Fig. 7. Tool feed sz during deep bore uEDM (tool ุ60 ตm)
It can be seen that with increasing depth the conventional process (f = 0 Hz) not only slows down, but also becomes unstable, which leads to retracting movements of the tool electrode caused by long short circuit periods.
For the direct ultrasonically assisted processes (f =21.8 kHz), no large retracting movements are observed.
This allows for the machining of high aspect ratio structures such as micro bore arrays (bore diameter 85 um, depth 1000 um, grid width 110 um, steel, Fig. 8) or complex external structures such as the cone array shown in Fig. 9.
The cone bottom radius is 75 um, the top radius 8 um, and its height 800 um.
The distance between the two cones (centre to centre) is 250 um.
In deep boring experiments, micro bores of diameter 100 um and depths of 5000 um could be successfully machined in steel.
Fig. 8. Array of 751 micro bores, diameter 85 um, depth 1000 um, grid width 110 um, steel
Fig. 9. Array of 121 micro cones (segment view); diameter 150/16 um, height 825 um, centre-centre-distance 250 um, steel