EDM

EDM is a manufacturing process also known as electrical discharge machining or EDM (for its English name, electrical discharge machining).

The EDM process consists of the generation of an electric arc between a piece and an electrode in a dielectric medium to remove particles from the piece until the shapes of the electrode are reproduced on it. Both, piece and electrode, must be conductive, so that the electric arc that causes the removal of material can be established.

Basically it has two variants:

• The process that uses the electrode form, known as ram EDMwhere the term ram In English it means "riete" and is illustrative of the "choke" of the electrode against the piece or vice versa (stone against the electrode).
• The one that uses the metal wire electrode or fine wire, WEDM (where the acronyms describe in English) wire electrical discharge machining)

The thermal erosion process in which metal is removed by a series of recurring electrical discharges between a cutting tool that acts as an electrode and a conductive part, in the presence of a dielectric fluid. This discharge occurs in a voltage gap (“gap”) between the electrode and the piece. The heat from the discharge vaporizes minute particles of the workpiece and electrode material, which are then removed from the gap by the continuously flowing dielectric. The expansion of EDM machining in the last 45 years has given rise to the three main types listed below, although the first two are the most widely used.

• Electroerosion by penetration
• Electroerosion by thread
• perforation electroerode (or electroerosion rectified)

This is the conventional type used by the first EDM machines and is based on the process that we previously described.

EDM process with shaped electrode

During the EDM process, the piece and the electrode are placed very close to each other, leaving a gap that oscillates between 0.01 and 0.05 mm, through which a dielectric liquid (usually low conductivity oil) circulates. By applying a continuous and pulsating voltage difference between the two, an intense electric field is created that causes a gradual increase in temperature, until the dielectric vaporizes.

When the insulation of the dielectric disappears, the spark jumps, increasing the temperature to 20,000 °C, vaporizing a small amount of material from the piece and the electrode, forming a bubble that acts as a bridge between the two.

When the pulse of the electrical source is cancelled, the bridge breaks separating the metal particles in gaseous form from the original surface. These residues solidify on contact with the dielectric and are finally carried away by the current together with the electrode particles.

Depending on the machine and settings in the process, it is possible for the entire cycle to be repeated thousands of times per second. It is also possible to change the polarity between the electrode and the piece.

The desired result of the process is the uniform erosion of the piece, reproducing the shapes of the electrode. In the process, the electrode wears out, which is why it is necessary to move it towards the part to keep the gap constant. In case the wear is severe, the electrode is replaced. If a precise finish is required (shape tolerance +-0.05 mm, the use of two electrodes is necessary).

Among the main characteristics of sinking EDM we can mention: The dielectric fluid is mineral oil, although some machines can use water or other special liquids. Both through shapes and blind shapes of complicated geometries can be obtained. Extraction capacity in steels: up to 2000 mm³/min. Minimum roughness in steels: up to less than 0.4 m Ra. Applications: manufacture of molds and drawing dies

The shaped electrode

The electrode is commonly made of graphite since this, because it has a high vaporization temperature, is more resistant to wear. It can be worked on a specific milling machine in order to create either a male electrode or a female electrode, which means that the electrode will have the opposite shape to the desired and resulting shape on the work piece.

It is good practice to have a rough erosion electrode and one that consumes in a fine and final form, but this can be determined by the dimensions and characteristics of the piece to be achieved.

Electrodes can be manufactured so that multiple shapes belong to the same piece of graphite.

Copper is also a favorite material for manufacturing precise electrodes, due to its characteristic conductivity, although being a soft metal it wears faster. The copper electrode is ideal for making holes or round and deep holes. Commonly these electrodes are found in diameters with millimeter sizes in increments of half a millimeter and various lengths. This particular process is widely used before the wire EDM process, to produce the initial hole where the wire passes through a thickness of material that is inconvenient for conventional drilling. If we want a good finish on the object to be eroded, regardless of the material in which the electrode is made, it must be reviewed by hand after being machined in a milling machine or lathe due to the marks that the cutting tools used in these machines produce. small marks on the electrodes.

Advantages of the EDM process with a shaped electrode

• By not generating cutting forces as in machining, turning and drilling processes, it is applicable for fragile materials.
• Very tilted holes can be produced on curved surfaces without slip problems. As well as of high aspect ratio (quotient between length and diameter), that is, with small diameter and great depth impossible with a conventional drill.
• Being an essentially thermal process, you can work any material while driving
• The tolerances that can be obtained are very adjusted, from ±0,025 to ±0,127 mm.
• It is a unique manufacturing process to achieve complex configurations that are otherwise impossible.
• They sometimes save the realization of a rough finish in the piece by means of acid attacks, passing to the name "Euroerosion finish". It is not a perhaps as perfect finish as the one that would be obtained with the acid attack but for costs and deadlines it is satisfactory in the industry

Disadvantages in the EDM process with a shaped electrode

• After the process there is usually a surface layer of molten metal, fragile and extreme hardness, which should be removed in those parts that require resistance to fatigue. It has more fatigue resistance a piece finished by chip start (fresh, lathe, planner...) than a piece finished by electrical penetration (electroerosion).
• The graphite is a fragile material, so the manipulation of electrodes should be very careful.
• The electrodes, usually, need to be manufactured, for example, machined in a milling machine to work graphite.
• The roughness left on the surface can be very high depending on the type of application and the reduction of the application using minor intensities takes a long time and at times undesired defects can be produced such as carbon or stain formation.
• The rough surface finish is not perfect resulting more rough on the flat faces than on the vertical walls by effect of the sporadic sparks that occur when evacuating the remains of material.

