cover edm spark erosion

Technology of spark erosion and Dielectrics for all your applications


Spark erosion

Spark erosion is a modern machining technique with decisive advantages as a result of which its use is becoming more and more widespread. Only one practical example is given here out of its countless applications in the machining of metal.

lt is a moulding die for glassware. In the bottom is the ejector opening. To the right it is the ejector. Both were eroded in a single operation. Difficult workpieces, machined quickly and accurately. But how does the process work? How can we visualize the removal of material by spark erosion? Unfortunately most of the processes are invisible.

edm dielectic fluid

The principle of spark erosion

The principle of spark erosion is simple. The workpiece and tool are placed in the working position in such a way that they do not touch each other. They are separated by a gap which is filled with an insulating fluid. The cutting process therefore takes place in a tank. The workpiece and tool are connected to a D.C. source via a cable.

There is a switch in one lead. When this is closed, an electrical potential is applied between the workpiece and tool. At first no current flows because the dielectric between the workpiece and tool is an insulator. However, if the gap is reduced then a spark jumps across it when it reaches a certain very small size.

In this process, which is also known as a discharge, current is converted into heat. The surface of the material is very strongly heated in the area of the discharge channel. If the flow of current is interrupted the discharge channel collapses very quickly . Consequently the molten metal on the surface of the material evaporates explosively and takes liquid material with it down to a certain depth. A small crater is formed. lf one discharge is followed by another, new craters are for med next to the previous ones and the workpiece surface is constantly eroded.

dielectic fluid edm

Spark Gap

The voltage applied between the electrode and workpiece and the discharge current have a time sequence which is shown under the illustrations of the individual phases. Starting from the left, the voltage builds up an electric field throughout the space between the electrodes. As a result of the power of the field and the geometrical characteristics of the surfaces, conductive particles suspended in the fluid concentrate at the point where the field is strongest.

This results in a bridge being formed, as can be seen in the centre of the picture. At the same time negatively charged particles are emitted from the negatively charged electrode. They collide with neutral particles in the space between the electrodes and are split. Thus positively and negatively charged particles are formed. This process spreads at an explosive rate and is known as impact ionization. This development is encouraged by bridges of conductive particles.
edm dielectic fluid

Here again we see what in fact is invisible. The positively charged particles migrate to the negative electrode, and the negative particles go to positive. An electric current flows. This current increases to a  maximum, and the temperature and pressure increase further. The bubble of vapour expands, as can be seen.
edm dielectic fluid

Connection between the path of electric power and heat

The model shows how the supply of heat is reduced by a drop in the current. The number of electrically charged particles declines rapidly, and the pressure collapses together with the discharge channel. The overheated molten metal evaporates explosively, taking molten material with it. The vapour bubble then also collapses, and metal particles and breakdown products from the working fluid remain as residue. These are mainly graphite and gas.

edm dielectic fluid

By means of the model we will now try to demonstrate the relationship between the flow of current and heat. In a detail enlargement below we see the negative electrode surface, and above it a part of the discharge channel. Positively charged particles strike the surface of the metal. These are shown in red. They impart strong vibrations to particles of metal, which correspond to a rise in temperature.

When a sufficient velocity is reached, particles of metal, which are shown in grey and yellow here, can be torn out. A combination of positively charged particles, which are shown in red, and negatively charged particles, which are shown in blue, augments the vibration and thus raises the temperature of the particles, which are now uncharged.

edm dielectic fluid

We know that electrical energy is converted into heat when the discharge takes place. This maintains the discharge channel, leads to the formation of discharge craters on the electrodes, and raises the temperature of the dielectric.

spark erosion

Polarity

Now let us examine the question of polarity. The exchange of negatively and positively charged particles, which are respectively shown in blue or red, results in a flow of current in the discharge channel. The particles thus generate heat which causes the metal to melt. With a very short pulse duration more negative than positive particles are in motion. The more particles of one kind move towards the target electrode, the more heat is generated on it. It is also important that as a result of their greater size the positively charged particles generate more heat with the same impact velocity.

novick edm

In order to minimize the material removal or wear on the tool electrode, the polarity is selected so that as much heat as possible is liberated on the workpiece by the time the discharge comes to an end. With short pulses the tool electrode is therefore connected to the negative pole. Its polarity is With short pulses the tool electrode is therefore connected to the negative pole. Its polarity is thus negative. With long pulses, however, it is connected to the positive pole so that its polarity is positive. The pulse duration at which the polarity is changed depends upon a number of factors which are mainly connected with physical characteristics of the tool and electrode materials. When steel is cut with copper the marginal pulse duration is about 5 microseconds.

