Saturday, November 26, 2022

Introduction to EDM Machining

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The EDM machining, or by electric shock, is a machining process by chip removal to electrically conductive materials employs precisely controlled sparks produced between an electrode, usually of graphite -which can be considered as the cutting tool- and a workpiece, in the presence of a dielectric fluid.

The origin of EDM machiningn dates back to the mid-eighteenth century, when the erosive effect of electrical discharges. Almost 200 years later and in the midst of World War II, the Soviet scientists B. and N. Lazarenko took advantage of this effect in order to develop a controlled process for machining those materials that were conductors. With that idea was born the process of EDM in 1943. The Lazarenkos perfected this process and designed a circuit that bore their name and that consisted of a succession of discharges that occurred between two conductors separated from each other by a film of non-conductive liquid called dielectric. Today, many EDM machines use an advanced version of the Lazarenko circuit.

The EDM differs from most metal-cutting operations
in which the electrode extracts the material without physical contact with the work piece. This feature eliminates the inherent force of the tool that is exerted, for example, with a milling machine or a grinder and, for that reason, with the EDM shapes can be produced that would or could be broken by conventional cutting tools.

Basis

The EDM It is a machining process that uses thermal energy, that is, the material is extracted by heat, which is introduced in the form of a spark by the electrical flow between the electrode and the work piece.

How does all this happen? Let’s first look at the essential components of the process in the figure below.

EDM machining

A pulsed power supply controls the timing and intensity of electrical charges, as well as the movement of the electrode relative to the workpiece. For a spark to form, the electrode must always be separated from the piece for a certain distance. This distance, known as spark distance, overcut or more simply, English, gap, is maintained thanks to a dielectric fluid which, depending on the type of machine used for the EDM, can be paraffin, light mineral oils or deionized water.

The dielectric fluid behaves as insulating until sufficient voltage is applied to transform it into driver. Since the surface of both the electrode and the workpiece contain small irregularities, the electric field generated between the closest points between the electrode and the piece is more intense and, therefore, a download between both points, as we see in the following figure.

EDM machining

EDM machining

Under the effect of this electric field, electrons and positive free ions are accelerated to high speeds and quickly form a dielectric fluid ionized column that conducts electricity. At this stage, current can flow and between the electrode and the workpiece a spark, causing a large number of collisions between the particles.

During this process, a gas bubble, whose pressure rises constantly to create a plasma zone. The plasma zone quickly reaches very high temperatures, between 8,000 and 12,000 ÂșC, due to the increasing effect of the number of collisions. This causes the vaporization instant localization of a certain amount of material on the surface of the electrode and on that of the workpiece.

When the spark goes out, the dielectric fluid deionizes, becoming insulating again, and the sudden drop in temperature causes the gas bubble to implode, dislodging vaporized material from the part, forming a cloud in the dielectric and leaving a small crater on eroded surface of the piece (hence the name EDM).

This cloud suspended in the dielectric cools, solidifies in the form of small spheres called EDM chip and is removed from the sparking area by the same dielectric flux. The process in which the dielectric fluid transforms from an insulator to a conductor and returns to an insulator again is repeated for each spark formed and, therefore, is highly dynamic. In the following figure we observe three instances of this process.

EDM machining

EDM machining

Summing up, the dielectric fluid plays important roles in the process of EDM, What are they:

  • Check the spacing of the “gap” between the electrode and the piece.
  • Cool the heated material to form the EDM chip.
  • Extract the EDM chips from the spark zone.

Applications and generalities

The EDM It is used to produce very small and precise parts as well as large parts such as automotive stamping dies and aircraft fuselage components. All materials undergoing EDM machining must be electrically conductive or semi-conductive, with no non-conductive cutting zones. These materials include hardened and heat treated steels, carbide, polycrystalline diamond, titanium, hot and cold rolled steels, copper, bronze and high temperature alloys.

There are many advantages that brings together the machining by EDM, among which we can mention:

  • It is a non-contact process that does not generate vibration or shear forces, allowing the production of very small, fragile and complex shaped parts.
  • Tighter tolerances, intricate details, and superior finishes can be achieved on a wide range of materials that are difficult or impossible to manufacture using traditional processes.
  • Burr-free edges are produced.
  • Very hard metals can be worked because the process vaporizes the metal instead of cutting it.
  • Explosive or flammable materials can be machined, because the process takes place inside a fluid.
  • EDM machines with a process awareness feature allow complex parts to be produced with minimal operator intervention.

Like any manufacturing process, the EDM also features some disadvantages or limitations, for example:

  • It cannot be applied on non-conductive materials.
  • It has low metal removal rates compared to traditional metal removal machining methods.
  • Some processing time is required to produce specific shapes of graphite electrodes. In addition, graphite is a brittle material, so the handling of the electrodes must be very careful.
  • After the process, there is usually a superficial layer of molten metal, fragile and extremely hard, which must be eliminated in the pieces that require resistance to fatigue.
  • The rough surface finish is not perfect, as it is rougher on flat faces than on vertical walls.

there are three main types of machining EDM, as well as various types of machines, which are developed within the Machines category.

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