Advances in control, sensor and automation technologies have made electrochemical milling more precise and efficient than ever before, opening up new applications for traditionally niche processes. #base
The ECG process requires a constant current source, a conductive grinding wheel (usually consisting of abrasives, copper and resin binder), an electrolyte (usually sodium nitrate) and a workpiece made of conductive and reactive materials such as steel, stainless steel. , chromium-nickel alloy or heat-resistant alloy.
Although electrochemical comminution (ECG) has seen steady growth, it is largely considered a niche process. It was developed in the 1930s and became popular in the US in the 1950s for grinding carbide cutting tools. At that time, the only way to grind carbide was with expensive natural diamond grinding wheels. ECG is capable of grinding difficult-to-cut materials such as carbide, and the process has become popular in the cutting tool industry. But with the development of disposable instrument inserts and molded instruments similar to mesh, the popularity of ECGs has declined. Today he is best known for his work with complex materials and thin-walled, fragile workpieces, including pipe cutting, medical equipment and aircraft engine parts.
But Tom Travia, vice president of business development for ECG equipment supplier Tridex (a Glebar company), said that as technology advances, the process’s potential applications are increasing again. Today’s ECG provides an increasingly efficient option for machining complex alloys, and OEMs in industries such as medical are finding it an effective method for machining precision parts made from these materials.
In electrochemical grinding (ECG), the workpiece becomes the anode and the grinding wheel becomes the cathode, removing material from the workpiece both electrochemically and mechanically.
Electrochemical machining is an electrolytic operation in which the workpiece becomes the anode and the cutting tool (in ECG, a grinding wheel) becomes the cathode. When a direct current flows between the anode and cathode, a reaction similar to electroplating occurs, but instead of taking material from the anode and depositing it onto the cathode, the material is removed from the anode and washed away by the electrolyte. ECG takes electrochemical machining a step further by using a grinding wheel to mechanically cut metal while electrochemically dissolving the material. “Theoretically, you can reduce the hardness of the material and break up some of it when grinding, allowing the wheel to cut with less force,” explains Travia.
In some ways, ECG is similar to traditional grinding—some of the same rules apply. For example, programming and configuration work very similarly. The mounting of the workpiece is also very similar, with the only difference being that the ECG fixture must be made of a corrosion-resistant material and allow electrical contact with the workpiece. “Some things are a little different than traditional sanding,” Travia said, “but if someone has experience sanding, they will notice the difference very quickly.” I have no grinding experience at all.
This process requires special equipment, including a DC power source. A conductive grinding wheel is also required. A standard EKG wheel consists of an abrasive (such as cubic boron nitride, diamond, aluminum oxide or silicon carbide), copper and a resin binder. Binders and particle sizes vary depending on the application. Tridex uses grinding wheels ranging from 0.004 to 4.0 inches wide. These standard grinding wheels are suitable for most applications, but materials such as copper or titanium require grinding wheels made from these materials.
Electrolytes are an important part of the ECG process. It is sprayed onto the grinding wheel and workpiece like coolant. But according to Travia, its functionality is completely different. Despite some cooling properties, ECH does not generate much heat, so the goal is to not inject as much liquid throughout the process as is the case with other processing operations. “The electrolyte is part of the process, which means that if you change the flow rate of the electrolyte, it will affect the way you cut,” he explains. Therefore, this process requires maintaining a certain electrolyte flow rate to ensure that the cuts are burr-free and within tolerances. “There’s more to it than just turning a valve,” he added. “Flow is an important variable in the electrocardiogram.” The electrolyte itself is a type of salt water. Sodium nitrate is often dissolved in this liquid because it is not too harsh, is gentle enough for operators to use, and is economical. Like any other metalworking fluid, electrolyte can become contaminated with metal, so it must be filtered and then replaced and disposed of properly. “Our machines typically have a centrifuge for filtration so that as many small particles as possible can be removed,” he said. “This often significantly extends the life of the electrolyte, in some cases even doubling it.”
ECG can only be performed on products made of conductive materials. This includes a variety of materials, including tool steels, stainless steels, most chromium-nickel alloys, and high-temperature alloys. In addition to being electrically conductive, the material must also be electrochemically active. For example, although platinum is electrically conductive, it is not electrochemically active enough to be used in electrocardiograms.
ECG offers many advantages over grinding and other forms of machining. The electrochemical process reduces cutting forces, extends wheel life and eliminates the need for dressing. And because ECG does not leave burrs or layers on the workpiece, secondary operations can be eliminated.
One of the biggest advantages of ECG is that the electrochemical process oxidizes the workpiece material (regardless of its hardness) and reduces the force required for cutting. This means that the ECG process has a longer grinding wheel life than traditional grinding processes. According to Travia, the G-factor (the amount of metal removed compared to the amount of grinding wheel consumed during grinding) for conventional grinding operations is typically 1 or less, while the G-factor for ECG applications ranges from 20 to well over 1. 100.