Machining time

As in all machining processes, in spark erosion time and accuracy are important factors. The erosion time is determined by the volume of material to be removed from the workpiece and the rate of removal, which is represented by Vw. This is measured in cubic millimetres per minute or cubic inches per hour.

The wear on the tool electrode is another factor influencing the machining accuracy. It is represented by a small Greek theta ( ) and a v. This figure is the volume of material lost from the electrode by wear, expressed as a per centage of the volume removed from the workpiece.

edm surface

 

Surface finish

In a similar way to conventional machining methods, spark erosion does not produce a completely smooth surface but a slightly rough, indented one. This surface is typical of spark erosion, and its quality must be known for the function or fitting of individual workpieces. For the purpose of measurement a reference system and surface dimensions have been created so as to allow the surface quality to be specified. Frequently used measurements and characteristics are Rmax and Ra.

Rmax represents the greatest roughness height. In Germany and France this value is also known as Rt, and in USA it is known as Hmax. Rmax becomes an important characteristic if, for example, a part has to be polished or lapped. The arithmetical mean roughness is represented by CLA in Britain. This value is always important when a part is being machined in order to achieve a fit. In the USA it is represented by AA, and in Switzerland by Ra.

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In exactly the same way as with cutting operations, fine or coarse surfaces can be produced by erosion. The following two examples show how wide a range of roughness the eroded surface can have.

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Different spark gaps

The spark gap separates the workpiece from the tool electrode. Even at a small cutting depth a distinction must be made between the frontal and the lateral gap. The frontal gap is determined by the control system, while the lateral gap depends upon the duration and height of the discharge pulses, the combination of materials, the no-load voltage and other predetermined values.

edm spark gaps

Power supply unit

The power supply unit is an important part of any spark erosion system. It transforms the AC supply from the mains and provides rectangular voltage pulses. This can visualized by plotting a graph of voltage against time. By a number of switching devices the size of the rectangles and the distance between them can be adapted to any operational requirements.

edm supply

The sequence of the rectangle is a graphic representation of the opening and closing of the switch, or in other words the pulse duration and pulse interval, or of the discharge time and pause, and also of the voltage and current at the spark gap. In the AGIEPULS-L power supply units the discharge current, pulse duration and pulse interval can be set completely independently of each other.

The discharge current is proportional to the height of the rectangle, and the width corresponds to the pulse duration, which is measured in micro seconds or millionths of a second. The distance between the individual pulses can also be altered so as to set the length of the intervals during which the flow of current is interrupted.

The pulse interval is expressed as a percentage of the pulse duration. For example, if the interval lasts 25 micro seconds and the pulse 100 micro seconds, Tau is 80 per cent. This means that the pulse lasts for 80 per cent of a switching cycle and the interval for 20 percent of the cycle.

edm supply

Electrode wear

Erosion with a light current gives a low rate of removal, while conversely a heavy current gives a high rate of removal. But the wear on the tool electrode expressed as a percentage of the volume also increases if steel workpieces are eroded with copper electrodes. Graphite electrodes behave differently. The wear declines up to a certain current level and then remains more or less constant.

electrode wear

Eroding with short pulses means increasing electrode wear. Conversely the wear is smaller when the pulses are long. In practice, when roughing with copper and graphite electrodes into steel a pulse length Iying between maximum removal and minimum wear is selected.

electrode wear

Off time

Not least, the interval between two discharges is a factor of considerable importance. In general we can say that rapid removal with little wear can be achieved with small intervals, or in other words a high duty factor. The limit must not be exceeded because a point is then reached beyond which the process is impaired resulting in reduced erosion and greater wear. This critical value is also known as the marginal duty factor.

