Direct ultrasonic vibration of the tool or workpiece (Fig. 6) is regarded as the optimal strategy for improved flushing and stabilised uED-machining of high aspect ratio structures.
The high frequency in the range from 20 to 60 kHz with amplitudes of 2 to 10 um peak-peak significantly influences the state of the frontal discharge gap and therefore the process itself.
Two major effects can be distinguished: 1) the influence of the vibration on the dielectric and 2) the immediate influence to the working gap width through periodic feed-retraction-movements of the vibrated part.
The first effect can be described by a very high velocity movement of the dielectric.
Its speed can reach more than 0.5 m/s with accelerations of over 60 km/s2, efficiently moving and stirring the dielectric.
An agglomeration of particles is reduced, leading to a more uniform gap condition and therefore increasing the amount of efficient discharges.
Fig. 6. Direct vibration of tool or workpiece in uEDM
The second effect relates to the periodic change in the gap width through vibration. There, a retracting movement ends longer arc discharges that cause geometric deviation and process instabilities [3].
As a result, the process speed is significantly enhanced and, additionally, more complex structures can be reproducibly machined. Fig. 7 shows the tool feed s2 for the machining of very deep micro bores with an optimised conventional process and two direct ultrasonically assisted processes.
The used tool electrode diameter is 60 ตm (a solid tungsten carbide rod, machined by centerless grinding).