This article will give a detailed discussion on wire electrical discharge machining (EDM)
It is expected that after reading, one should understand;
A method of precision machining called electrical discharge machining (EDM) removes material from a workpiece using thermal energy rather than mechanical force. A thin, single-strand metal wire and deionized water used to conduct electricity are used in the electrical discharge machining method known as wire electrical discharge machining (EDM) to cut through metal while preventing rust. In wire EDM, the tool electrode is a metallic wire, typically constructed of brass or layered copper. To cut or shape a workpiece, utilize the metallic wire. The thin electrode wire travels along a predetermined route. Although smaller and larger diameters are available, the typical electrode diameters range from 0.004 inches to 0.012 inches (0.10mm to 0.30mm). The wire is coiled between two spools so that the active portion of the wire is continually changing. This design prevents the wire from eroding to the point where it breaks.
Wire electrical discharge machining (EDM) can machine parts resistant to conventional machining methods as long as they are electrically conductive. Typically, non-ferrous metals like steel, titanium, super alloys, brass, and many others fall into this category. EDM produces relatively small chips and a precise cut line by melting or vaporizing the material rather than cutting it. EDM is widely accepted in the industry and has a wide range of applications since it is extremely adaptable, can cut strong metals, and takes up only a small amount of space.
One of the most successful and economical ways to accurately process conductive and hard materials is wire EDM. This technique allows complicated structures to be swiftly and readily sliced while achieving high tolerances. It is also a no-impact cutting technique, allowing for the distortion-free machining of hard, brittle, and delicate materials.
The material is cut, trimmed, and removed from the workpiece during the wire EDM process. Wire EDM machining generates an electrical current discharge between the wire or electrode and the conductive workpiece. Material is removed from the workpiece, and the electrode as the spark jumps across the gap. Hard conductive materials can be easily machined with wire EDM because of the intrinsic characteristics of the technique. A non-conductive fluid or dielectric is also employed to prevent the sparking from shorting out. The dielectric then removes the waste material, and the procedure resumes.
A hole in the workpiece is required before beginning wire machining, or one can start from the edge. Every discharge on the area being machined leaves a crater in the workpiece and an imprint on the tool. It is possible to create parts with taper or various profiles at the top and bottom, thanks to the wire's ability to be inclined. The electrode and workpiece never make contact mechanically. Depending on the accuracy and surface polish required, a part may be cut, roughed, and skimmed. On a single cut, the wire should go through a solid component before dropping a slug or piece of scrap. This design will provide acceptable precision for some tasks, but skimming is usually required.
Potential difference is used in wire EDM, which is pulsed onto the electrode and workpiece. As a result, electrons from the negative electrode flow toward the positive workpiece and strike the deionized water molecules. The number of ions and electrons between the workpiece and the electrode rises due to the electrons' conversion of the molecules into ions. An electric current is produced when the ions go toward the workpiece, and the electrons move toward the electrode.
The temperature rises to almost 10,000° C when the electric current passes between the workpiece and the electrode. The material farthest from the workpiece is melted and vaporized by extreme heat. The molten material is transported or flushed away by the moving dielectric fluid after the current stops. The cutting parameters and speed significantly impact the wire EDM process's accuracy. The wire may move slightly or bend at higher speeds, reducing accuracy overall. Therefore, keeping lesser power and speed guarantees the highest possible precision. Tolerances of up to +/- 0.0002 inches are possible with lesser power and speed, whereas tolerances of +/- 0.001 inches are possible with greater speeds.
The diameter of EDM wire varies from 0.0008 to 0.013 inches. Smaller wires require lower power settings and cutting more slowly. A 0.010-inch diameter plain brass wire is used in more than 80% of EDM work. One should select a wire suitable for the material they want to machine.
