MAY 2018

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Page 6 of 23 | The Machine Tool Search Engine 5 must be selected. The wire must have the right diameter and tensile strength. The power setting and tension of the wire must also be right. The condition of the deionized water and flushing arrangements must be optimized, as well. Machine Accuracy When attempting to hold ±0.0001-inch positional accuracy with wire EDM, the shop environment becomes a factor. For example, both steel and carbide have a thermal expansion coefficient of ~6.8 ppm per degree Fahrenheit. This means that, for every 2°F change in shop temperature, a 12-inch part could grow as much as 0.00016 inch, putting the operation over the 0.0001-inch tolerance it is trying to hold. To be successful under these conditions, a shop must be able to hold its ambient temperature within 1°F in either direction during an eight-hour period. Controlling the temperature of the dielectric solution to ±1°F also helps control the temperature of the machine and the workpiece. The two most common machine designs use either ballscrews or linear motion systems. In terms of machine accuracy, each design has pluses and minuses, which must be explored when choosing a wire EDM unit. High-precision glass scales are used to negate the effects of pitch error or backlash on the linear feedback. On the best machines, high- resolution servodrives with fine increments are used to position the wire, thus improving surface finish and accuracy. Adaptive controls can compensate for thermal growth. High- speed circuitry in servomotors enables them to react instantaneously for finer control of the spark. High-peak power supplies can now put more electrical energy into the wire, greatly enhancing productivity. Dielectric Fluid and Flushing The dielectric fluid, or coolant, used in the wire EDM process, is deionized water. It serves several purposes. First, it acts as a semiconductor between the energized wire and the workpiece to maintain stable and controlled conditions for ionization in the spark gap. Second, it can be chilled to keep the wire, workpiece, worktable and fixtures at a steady temperature. This limits thermal growth of the workpiece and machine in order to hold tight tolerances. Third, it acts as a flushing agent to wash away the ashy debris created as cutting occurs. When a machine is commissioned, distilled or deionized water with low conductivity is used first. Subsequently, tap water can be used if first passed through a deionizing resin bottle to filter out any contaminants and neutralize particles with an electrical charge. The water then circulates through a 3- or 5-micron paper filter to remove any remaining particles. Most machines are equipped with 5-micron filters. Because EDM creates microscopically small particles during the cutting process, removing these "chips" becomes a key factor in maximizing cutting speed as well as attaining part accuracy and surface finish. To be an effective flushing agent, the dielectric fluid must flow freely into the zone where the cutting action occurs. Because each spark melts away a microscopic bit of the workpiece, the fluid helps solidify the molten particle and keep it from adhering to the wire or the workpiece surface. Adjustable flushing nozzles, which are located close to the top and bottom of the workpiece, direct a stream of fluid into the cutting zone from opposite directions. When the flushing pressure and nozzles are set properly, the two streams meet in the middle, creating a "rooster tail" effect that is visible on the slug removed after the roughing pass. If debris is coming mostly out of the bottom of the part when cutting a solid, flushing may be unbalanced. This creates poor cutting

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