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. It is a moulding die for glassware. In the bottom is the ejector opening. To the right of it is the ejector to fit. 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. (Fig. 1)

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Fig. 1

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.  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. Aser. 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. If one discharge is followed by another, new craters are formed next to the previous ones and the workpiece surface is constantly eroded. (Fig. 2)

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Fig. 2

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 center 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. (Fig. 3)

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Fig. 3

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 Figure 4.

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Fig. 4

 

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. (Fig. 5)

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Fig. 5

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. (Fig. 6)

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Fig. 6

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. (Fig. 7)

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Fig. 7


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