An important factor in deciding if a design is appropriate for wire EDM is part geometry. It also affects how challenging it will be to manufacture the component itself. Designers must first determine whether wire EDM can be used to manufacture the features. Not all geometries work well with EDM and can cause issues with wire erosion, surface finishes (due to variations in material thickness), and even the level of tolerance that can be achieved. Engineers and designers must consider geometry as early in the design process as possible when designing a part for manufacturability.
The requirements for the surface finish should be carefully considered when designing a product for wire EDM machining. Surface finishes created by wire EDM can have an accuracy of up to 3 microinches. Only some components, though, need to be extremely accurate. Only a small percentage of parts require such a high level of precision and polish; thus, designers should consider the minimum and maximum tolerance variations when engineering an object. The establishment of minimal specifications will significantly impact the final product's expenses.
Cost optimization, project lead time reduction, and the finished part's repeatability requirements all play a significant role in design. For example, the greater the surface polishes in wire EDM, the narrower the tolerance. However, the cost will increase because the part will require more time to cut overall. In addition, the type of material or part height may impact machining precision. Taller pieces require greater force to remove material, but precision and surface polish will also be impacted; therefore, these factors must be considered.
When assessing a project design, the designer should question the client regarding the material selection. Another choice that needs to be carefully considered is the choice of the right conductive material to meet the part design specifications and be suitable for wire EDM. Selecting the right material conducive to wire EDM machining can reduce the cost of the finished product if material hardness, dimensional accuracy, and tight tolerances are required. Each component of the part design must be assessed and optimized for proper functional performance and the lowest possible product cost for wire EDM design to be most effective.
The time factor in manufacturing is another important consideration when designing a part for the best manufacturability. Since EDM machining takes longer, customer orders frequently call for a quick turnaround.
The entire process is focused on wire EDM programming. The numerical control program directs how the machine tool is processed. The program's correctness directly impacts the processing shape and accuracy. The CNC program should be checked and verified to ensure it is correct after the programming is finished and before the official cutting process. The wire EDM machine tool's numerical control system offers a program verification technique. The program is examined for syntax mistakes and compliance with the pattern processing contour.
The wire EDM machine's various components interact to shape material appropriately. These parts work in tandem with one another and are all necessary for the machine to function.
The wire serves as the cathode, and the workpiece as the anode of the machine's electrodes. When cutting with wire EDM, the servo motor controls the wire electrode to ensure that it never comes into contact with the workpiece.
The wire running system regulates the speed and tension of the electrode wire feed, the electrode wire's reciprocal feeding, and the electrode wire coiling on the wire drum without overlapping.
The wire serves as the electrode to create the electrical discharge. Therefore, the workpiece's thickness and form directly impact the wire's diameter.
The workpiece is supported and held by the work table. Two stepper motors regulate their movement. Relative movement between the working table and cathode wire completes wire EDM machining. The high-speed feed wire cut EDM machine's working table has X and Y axis slides and uses a priceless linear guideway and priceless ball screw as its moving parts. Since the XY cross structure has been in use for many years, its mechanical rigidity and controllability have been established and widely accepted. Today, its production and design processes are well-advanced and frequently used for many machine tools.
High-precision stepper motors with strong coupling and programming instructions are essential components of CNC systems. In addition, CNC tools manage the entire wire EDM machining process. Controlling the entire procedure entails being able to handle the cutting process automatically and sequence the wire path.
The tank used for the wire-cut EDM technique must contain dielectric fluid. With the help of this liquid, the workpiece's minute particles are kept from adhering to the wire electrode. Deionized water is the most used medium since it cools the process and produces a workpiece with a smooth surface.
The power supply unit sends pulses between 100V and 300V to the wire electrode and the workpiece. The electrical charges that flow through the wire electrode to interact with the workpiece are also controlled in frequency and intensity. Therefore, a highly-developed power supply unit is required to deliver the right kind and quality charges during wire EDM machining.
The working solution (deionized water) is drawn out of the tank by a pump, passed through a filter to remove impurities, then directed into separate up and down nozzles before returning via a filter next to the tank. If the working solution and filter impact cutting quality, they must be changed.
