This method is also a very precise method, but it is much more suitable for mass production. Like the EDM process, the ECM process can also be used on hard materials that cannot be machined by other more mechanical processes. Unlike the EDM process, no sparks are generated between the cathode and anode. The best way to understand ECM is to think of it as reverse-electroplating (i.e.) instead of adding material, we remove it.
Since 1993, Smith & Wesson has been using ECM to manufacture most of their revolver barrels. They use machines manufactured by Surftran to do their work. The barrels are hardened and annealed before the rifling process. The hardened barrels are then placed in the ECM machine and held stationary. The electrode is a plastic cylinder with metal strips circling around the exterior. The metal strips are a reverse image of the desired rifling and are inset into the plastic cylinder. This way, only the plastic part of the cylinder touches the barrel and not the metal strips. The electrode is placed inside the barrel and the whole is immersed into an electrolytic solution of sodium nitrate which is constantly circulating under pressure. The electrode is moved down the barrel and rotated at the desired rate of rifling twist. As current flows from the cathode (the electrode) to the anode (the barrel), the material is removed from the anode to duplicate the grooves in the shape of the electrode. Because the metal parts of the electrode never actually touch the barrel (only the plastic core does) and because the flowing electrolyte removes any material from the barrel before it has a chance to accumulate on the metal strips, the electrode usually lasts a very long time and needs no cleaning or maintenance. In fact, the electrode is replaced only when the plastic core which contacts the barrel to provide proper centering and spacing of the metal strips, wears out.
The advantages of this are that the process is extremely precise and can be used to machine hard materials like hard steel alloys, titanium alloys etc. Similar to the EDM process, it also produces no heat or stress on the barrel during the rifling process and also produces an excellent finish. Unlike the EDM process though, it is much faster to machine parts using this technique. A typical rifling job for a 357 magnum revolver barrel can be done in about one minute using this process, making it ideal for mass production. The tool can also be repeatedly used as there is very little tool wear.
The disadvantages are that these machines have high tooling cost and also use large amounts of electricity.
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