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Posted on Sep 15, 2020
CNC wire cutting or electrical discharge electrical discharge machining (EDM) is a metalworking process in which a tool projects thousands of sparks onto a metal object. An unconventional process, although not new, the wire EDM machine operates on parts resistant to conventional machining processes, but only if these parts are electrically conductive; they are usually non-ferrous and contain steel, titanium, super alloys, brass and many other metals.
Instead of cutting the material, the EDM melts it or vaporizes it, producing relatively small chips and providing a very accurate cut line. Industry acceptance has led to a wide variety of EDM applications as it is very versatile, can cut hard metals, and uses a relatively small working space.
There are two main types of EDM machines: conventional or sinker, and wire or cut. Traditional EDM, as described above, uses the tool to disperse the electric current. This tool, the cathode, runs along the piece of metal, the anode, and the electric current reacts to melt or vaporize the metal. As a result of the action of a dielectric fluid, usually a hydrocarbon oil, in which both the cathode and the workpiece are immersed, the fine chips produced in the process are washed away from the component.
Wire-cut EDM (or WCEDM) carries away the electrified current by means of a tensioned thin wire that acts as a cathode and is guided along the desired cut path or gap. The dielectric fluid in this case - usually deionized water - is passed through the incision during its duration, again serving to drive away particles and control sparks. The thin wire allows precise cuts, with narrow gaps (routinely ~ 0.015 inch, with finer cuts available) and possible tolerances of +/- 0.0001 inch. This enhanced precision enables complex three-dimensional cuts to be made and produces highly accurate punches, dies and stripper plates.
Wire cutting EDM equipment is computer controlled (CNC) instruments, which can control the wire on a three-dimensional axis to provide greater flexibility. Straight cuts are made by changing the x-y coordinates of the knife, and more complex cuts are obtained by adding an axis of motion to the wire guides. Both four and five axis wire EDM machines and services are available. While conventional EDM cannot always produce tight corners or very complex patterns, the increased precision of the wire EDM allows for complex patterns and cuts. In addition, the wire EDM machine is capable of routinely cutting metals up to 0.004 inch thick and over 16 inch thicker, with thicker sections possible.
An ED wire at a certain material thickness will simply evaporate the metal, thus eliminating any potential contamination. The WCEDM wire emits sparks from all sides, which means that the cut must be thicker than the wire itself. In other words, since the wire is surrounded by a current ring, the smallest and most accurate possible cutting path is the extra diameter of the ring and wire; technicians can easily accommodate this extra dimension. Manufacturers continue to produce thinner and thinner wire to allow for smaller kerfs and even finer precision.
Due to its versatility, manufacturers use EDM machines for a wide range of applications. As this process can cut very small parts, it is often the ideal choice for producing small, very detailed parts that would normally be too delicate for other machining options. In addition, the process is cost effective for low volume designs and can prove beneficial for prototyping even if the actual design is done differently.
Please note that the wire in the process is constantly moving and cannot be reused. As a result, copper, brass, or other metal wire can be miles long, adding to the cost of the process. And while the process does not use force and therefore does not burr and can be used on delicate items, there is certainly the possibility of thermal stress.
Most wire EDM operations start with a rough cut with a fairly high feed rate and high dielectric flow. Subsequent degreasing passes require smaller cuts with reduced dielectric flow to achieve tolerance of the finished surfaces. The reduced dielectric flow prevents distortion of the wire during these transitions.
Cuts that do not start along the edge of the part (such as holes) require pre-drilling to allow the wire to pass through. Many wire EDM machines have small hole drilling electrodes for this purpose, which allows starting holes to be made in hardened steel without the use of conventional drill bits. Small hole drills typically use an EDM electrode mounted in a rotating mandrel with a dielectric pumped through the electrode to flush the hole. The process of making small holes in a hardened material using standalone EDM drills is sometimes called hole pop-up.
:: Read more : Introduction to wire cutting
The main advantage of a wire EDM machine over a conventional EDM machine is that the continuous wire feed minimizes the wear that occurs with a stationary electrode. However, a wire EDM machine can use a lot of wire, which increases costs. Many wire EDM machines are self-threaded, so if the wire breaks, the process can continue almost continuously.
Another important advantage is the ability to cut parts after heat treatment, which eliminates the possibility of deformation resulting from post-treatment. In addition, since wire EDM does not apply pressure to the tool on the workpiece, small, delicate parts are easily machined. Fixing requirements are minimal compared to conventional machining methods. Very fine surfaces are possible.
Compared to many machining methods, EDM is a slow process, although speeds have increased over the years due to technological advances. Productivity can be increased by stacking identical parts and cutting them all at once. Multi-head machines are available to cut multiple identical parts simultaneously. Because wire EDM is a thermal process, some stresses can be imparted to the workpiece.
Although relatively straightforward to perform, maintenance of wire EDM machines can be more demanding than that required for conventional machine tools. Consumables can be more costly, too. These costs can sometimes be softened by the fact that almost all wire EDM machines are designed to operate unattended, save for loading and unloading. The absence of burrs on the finished parts can also save by labor or time by eliminating the deburring step.
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