MAY 2018

Techspex provides metalworkers free research and analysis tools to help them find the right machine for their job.

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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 4 stream of water guides the wire through the workpiece. Automatic threading enhances unattended operation by making it possible to produce multiple openings or features within a part in a single setup. Cutting and rethreading of the wire are controlled by codes in the program. If the wire breaks accidentally, the machine senses the break, rethreads the wire and resumes cutting where the wire broke. If wire smaller than 0.004 inch is required, a fine wire kit is available as an option. Cutting Speed, Accuracy and Surface Finish The two things every wire EDM user wants are speed and accuracy. Unfortunately, these objectives are usually incompatible. You don't get speed with precision and you can't achieve high accuracy without also achieving a fine surface finish. Accuracy and surface finish go together. Speed and accuracy do not. EDM units from the early 1980s might achieve cutting speeds of 3 to 4 square inches per hour. With changes in machine design and power supplies, speeds of 17 square inches per hour became attainable in the 1990s. Today, with improved power supplies, working in conjunction with sophisticated adaptive controls, it is not uncommon to achieve 24, 37 and in some cases 45 square inches per hour. The type of material and the height of the part being cut are critical as well. It is generally easier and faster to cut hardened tool steel than cold-rolled steel, for example. The harder the material the better. Typically, tool steels, carbide and special alloys have fewer impurities and lower porosity, making them easier to cut. Cold-rolled steel may contain impurities, so wire cutting is slower and the surface finish is poorer. Although aluminum is easy to cut at higher speeds, the material is so soft that it is very difficult to get a good surface finish. Even a 30-microinch surface finish is difficult to achieve in aluminum. In contrast, it is possible to cut a three-inch thick carbide workpiece, with accuracies of ±0.0001 inch, and still produce a of 5-microinch Ra surface finish. A typical wire EDM process consists of several passes, traveling at varying speeds. The first pass is generally a roughing pass designed to cut as quickly as possible, while accuracy and surface finish are less of a concern. Each subsequent skim cut travels at progressively faster speeds, takes less and less material while steadily improving dimensional accuracy and quality of the surface finish. During the finish cuts, the tension on the wire is increased, the current is reduced and the voltage gap narrowed, allowing the user to refine the spark and the distance the spark jumps from the wire to the part. The offset applied to the last finish pass might be as small as 3 microns. To achieve a 4- or 5-microinch Ra finish, as many as six or seven skim cuts might be necessary. Whereas the diameter of a cutting tool determines the offset in milling, the EDM controller applies a cutter comp based on the diameter of the wire. For example, if a 0.010-inch diameter brass wire is used, the cutter comp will approach 0.005 inch plus a spark gap as the wire gets closer and closer to the part surface, and possibly finish at 0.0051 inch. To achieve these close tolerances and super- fine surface finishes, every parameter must be properly set. The right type of EDM wire EDMs from the 1980s might achieve cutting speeds of 3 to 4 square inches per hour. Today, it is not uncommon to achieve 24, 37 and in some cases 45 square inches per hour.

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