Spindle
The spindle (Fig. 5) rotates the electrode during machining in
order to reduce the influence of the asymmetric imperfections
of the tool-electrode (run out). This spindle uses a stepper
motor, which allows precise positioning when tool-electrodes
with different shapes are needed (triangle, square etc.).the model of the stepper motor is ST4218L1806-LC-NEU from
Nanotec which can be controlled using a SMCI33-1 drive.
The step angle of the motor is 1.8 with an accuracy of ±5 %.
The drive communicates with the HMI using a USB port.
In μECMtool electrodes with simple shapes (cylindrical or
semi-cylindrical) are used in most cases. The electrical connection
of the cathode is complex to achieve because the tool
is spinning: energy pulses must be reliably transmitted to the
rotating elementIn the case of high-frequency power supply and high
current, it is not suitable to use the traditional carbon or silver
brushes because the transmission of high current has a detrimental
effect on them and they do not work well at high
frequency (parasitic inductance). At the moment, there is no
good solution on the market such as a system transmitting
high-frequency signals and high current to a spinning electrode
and at the same time to be located as close as possible to
the IEG (with a minimal length of metal conducting the
pulses).
The solution adopted by the authors was to develop a
purpose built spindle (Fig. 5) using precision bearings and to
have the transmission of the signals to be done via a ring with
microfiber carbon brushes. This type of ring is normally used
for shaft grounding and bearing protection, and is able to
handle high current at high frequencies [37]. To improve the
electrical contact between the brushes and the tool-electrode, a
silver coating was applied on the surface of the electrodeholder.
This setup makes a reliable connection between the
PSU and the rotating parts of the spindle which holds the toolelectrode.