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Impulse current

This diagram shows that the surface roughness and the size of the spark gap are decisively influenced by the discharge energy, which is represented by the area of a current pulse in the picture. The energy contained in a pulse is proportional to the orange-coloured area. It can clearly be seen that the roughness is less marked with a small discharge energy than high discharge energy.

For example, in pre-finishing and finishing a certain surface quality must be attained. This corresponds to a given discharge energy which must be found by suitable adjustment of the discharge current or pulse height and the discharge time or pulse width A compromise between maximum erosion and minimum wear is chosen from the range of possible settings.

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Surface quality in relation to current

A rougher surface is machined to a finer one by eroding with reduced discharge energy. The roughness is reduced, while the electrode wear is some what increased. The picture shows how big a difference there can be in practice between two subsequent machining stages.

edm surface quality

edm surface quality

In workshop practice, in roughing or pre-machining a degree of roughness should be attained which needs only to be evened out in the next machining stage. Experience has shown that the roughness of the subsequent stage is about a third to a fifth of the initial roughness. This procedure gives a very economic overall eroding time in relation to the degree of accuracy attained.

The use of Dielectrics in spark erosion

In 1943 the Russian research scientists, Mr. and Mrs. B. R. and N. J. Lazarenko, discovered that the erosive effect of capacitor discharges could be utilized in the processing of metals. At first they used ordinary air as a dielectric. Very soon it became clear, however, that liquid mineral oil derivatives had considerable advantages. Disruptive strength was greater. Smaller spark gaps could be used, making higher precision possible. Spark frequency could be increased and metal particles could be removed without difficulty. Without these mineral oil products the industrial utilization of spark erosion would never have become possible. Initially products containing petroleum and products derived from white spirit (e. g. Kristallöl 60) were used.

Dielectrics in spark erosion

From 1960 onwards the mineral oil industry began developing industrial fluids specifically for use in spark erosion machines.

Functions of the Dielectric

Insulation One important function of the dielectric is to insulate the workpiece from the electrode. The disruptive discharge must take place across a spark gap which is as narrow as possible. In this way efficiency and accuracy are improved.

lonization As quickly as possible optimum conditions for the production of an electrical field must be created and then a spark path must be provided. After the impulse the spark path must be deionized quickly so that the next discharge can be made. The dielectric ought to constrict the spark path as much as possible, so that high energy density is achieved, which increases discharge efficiency at the same time.

Cooling The spark has a temperature of 8000–12000° C when it punctures the workpiece and so the dielectric must cool both the electrode and the workpiece. Overheating of the electrode must be avoided, so that excessively high electrode wear cannot occur. It must be possible for the metal gases which develop during spark erosion to condense in the liquid.

Removal of waste particles Metal particles that have been eroded away must be removed from the area of erosion by the dielectric to avoid disruptions in the process.

Requirements for Dielectric

Theoretically all insulating liquids can be used as dielectrics. However, due to the requirements set out below, only de-ionized water (for polishing) and hydrocarbons are used for this purpose today. These hydrocarbons can either be produced by distillating and refining mineral oil, or synthetically by processing gases in a synthetizing oven with the help of a catalyst. Synthetically produced hydrocarbons are characterized by otherwise unparalleled purity. In addition, precisely those chains of hydrocarbon molecules can be synthetized which have the best possible erosive effect as well as offering optimum protection against electrode wear. In this way they are far superior to those mineral oil products which are produced from certain mineral oil fractions.

Criteria for assessing Dielectrics

The following criteria are generally used today to assess different dielectric fluids:

a) Degree of metal removal and electrode wear

b) Effects on health: skin irritation toxicity smoke odours

c) Flash point

d) Density

e) Evaporation number

f) Viscosity

g) Conductivity

h) Dielectric constant

i) Disruptive

j) Particle suspension

k) Filterability

l) Compatability with other machine components (machine parts, varnish, sealing material)

m) Aging stability

n) Constancy of quality

o) Availability

p) Price

In general it can be said that it is easy to develop a product which achieves excellent results according to one or another of the above criteria. However, it is important for the utilized product to achieve an optimum in them all, if possible. Thus it is possible for a product of the highest mechanical efficiency, combining high metal removal with low electrode wear, to be unusable in practice, because of physiological reasons, or because it eats into engine parts.

