With advances in control, sensors and automation, electrochemical milling is more precise and efficient than ever before, opening up new application possibilities for traditionally niche processes. #base
The ECG process requires a DC power source, a conductive grinding wheel (usually made up of abrasives, copper and a resin bond), an electrolyte (usually sodium nitrate) and a work piece made from conductive and reactive materials such as steel, stainless steel. , nickel-chromium alloy or superalloy.
Despite steady growth, electrochemical grinding (ECG) is mostly considered a niche process. It was developed in the 1930s and became popular in the US in the 1950s for grinding hard metal cutting tools. At the time, the only way to grind carbide was expensive natural diamond wheels. ECG is capable of grinding difficult-to-machine materials such as carbide, and this process has become popular in the manufacture of cutting tools. But with the development of disposable instrument inserts and near-clean shape molded instruments, the popularity of the ECG has declined. Today, it is best known for applications that process complex materials and thin-walled, brittle parts, including pipe cutting, medical equipment, and aircraft engine parts.
But Tom Travia, vice president of business development for ECG equipment supplier Tridex (Glebar), says that as the technology improves, the potential uses for the process are expanding again. Today’s ECG offers an increasingly efficient option for machining complex alloys, and OEMs in industries such as the medical industry are finding it an effective method for machining precision components 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 electrochemically and mechanically.
Electrochemical machining is an electrolytic operation in which the workpiece becomes the anode and the cutting tool (grinding wheel in ECG) becomes the cathode. When a direct current flows between the anode and cathode, a reaction occurs similar to electroplating, but instead of taking material from the anode and depositing it on the cathode, the material is removed from the anode and washed away by the electrolyte. ECG is taking electrochemical machining to the next level by using a grinding wheel to mechanically cut metal while electrochemically dissolving the material. “Theoretically, you can reduce the hardness of the material and partially break it down while grinding, allowing the wheel to cut with less effort,” explains Travia.
In some ways, an ECG is like a traditional resurfacing—some of the same rules apply. For example, programming and tuning work very similarly. The fixture is also very similar, with the only difference being that the EKG must be made of a corrosion resistant material and make electrical contact with the workpiece. “There are some things that are a little different from traditional sanding,” Travia said, “but if someone has experience with sanding, they can notice the difference pretty quickly.” Tridex customers are adding their first ECG machine, he adds. Grinding experience is generally quite common.
This process requires special equipment, including a DC power source. Conductive grinding wheels are also required. Standard ECG wheels consist of an abrasive (such as cubic boron nitride, diamond, alumina, or silicon carbide), copper, and a resin binder. The binder and particle size vary depending on the application. Tridex uses grinding wheels from 0.004″ to 4.0″ wide. These standard grinding wheels are suitable for most applications, but materials such as copper or titanium require grinding wheels made from these materials.
Electrolyte fluids are an important part of the ECG process. It is sprayed onto the grinding wheel and workpiece like a coolant. But according to Travia, it works very differently. Although ECH has some cooling properties, it does not generate much heat, so the goal is not to inject as much liquid as possible throughout the process, as we do in other processing operations. “The electrolyte is part of the process, which means that if you change the flow rate of the electrolyte, you can affect the way you cut,” he explained. Therefore, this process requires maintaining a certain electrolyte flow rate to ensure that the cuts are burr-free and within tolerance. “There is something to it, and not just the rotation of a valve,” he added. “Flow is an important ECG variable.” The electrolyte itself is essentially a form of salt water. Sodium nitrate is often dissolved in this fluid because it is less corrosive, mild in nature, accessible to operators, and economical. Like any other metalworking fluid, electrolyte can become contaminated with metal, so it must be filtered and then replaced and disposed of properly. “Usually our machines are equipped with a centrifuge for filtration, so as many small particles as possible can be removed from it,” he said. “This often greatly extends the life of the electrolyte, and in some cases even doubles it.”
Electrocardiograms can only be performed on products made of conductive materials. This includes a wide range of materials including tool steels, stainless steels, most chromel alloys and superalloys. In addition to being electrically conductive, the material must also be electrochemically active. For example, although platinum is conductive, it is not electrochemically active enough for electrocardiography.
ECG has 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 leaves no 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 to cut. This means that the ECG process has a longer wheel life than traditional grinding processes. According to Travia, traditional grinding applications typically have G-factors (the ratio of the amount of metal removed to the amount of wheel consumed in the grinding process) of 1 or less, while ECG applications have G-factors ranging from 20 to well over 1.100.
