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 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. (Fig. 8)

Fig. 8

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 percentage of the volume removed from the workpiece. (Fig. 9)

Fig. 9

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. (Fig. 10)

Fig. 10

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. (Fig. 11)

Fig. 11

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. (Fig. 12)

Fig. 12

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 be 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. (Fig. 13)

Fig. 13

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 th AGIEPULS-L power supply units the discharge current, pulse duration and pulse interval can be set completely independently of each other. The discharge curren is proportional to the height of the rectan gle, and the width corresponds to the pul se duration, which is measured in microseconds 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 microseconds and the pulse 100 microseconds, 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. (Fig. 14)

Fig. 14