Effects on health

In the present, and certainly even more so in future, the effects of industrially used hydrocarbon fluids on health are becoming increasingly important. Smoke, odours and skin irritation have a decisive influence on working conditions at spark erosion machines.

Skin irritation

Products, which are so pure that they are unharmful from a dermatological point of view, should always be given preference over others. As far as possible these products ought to consist of completely saturated hydrocarbons and should contain as few aromatic compounds as can be. An aromatic content of less than 1% in vol. is desirable. Hydrocarbons from the normal paraffin series of C12 to C14 often cause skin irritation and ought not to be used. If at all possible only such products ought to be used which have been proven to be unharmful to the skin by independent medical tests.

Toxicity

There are as yet no legal provisions i.r.o. toxicity (or rather physiological properties) for the industrial utilization of dielectric fluids. Low aromatic content in an unused product is not on its own an indication of good quality. Far more important is the question, to what extent there is a tendency for aromatic compounds to develop during erosion (aging stability). Even after the product has been in use for some time it must not develop any polycyclic aromatics (e.g. benzpyrene)  which are today considered to be carcinogenic.

Smoke

The amount of smoke given off during erosion is largely dependent on the varying rates of metal removal. Thin-bodied dielectrics usually give off less smoke than more viscous ones. The higher the flow of the dielectric over the place of erosion, the less it smokes. (According to German engineering guidelines – VDI 3402 – the dielectric level must be at least 40 mm above the place of erosion.) A ventilator should always be provided at a spark erosion machine, unless it is used exclusively for fine work.

Odours

The unused dielectric should be odourless and should not begin to smell, even when heated. After it has been used for some time, it is quite usual for a faint ozonic smell, caused by the electrical discharges, to develop. A sour, acrid smell, however, is often an indication that the dielectric ought to be renewed.

Flash point (German standard – DIN 51755)

The flash point is the lowest temperature at which a dielectric gives off sufficient vapours to produce an inflammable mixture of air and gases in a standardized apparatus. The higher the flash point, the safer is the use of the dielectric. Dielectrics are divided into different danger classes according to their differing flash points.

Danger class:

A I under 21° C e. g. benzine

A II 21–550° C e. g. crude petroleum, white spirit

A III 55–100° C e. g. diesel, light fuel oil

According to German engineering guidelines – VDI 3402 – substances with flash points below 21° C may not be used in spark erosion machines. It must also be pointed out that crude petroleum and white spirit are in danger class A II and that special safety regulations must therefore be complied with when they are used. Most of the dielectrics in use today are in danger class A III. Dielectrics whose flash point is over 100° C are not considered to be inflammable as defined by German law. No special safety measures are therefore needed for them. To determine the flash point of fluids in accordance with the German legal provisions for industrial substances, flash points up to 50° C must be measured with the Abel-Pensky apparatus, while flash points of over 50° C must be measured with the Pensky-Martens apparatus (Flp. PM). It is not permissible to use an open cup apparatus, such as the one developed by Cleveland.

Density (German standard – DIN 51757)

Irrespective of viscosity, the influence of density is greater during the finishing process than in rough cut operations. „Heavy“ products remove more metal. The density of a substance is the ratio of its mass to its volume (usually measured at a temperature of 15° C). Dielectrics normally used today have a density of 0.750–0.820. The shorter the chain of hydrocarbon molecules, the lower usually is its specific gravity. Changes in the specific gravity of a dielectric before and after use indicate that alien substances, such as hydraulic fluid, have entered it. Density increases in a dielectric which was blended from different fractions show to what extent the more volatile parts have evaporated. Density can easily be checked with a densimeter (hydrometer). This is a floating glass instrument with a density scale (units of 0.001) also containing a thermometer.