Applications of the EDM process with a shaped electrode

As an example, we can cite the drilling of injector nozzles in the automotive industry, as well as in the manufacture of molds and dies for molding or plastic deformation processes.

Wire EDM process

1.Hilo (Conductor) 2.Electrical Arc (Erosion by Electric Download) 3.Electrical Potential 4.

It is a development of the previously described process, born in the 1970s, and therefore more modern than the previous one, which replaces the electrode with a conductive wire; In addition, this process has better mobility. Wire removal rates are around 350 cm³/h.

The quality, material and diameter of the wire, in conjunction with the voltage and amperage applied, are factors that directly influence the speed with which a piece can be worked. Also, the thickness and material of the piece dictate adjustments for the compliance of the cut.

The finish desired in the process is also a consideration factor that affects the manufacturing cycle time, since the finish that this process leaves on the piece can be improved the more semi-repetitive cutting passes are made on the same surface. executed.

Leading thread

The metallic wire can be made of brass or zinc (and molybdenum, in the case of recirculating wire machines). In environmental protection practices, after the use and disposal of the used yarn and its waste, the material of the yarn, either in the form of yarn or pulverized, is accumulated separately in order to be recycled.

There are several diameters on the market, including 0.010” (0.25mm) and 0.012” (0.30mm). Thread is generally sold in spools and by weight, rather than length.

Thread tension is important to produce an effective cut, and therefore a better part; over-tensioning the thread results in the thread breaking when not desired. But the breakage of the thread is common during the process, and it is also necessary. In some workshops, common lighters are used as a practical way to cut the thread.

Initially, the position of an upper head and a lower head through which the thread passes are in a vertical and concentric alignment with each other; the thread in use is located between these two mechanical components. The thread is mostly a fine thread.

Wire EDM machines

Unlike EDM machines with shaped electrodes to which the applied polarity can be reversed, the polarity in the wire EDM process is constant, that is, the "table" or frame where the pieces are assembled to be worked on is land; this means that it is of negative polarity. The thread, therefore, is the mechanical component to which the positive charge is directed.

All machines receive a wire so that it is tensioned vertically (axial "Z"), to produce cuts and movements in axial "X" and "Y". But for the most part, wire EDM machines have the ability to move their components to adjust the vertical wire and produce a limited cutting angle (axials "U" and "V")..

Internal and external cut

In the internal cut, the wire held at its ends starting with a previously drilled hole and using a back and forth movement, like that of a saw, undercuts the piece until the desired geometry is obtained.

In the external cut, the thread can start the movement from the outside of the perimeter of the piece until it engages the arc; it continues its movement until it reaches the desired periphery.

Advantages of the wire EDM process

• It does not require the previous machining of the electrode.
• It's a high-precision process.
• Complex forms can be achieved.
• Constant results.
• Depending on the machine's ability, wire work may include controlled variable angularity or independent geometry (axis four).
• Pre-tempered materials can be machined and thus avoid the deformations produced in the case of making this thermal treatment after the end of the piece.

Ancillary Items and Artifacts

• Imanes. Because of the fact that the piece must be of a electrical conductor material, such as steel, and many varieties of this are magnetic, the use of magnets is most practical. There are magnets specially designed to prepare the pieces to be worked.
• Specific artefacts. For reason that the geometry of some parts is complicated and the necessary cutting work on them can be positioned in a difficult way, specifically manufactured devices are required in order to hold the pieces during the process and, generally, must be made of stainless steel.
• Rotational motor. If the machine does not have the capacity to move in the axial room, and the cut to the piece requires it, an independent rotational motor can be added in order to turn the piece.

In the 21st century a high-speed turning-like process can be produced using thread for whimsical configurations, difficult dimensions and satisfactory finishes.

Planting the cycle

When one of these two forms of process is chosen to be applied, the aim must be that the manufacturing cycle is as short as possible (reduction of cycle time), that the finish on the piece has the roughness and desired quality, and that the precision in dimensions and geometric tolerances are those planned, all this included with the general and accepted practices in good manufacturing, manufacturing and production.

Planting an intelligent cycle and, when possible, preparing multiple pieces in order and assembled in order to be worked in cycles that require minimal attention, are two ways that contribute to saving time and resources. Obviously, the protection and safety of the operator is the most important thing and, therefore, it also contributes to prosperity and savings.

Precautions and preventive considerations

• The use of electric current, water and high voltage present a danger of electrocution.
• It is feasible that sparks jump out of the container.
• Melts during filling and emptied tank or use of pressure liquid.

Precautions and preventive considerations must always be observed, and regulations dictated by good practices, by instructions and manuals for machines and other equipment, and by the workshop or factory where the EDM process is practiced.