables doubling the cut speed. However, the price of the wire can be double that of other types. Stratified wire is recommended for roughing cuts. Plain copper wire is now rarely used because it is too soft and is too expensive. No matter which type of wire is selected, it can only be used once. All wire is degraded by the wire EDM process and can be sold for scrap afterward. Wire Tensile Strength Specifying the tensile strength of EDM wire is also important. Even though the wire does not touch the part during cutting, it is stretched by the machine's wire drive feed mechanism, which consists of wire tensioner, roller guides, and upper and lower feed contacts (where the electric current is applied). Tension is preloaded onto the wire and can then be increased or decreased to accomplish different cutting techniques. Tensile strength determines the ability of the wire to withstand the tension imposed during cutting. The lower the tensile strength, the easier it is to break. Wire with less tensile strength will cut angles without breaking. By displacing or offsetting the wire guides in the UV axis, large tapers and angle cuts of up to 45 degrees can be generated. Wire with more tensile strength can be stretched more tightly when using the UV axis to closely control the perpendicularity of vertical walls for precise cutting. For roughing, lower wire tension enables the machine to cut faster without breaking the wire. Skim cuts require higher tension, so slower speeds and less power are applied to achieve maximum surface finish and precision. EDM wire is sold by the pound and typically comes in an 8-pound spool. Under normal cutting conditions, one spool will last eight to 10 hours of uninterrupted cutting. Most EDM manufacturers offer a large-spool option which will hold a spool of wire weighing as many as 35 or 50 pounds, making it possible to cut more than 40 hours without interruption. Provided the EDM unit has an auto rethreading system, the machine can operate for a week or more with little attention. However, a robot or automatic workpiece loading device is needed to support such long run times. Automatic Rethreading All wire EDM units now come with an automatic wire rethreading system. This feature works exceptionally well with wire as small as 0.002 inch in diameter, but the larger the wire diameter, the better. A high-pressure

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