ECG also makes it easy to cut difficult-to-cut materials. This is because these materials, including chromium, cobalt and nickel, are highly reactive and readily dissolve during electrochemical processes. Higher material grades such as Inconel, Hastelloy and Waspar can be freely cut with ECG, but are difficult for conventional machining. “For example, when you put a piece of carbide into inconel, you have to cut slowly, you don’t get very good tool life, and that hardens,” Travia said. “It is not easy to process, 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 many ways,” he notes.
ECG cuts material at a relatively low temperature, unlike processes such as EDM and laser cutting that 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 prone to cracking. This is a common problem when cutting pipes, especially aircraft engines. The ends of these tubes are usually widened to allow fittings to be added. If it has recast layers, it can crack and cause expensive parts to be scrapped at the end of the manufacturing process. Recycled layers can be removed, but this adds an extra step to the process.
Another advantage of ECG is that its grinding wheels do not require sharpening. Since the electrochemical process softens the workpiece material and the electrolyte washes away some of it, the material does not enter the grinding wheel as it does with traditional grinding. “So your wheel may initially cost more, but it will last longer,” says Travia. This also means that ECG is faster than traditional grinding processes, especially those that require continuous dressing, such as creep feed grinding. Although wheels last longer, they don’t last forever – they get smaller and profiled wheels lose their shape over time and need to be repaired.
ECG also allows you to exclude many secondary processes. It does not leave burrs, and while EDM and laser also allow burr-free cutting, ECG can provide low edge breakage and good surface finish.
Because of the ease with which ECG can cut superalloys, it is widely used in the aerospace industry. ECGs are also finding increasing use in the medical industry, such as in hypodermic needles and arthroscopic razors.
application. According to Travia, the first question to ask when determining whether an ECG is suitable for a particular application is: “Can a grinding wheel create the shape you want?” If yes, then it is worth studying the ECG. This process can replace many traditional grinding methods, including traditional profile grinding. “In most cases, ECG removes more metal faster than conventional grinding, while increasing wheel life,” he says.
According to Travia, the first question to ask when determining whether an ECG is suitable for a particular application is: “Can a grinding wheel create the shape you want?” If yes, then it is worth studying the ECG.
ECG cannot replace all machining and grinding operations. For applications that require significant material removal, including many milling, turning and grinding processes, this does not make sense. “If you increase the size of the notch, you increase the amount of current required by the power supply,” Travia explained. “We usually don’t make power supplies over 1000 amps, and 1000 amps is a pretty big discount.”
The two most common end markets for ECG are medicine and aerospace. Although the part geometries in these two areas can be very different, they use many common materials, especially superalloys, which can be cut by ECG more easily than traditional machining methods. These areas also have thin-walled and fragile parts, such as needles and honeycombs, which the ECG excels at.
One of the largest areas of ECG development is the resurfacing of needles in the medical field, including hypodermic needles, surgical needles, trocars, and biopsy needles. Traditional grinders leave burrs that need to be removed by sandblasting or electropolishing. However, these procedures can lead to blunting of the needle tip. “Injecting with a blunt needle is uncomfortable,” Travia said. With ECG, you can effectively sharpen most of the needle and then adjust the settings at the tip to create a sharp tip without large burrs.
Another area of ECG development is tube cutting. Tridex ECG machines with standard attachments can cut tubing 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 maximum control over all the variables in the process,” Travia says of Tridex’s strategy. The control system as a whole has been improved, making this machining process, like any other, more accurate and repeatable.
Electrolyte management has also improved in recent years. “We can better control the flow,” Travia said. “We were able to control various parameters of the electrolyte. As the electrolyte changed, we were able to better control and adjust the cutting parameters.” For example, with use, the solution absorbs metals and becomes more conductive, possibly affecting cutting. The sensor can now measure the amount of dissolved salt in a solution and its conductivity, as well as flow, temperature and pH. The Tridex machine also uses a method to determine when a decision has changed, a decision that users have made in the past without much technical basis.
The acquisition of Tridex by Glebar also allowed the company to explore the possibilities of automation. Glebar has been automating its slide and centerless grinders for some time now, and Tridex is applying this experience to its own products. “Now that we have become a company, we are starting to explore all kinds of automation applications,” Travia said. The company now offers electrochemical pallet sanders that allow users to load and unload parts while the machine is running. This reduces the download time, sometimes to zero. According to him, “it was very popular because it significantly increased productivity.” In one example, a client was capturing 30 parts per hour on a traditional ECG and upped the production to an hour on a 100 part shuttle with no other changes. Users can further enhance the pallet handling system by adding robots to load and unload the machine. Tridex also develops automation systems for its spot grinders, including machine maintenance robots.
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Post time: Sep-04-2023