Due to their high conducting qualities, brass is the most popular material for EDM wires. It is a copper and zinc alloy, and the wire cuts more quickly the more zinc there is in the alloy. There should be a balance, though, as brass wire's corrosion rate is decreased when the zinc concentration exceeds 40%.
As the name suggests, these are produced by coating the surface of the wire with pure zinc or zinc oxide. Manufacturers use zinc-coated wires because they increase machining speed.
The first wire to be used in wire EDM was copper. At the time, copper seemed a sensible option for an EDM electrode due to its accessibility and high conductivity. Still, copper's slow cutting speed and low tensile force quickly became apparent as generators got more powerful. As a result, this wire is rarely advised, except for older equipment that calls for the use of copper wire.
Wires with a greater zinc content can be produced with the diffusion annealing process. It entails applying layers of pure zinc coating to wires. These wires can process various materials and are excellent for mass manufacturing.
Wires made of molybdenum have a very high tensile strength. Molybdenum wire is a somewhat poor electrode material due to its extremely high melting and vaporization temperatures. However, moly wire helps maintain great wall straightness and reduces the frequency of wire breaks typical with many small and fine brass wires due to its high tensile strength.
This kind of wire contains a core made of carbon steel, which provides great strength and fracture resistance, and is covered in a thick layer of zinc-rich, diffusion-annealed brass, which improves cutting performance.
When attempting to address a challenging application, tungsten wire is frequently one of the last options considered. Sometimes this wire is the only option for an EDM issue, despite being very expensive, cutting poorly, and difficult to deal with. It is the least effective wire electrode in cutting performance, with even greater melting and vaporization temperatures than moly. Of all EDM wires, however, tungsten wire has the highest tensile strength. This quality enables it to define and sharply carve very small, straight-walled details.
It's crucial to specify the tensile strength of EDM wire. The machine's wire drive feed mechanism, which comprises a wire tensioner, roller guides, and upper and lower feed contacts, stretches the wire even when it does not come into contact with the part during cutting (where the electric current is applied). The wire has tension preloaded on it, which may be changed to perform various cutting procedures. The wire's capacity to withstand stress during cutting depends on its tensile strength. The easier it is to break, the lesser the tensile strength.
Angles can be cut in a wire that has reduced tensile strength without breaking. The U-V axis wire guides can be moved or offset to create massive tapers, and angle cuts up to 45 degrees. When employing the U-V axis to manage the perpendicularity of vertical walls for cutting precisely, the stronger wire can be stretched more tightly. Lower wire tension for roughing allows the machine to cut faster without breaking the wire. For skim cuts, slower speeds and less power are used to achieve the best surface finish and precision.
Since there is no real way to rate fracture resistance, it could be more accurate to refer to an EDM wire's fracture resistance as wire toughness or resilience. This alternate reference is because the wire can withstand the spark gap's extraordinarily dynamic environment.
Conductivity gauges a substance's electrical current-carrying capacity. More power can be transmitted to the workpiece during EDM if the wire's conductivity is higher. Increased cutting speeds are frequently the result of more efficient conductivity.
For quick vaporization, a low wire melting temperature will work best. Then, instead of polluting the gap with resolidified chips, the wire surface should evaporate and swiftly transform into gases.
Tensile strength is frequently confused with hardness. A wire's elasticity or capacity for extension, elasticity, or capacity for extension is referred to as its hardness or temper. EDM wires fall into one of two categories here: soft or hard. A strong wire will thread better than a softer wire on closed-guide machines, but a soft wire will taper-cut better. Additionally, a hard wire will offer the most reliable auto-threading performance.
This chapter will discuss the applications, advantages, and disadvantages of EDM machining. Aspects to consider when choosing the right EDM machining will be discussed.
However, it must be noted that the disadvantages of EDM machining are far outweighed by the advantages.
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