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. (Fig. 15)

Fig. 15

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 lying between maximum removal and minimum wear is selected. (Fig.16)

Fig. 16

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. (Fig. 17)

Fig. 17

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 a 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. (Fig. 18)

Fig.18

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 somewhat increased.  The picture shows how big a difference there can be in practice between two subsequent machining stages. (Fig. 19)

Fig. 19

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. (Fig. 20)

Fig. 20