Evaporation number (German standard – DIN 53170)

The evaporation number (VD) is the ratio of evaporating time for the dielectric to that for ether. Dielectrics for polishing work should have an eva poration number of 500–1000. For economic reasons, substances that evaporate more quickly (e. g. Petroleum VD 260) are not suitable as dielectrics.

Viscosity (German standard – DIN 51562)

Viscosity is the property of a fluid whereby it tends to resist the displacement of two neighbouring layers. The physical unit of measurement of absolute viscosity is the Pascal second. One mPa.s is equal to one Centipoise (cP). The ratio of absolute viscosity to density is called kinematic viscosity. The unit of measurement is the square metre per second (M2/s). A centistoke (cSt) is equal to 1 mm?/s. The viscosity of thin-bodied dielectrics is usually measured at a temperature of 20° C. Dielectrics of 2 to 3.5 cSt at a temperature of 20° C are suitable for polishing work. 4 to 6.5 cSt at 20° C is suitable for rough cut operations. The disadvantage of dielectrics which have been produced from two fractions of differing viscosity is that the more volatile, less viscous components evaporate more quickly, leaving behind a dielectric which is so viscous after prolonged use that it is suitable only for rough cut operations. The surface roughness of the processed workpiece is also dependent on viscosity. Thus a narrow spark gap can be used with a thin-bodied dielectric, leading to a finer finish. When more viscous dielectrics are used, a larger spark gap must be chosen to avoid flushing difficulties. This leads to greater roughness in the processed workpiece.

Conductivity

Conductivity is equal to the reciprocal of volume resistivity. The unit is the Siemens. A conductivity AC bridge on the Whetstone bridge principle, at frequencies of 50 or 3000 Hz, is used for measurement. Hydrocarbon dielectrics for industrial use have a conductivity of about 2×10-14 ohm x cm-1 when new.

Dielectric constant (German standard – DIN 53483)

The relative dielectric constant (DK) of a particular dielectric shows to what extent the capacitance of an empty capacitor is increased by introducing that dielectric. A (dielectric constant) DK-meter is used to measure the dielectric constant. The capacity of a capacitor is measured by connecting it to a high frequency resonant circuit, both when filled with dielectric and when empty. The dielectric constant is the ratio of the two different values obtained. A dielectric suitable for spark erosion ought to have a dielectric constant of 2–2.5.

Disruptive voltage (German standard – DIN 53481/ German electrical guidelines – VDE 0303)

The voltage required to disrupt a 2.5 mm layer of dielectric between two spherical electrodes is called disruptive voltage. Good dielectrics should have a disruptive voltage of 50–60 kv when new. It must be noted that the least amount of moisture added to the dielectric (e. g. condensation water) will have a negative influence on this value.

Particle suspension

Waste particles eroded away from the workpiece and the electrode, as well as carbon particles resulting from electrical discharges, are impurities in the working substance. The dielectric must remove these particles from the work area. Adequate particle suspension is necessary for this task. However, particle suspension must not be too high, otherwise these impurities will not separate from the Dielectric during filtration. Too many impurities lead to arcing. On the other hand, a dielectric will only function in the best possible way if a few microparticles are to be found in the dielectric, as this is conducive to ionization. These tiny particles can even be added to the dielectric artificially when it is new to improve erosion from the start.

Compatability with other machine components

Dielectric fluids in industrial use must remain neutral towards other machine components with which they come into contact, e.g. sealing material, tubes and varnish used in containers. The dielectric must not cause these materials to swell up, shrink or dissolve.

Aging stability

Aging stability in dielectrics is very important for economic reasons. The longer a product can be used, the better is the relationship of price to performance. In ordinary erosion practice it ought to be possible to use a dielectric with paper filtration for one or two years. When using precoated filters, dielectrics have now been known to last for almost 20 years without having been renewed. In these cases nothing more was done than to replenish the dielectric tank as the need arose. Age can be assessed by means of infrared spectrographic analysis, but the alternative method, by which neutralization value is determined (NZ/German standard DIN 52558), has also proved to be reliable up to the present. Dielectrics with an acid number of more tha 1 mg/KOH/g ought to be renewed as soon as possible.