ECG also allows you to cut difficult materials with ease. This is because these materials, including chromium, cobalt and nickel, are highly reactive and easily dissolved during electrochemical processes. These higher grade alloys, such as Inconel, Hastelloy and Waspa, can be free-cut with ECG, but present challenges for conventional machining. “For example, when you put a piece of carbide in Inconel, you’ll have to cut it slowly, you won’t get good tool life, and it will harden,” Travia said. “It is not easy to machine, but it is suitable for electrochemical grinding.” ECG can also process more common materials including aluminum and copper, but this is not always the most cost-effective solution. “Aluminum is easy to cut in a variety of ways,” he notes.
ECG cuts materials at a relatively low temperature, unlike processes such as EDM and laser cutting, which operate at high temperatures. The rapid heating and cooling of the material during these processes can cause metallurgical changes that harden the material and make secondary operations such as drilling and tapping difficult. It also tends to leave a heat-affected zone, or remelted layer, that is susceptible to cracking. This is a common problem when cutting pipes, especially aircraft engines. The ends of these tubes are often flared so accessories can be added. If there is a recast layer on it, it can crack and cause the expensive part to be scrapped at the end of the manufacturing process. The recycled layer can be removed, but this adds an extra step to the process.
Another advantage of the ECG is that its grinding wheel does not require refilling. Because the electrochemical process softens the workpiece material and the electrolyte washes away some of it, the material is not forced into the grinding wheel as with traditional grinding. “So your wheel may cost more initially, but it will last longer,” Travia says. This also means that ECG is faster than traditional grinding processes, especially grinding processes that require continuous dressing such as creep feed grinding. Although grinding wheels last longer, they don’t last forever – they get smaller, and profile grinding wheels lose their shape over time and need to be reshaped.
ECG also allows us to exclude many secondary processes. It does not leave burrs, and while EDM and laser can also produce burr-free cuts, EDM can provide little edge breakage and good surface finish.
ECG is commonly used in the aerospace industry because it can easily cut high-temperature alloys. ECGs are also increasingly used in the medical industry, where they are used to produce parts such as hypodermic needles and arthroscopic razors.
application. Travia says the first question to ask when determining whether an ECG is suitable for a particular application is: “Can the grinding wheel create the shape you want?” If yes, then you should pay attention to the ECG. This process can replace many traditional grinding methods, including traditional form grinding. “In most cases, ECG can remove more metal faster than traditional grinding, and the grinding wheel will last longer,” he says.
Travia says the first question to ask when determining whether an ECG is suitable for a particular application is: “Can the grinding wheel create the shape you want?” If yes, then you should pay attention to the ECG.
ECG cannot replace all machining and grinding operations. This does not make sense for applications that require significant material removal, including many milling, turning and surface grinding processes. “If you increase the size of the notch, you increase the amount of current required by the power supply,” explains Travia. “We don’t typically make power supplies larger than 1,000 amps, and 1,000 amps is a pretty big reduction.”
The two most common end markets for ECGs are medical and aerospace. Although the geometric shapes of parts in these two areas can be very different, they use many common materials, especially high-temperature alloys, which ECG can cut more easily than traditional machining methods. These areas also typically contain thin-walled and fragile parts, such as needle and honeycomb parts, which ECG specializes in processing.
One of the most promising applications for ECG is in the resurfacing of needles in the medical field, including hypodermic needles, surgical needles, trocars and biopsy needles. Traditional grinders leave burrs that must be removed by sandblasting or electropolishing. However, these processes can lead to a dull needle tip. “It’s not very comfortable to inject with a blunt needle,” Travia said. The ECG can be used to effectively grind large parts of the needle and then adjust the settings on the tip to create a sharp tip without large burrs.
Another growing application for ECG is tube cutting. Using standard clamps, Tridex ECG machines can cut tubes and wires from 0.010 to 3 inches in diameter. Applications for these tubes range from hypodermic needles to small tubes in aircraft engines.
Like other data processing methods, ECG technology has advanced in recent years. “We want to have as much control as possible over all the variables in the process,” Travia said of Tridex’s strategy. The overall control system has been improved, making this process, like other machining processes, more accurate and repeatable.
Electrolyte management has also improved in recent years. “We can control the flow better,” Travia said. “We can control various parameters of the electrolyte. As the electrolyte changes, we can better control and adjust cutting parameters.” For example, as the solution is used, it will absorb metal and become more conductive, possibly affecting cutting. Sensors can now measure the amount of salt dissolved in a solution and its conductivity, as well as flow, temperature and pH. Tridex machines also use a method to determine when a decision has changed—a decision that users have made in the past without much technical basis.
Glebar’s acquisition of Tridex also allowed the company to explore automation opportunities. Glebar has been automating its guide and centerless grinding machines for some time, and Tridex applies this experience to its own products. “Now that we are one company, we started looking at different automation applications,” Travia said. The company now offers electrochemical grinding machines equipped with a pallet movement mechanism that allows users to load and unload parts while the machine is running. This reduces loading times, sometimes to zero. “It’s popular because it greatly improves productivity,” he said. In one example, a customer who was receiving 30 parts per hour using a traditional ECG increased throughput to 30 parts per hour using a 100-part pallet shuttle without any other changes. Users can further enhance the pallet handling system by adding robots to load and unload the machine. Tridex is also developing an automation system for its point grinders, including a robot to operate the machine.
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Post time: Sep-15-2023