The historical roots of EDM Filters date back to the discovery of electric discharges. Besides the discharges produced by natural phenomena, namely lightning, the production of artificial discharges has been closely related to the development of electrical energy sources. First investigations of electrostatic phenomena were performed with frictional machines, during the first half of the 18th century. After that, the first sparks and pulsed arcs were produced with “Leyden jars”, an early form of capacitor invented in Germany and in the Netherlands around 1745 [4] (see figure 1.4 (a)). More powerful discharges were created by putting several Leyden jars in parallel, creating thus a “battery”.
Although scientists of this period sensed that the nature of these artificial discharges was the same as the nature of lightning, the understanding of the observed phenomena was very incomplete. Joseph Priestley (1733¡1804), an English theologian and chemist, was the first to discover in 1766 erosion craters left by electric discharges on the cathode surface: “June the 13th, 1766. After discharging a battery, of about forty square feet, with a smooth brass knob, I accidentally observed upon it a pretty large circular spot, the center of which seemed to be superficially melted. (…) After an interruption of melted places, there was an intrie and exact circle of shining dots, consisting of places superficially melted, like those at the center.” (see figure 1.4 (b)) “June the 14th, 1766. (…) Examining the spots with a microscope, both the shining dots that formed the central spot, and those which formed the external circle, appeared evidently to consist of cavities, resembling those on the moon, as they appear through a telescope, the edges projecting shadows into them, when they were held in the sun.” [5] Priestley also investigated the influence of the electrode material and of the discharge current on the craters size.
Figure 1.4: (a) Engraved plate sent by Alessandro Volta to Joseph Priestley, showing the spark produced by short-circuit of a Leyden jar (1775) [6]; (b) sketches of erosion craters on cathode surface, observed by Joseph Priestley in 1766 [5]. Whereas the discharges studied by Priestley were pulsed and oscillating (because created by short-circuiting of Leyden jars), continuous discharges could only be produced with battery of electrochemical cells, invented later by Alessandro Volta (1745¡1827) in 1799. Developing very large voltaic batteries, the first continuous carbon arc was produced
by Vasilii Petrov in St-Petersburg in 1802 [7]. Published in 1803 but only in Russian, his discovery remained ignored and forgotten for over a century. The discovery of electric arcs is thus often attributed to Humphry Davy (1778¡1829). Unaware of Petrov’s work, he re-discovered independently carbon arcs around 1808, using the huge voltaic battery of the Royal Institution of London (see figure 1.5 (a)). By separating two horizontal carbon electrodes connected to the battery, Davy created a bright and stable discharge. The shape of this discharge was arched, giving its name to the phenomenon.
Development of devices using electric arcs for lighting purposes followed quickly. Swiss natural philosopher Auguste-Arthur de la Rive (1801¡1873) proved in 1820 that arcs can also burn in vacuum, by creating a discharge in an exhausted glass vessel. Figure 1.5 (b) shows examples of these early carbon arc lamps Figure 1.5: (a) Public demonstration of the carbon arc discharge, probably by Humphry Davy in the Royal Institution of London (early 19th century) [7]. The picture below shows the basement filled with a huge battery, used to create the discharge; (b) early carbon arc lamps in air (left) and in exhausted glass vessel (right), also known as “Davy’s electric eggs” or “de la Rive’s electric eggs” [7]. With sophistication of electric sources and industrialization, Auguste de Meritens (1834¡1898) developed in 1881 in France a second major application using electric arcs. He used the heat produced by an arc for joining lead plates, inventing the principle of arc welding. Nowadays, electric arcs are also used for coating deposition, metal processing, plasma spraying and as high power switches, for example [8]. The history of EDM Filters itself begins in 1943, with the invention of its principle by Russian scientists Boris and Natalya Lazarenko in Moscow. The Soviet government assigned them to investigate the wear caused by sparking between tungsten electrical contacts, a problem which was particularly critical for maintenance of automotive engines during the second world war. Putting the electrodes in oil, they found that the sparks were more uniform and predictable than in air. They had then the idea to reverse the phenomenon, and to use controlled sparking as an erosion method [9]. Though they could not solve the original wear problem, the Lazarenkos developed during the war the first EDM Filters machines
which were very useful to erode hard metals such as tungsten or tungsten carbide. The “Lazarenko circuit” remained the standard EDM generator for years. In the 1950’s, progress was made on understanding the erosion phenomenon [10–12]. It is also during this period that industries produced the first EDM machines. Swiss
industries were involved very early in this market, and still remain leaders nowadays. Agie was founded in 1954, and les Ateliers des Charmilles produced their first machine in 1955. Due to the poor quality of electronic components, the performances of the machines were limited at this time. 2005
In the 1960’s, the development of the semi-conductor industry permitted considerable improvements in EDM machines. Die-sinking machines became reliable and produced surfaces with controlled quality, whereas wire-cutting machines were still at their very beginning. With the introduction of numerical position control in the late 1960’s and early 1970’s, the movements of electrodes became much more precise. This major improvement pushed forward the performance of wire-cutting machines. Computer numerical controlled systems (CNC) improved further the performance of EDM in the mid 1970’s. During the following decades, efforts were principally made in generator design, process automatization, servo-control and robotics. Applications in micro-machining became also of interest during the 1980’s [13]. It is also from this period that the world market of EDM began to increase strongly, and that specific applied EDM research took over basic EDM research [14]. Finally, new methods for EDM process control arose in the 1990’s: fuzzy control and neural networks. EDM Filters