Quality and availability

The producer of a dielectric must be able to guarantee its quality for an adequate period of time. In addition, the quality of a dielectric sold under the same name in different countries must always be the same. Dielectric fluids for industrial use ought to be available in those quantities, in those localities, and within those time periods, in which they are required.

Prices

When prices are compared, all the above criteria must be taken into consideration, as the dielectric which is cheapest at first is often the most expensive in the long run.

The flushing process during spark erosion

Every experienced spark erosion expert knows that the flushing process is of utmost importance, when metals are subjected to this procedure. The dielectric must flush away the eroded particles from the gap between electrode and work piece, otherwise they may form bridges, which cause short circuits. Such arcs can burn big holes in the work piece and in the electrode. Modern spark erosion plants therefore have a built in power adaptive control system, which increases pulse spacing as soon as this happens and reduces or shuts off the power supply completely. The more thin-bodied a dielectric and the lower its surface tension, the better it is able to meet flushing requirements.

Open flushing

Open flushing is the most common form of flushing and is used when it is impossible to flush through the electrode or workpiece.

flushing process during spark erosion

Pressure flushing

Next to open flushing, pressure flushing is the most important form. The dielectric is either pushed through a flushing hole in the electrode from above, or through a flushing hole in the work piece from below. The amount of dielectric flowing through is more important for effectivity than the pressure of flushing.

flushing process during spark erosion

flushing process during spark erosion

 

Suction flushing

In suction flushing the eroded particles are sucked out of the gap between electrode and work piece. This type of flushing is best in those cases, where a fine finish and paralell walls are required in the work piece. When using this method with narrow gaps and small amounts of dielectric flowing through, care must be taken that enough dielectric gets into the spark gap, so that the spark erosion process will remain stable.

flushing process during spark erosion

Combined flushing

In very complex jobs it may be advisable to combine suction and pressure flushing.

flushing process during spark erosion

Interval flushing

In interval flushing the erosion process is interrupted after a while and the electrode is retracted. This improves the flushing out of the eroded particles. The retraction and return of the electrode has the additional effect of suction and pumping respectively, which improves the effectivity of the flushing process. This method is particularly suitable, when deep depressions or thin electrodes are involved, and also during finishing work.

Filtering the Dielectric

In order for the dielectric to perform its flushing function in the best possible way, eroded particles from the workpiece and the electrode, as well as the cracked parts of the dielectric itself, must be removed. In addition the dielectric, which heats up during spark erosion, has to be cooled down again to a normal working temperature of 20° C–30° C. If it is too hot, there will be inaccuracies in the work and much of the dielectric will be lost through evaporation. For this reason every spark erosion machine has a filtering plant, which has the following functions to perform:

a) Storing the dielectric

b) Cleaning the dirty dielectric coming from the work tank

c) Providing the required amount of clean fluid and the necessary pressure for rapid filling as well as for pressure and suction flushing

d) cooling the dielectric (by air, water or cooling plant)

e) processing backwashed fluid and filtrate

Cartridge filter system 

In practice cartridge filter systems have proved very effective for filtering dielectric in smaller spark erosion plants, in which up to approx. 450 mm2 / min are eroded. Cartridge filter systems are simple, and, as far as the cost of acquisition is concerned, inexpensive apparatuses. In the main they consist of a storage tank, filter pump, machine pump, cartridge filter, cooler and the requisite piping. The plant is operated manually. The filter element itself is housed in a pressure resistant container and consists of a piece of paper, folded like a star and arranged around a central pipe. The filter cartridge is not reusable. Once it has attained its maximum capacity for retaining dirt, it has to be replaced by a new one. The fineness of the filtering effect of such a plant lies between 1 and 5 µm, depending on the paper used. Under normal conditions the dielectric IME can be used with a paper filter plant for about 1–2 years.

