Technology of spark erosion

Introduction

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. We shall try to obtain a picture of them with the aid of models and diagrams.

Principle

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.

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.

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 in the following picture.

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.

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.

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.

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. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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