filter system

Precoated filter system 

In big spark erosion plants it is advisable to mount a so called precoated filter system. In these systems the filter elements are coated with an even layer of filter aid, before filtering begins. This layer may consist of diatomite, Rixid or cellulose. After precoating is completed, the filter cycle of the plant is started, either by hand or by machine. After a maximum differential pressure has been reached, the entire filter system is flushed back and all the dirt on the filter elements, plus the filter aid, are expelled via a mud valve into the after-filter. After the flushing back process is completed, the filter plant can be precoated anew and the filter cycle restarted. The filter area should be large enough, so that all the dirt accumulating during one shift can be absorbed, before flushing back becomes necessary. A fineness of up to 1 µm can be achieved with precoated filter systems. On the average 1 kg of diatomite or 0.5 kg Rixid are required for 1 m2 filtering area. The residual moisture of a dry sludge cake discharged from a precoated filter system lies between about 20 % and 30 % of the weight, depending on the type of dielectric used. The service life of the dielectric in precoated filter systems is very long, since diatomite and Rixid not only have a mechanical cleaning effect, but also filter out acid components from the dielectric to a certain extent. In precoated filter systems bleaching earth may also be used as a filter aid, in order to clean the dielectric even more thoroughly. There is data available from precoated filter systems, which were filled twenty years ago with a quantity of the dielectric IME, which is still fully operative today. Merely the amounts lost through drag-out and evaporation had to be replaced.

filter system

between about 20% and 30% of the weight, depending on the type of dielectric used. The service life of the dielectric in precoated filter systems is very long, since diatomite and Rixid not only have a mechanical cleaning effect, but also filter out acid components from the dielectric to a certain extent. In precoated filter systems bleached earth may also be used as a filter aid, in order to clean the dielectric even more thoroughly. There is data available from precoated filter systems, which were filled twenty years ago with a quantity of the dielectric IME, which is still fully operative today. Merely the amounts lost through drag-out and evaporation had to be replaced.

The Transor filter system 

The Transor filter system is able to produce a filtering effect of 1 µm without the use of filter aids by employing the edge filter principle. Filtering rods, on which thousands of extremely fine special paper discs are mounted, are installed in a pressure tank. The dirty dielectric is pumped into the pressure tank and pressed through the filtering rods from the outside to the inside. As this system works without filter aids, no precoating is necessary. The gaps between the paper discs are so narrow, that all particles that are larger than 1 µm are deposited on the surface of the filter rods. When the rods are dirty, backflushing occurs, and the dielectric, which has already been filtered, is pressed back through the filter rods in the opposite direction. The dirt layer on the filter rods is blasted off and can be taken out of a sludge tank. There is little sludge in comparison to the precoated filter system, because no filter aids are used. The service life of the filter rods is on the average about 8,000 working hours. In a Transor filter system one must make sure that the viscosity of the dielectric does not excede 4.0 cSt at 20° C.

filter system

Diagram of a filter system for dielectrics working according to the edge filter principle a) filter container, b) filter rods, c) filter pump, d) sludge tank, f) clean oil tank, g) machine pump, h) oil air cooler, i) water trap and reducing valve for compressed air, j) central valve with single-lever operation

a) filter container,

b) filter rods,

c) filter pump,

d) sludge tank,

f) clean oil tank,

g) machine pump,

h) oil air cooler,

i) water trap and reducing valve for compressed air,

j) central valve with

Dielectric fluids

Whether for roughing work or use in ultrafine finishing – the dielectric has to satisfy the very specific requirements for every application. The high-performance dielectrics  are made from synthetic base oils and contain discharge-intensifying, wear-reducing additives and ageing inhibitors for use in electric discharge machining (EDM).

The dielectrics have very high disruptive strength, are as clear as water and practically odourless. Furthermore, they achieve the degree of purity of pharmaceutical white oils and are more or less free of aromatics.

The individual dielectrics are specially formulated in their different viscosities and additives to the machining processes and materials in question. In each product series you will find a variant specially formulated for polishing, roughing and universal application.

Benefits

  • Competitive performance
  • Good discharge properties
  • Non-irritating to the skin
  • Low odour
  • Competitive price

Advantages

  • High discharge properties, Up to 20% higher discharge capacity
  • Low electrode wear
  • Good surface qualities
  • Excellent polishing properties
  • Long service life of the dielectrics
  • No corrosion on workpieces and the machine

 

 

 

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