On Overlapping the Pulses on Cumulative Microjets Stage at Ultrasonically Aided Electrical Discharge Machining

Abstract

The paper deals with a solution to increase the machining rate at electrical discharge machining aided by ultrasonics (EDM+US), i.e. the overlapping the voltage pulses provided by EDM generator on the cumulative microjets stage (CMS), which occurs at every final of an ultrasonic period by collective implosion of the gas bubbles from the working gap. This overlapping moves the limit of the removed material by a discharge from the boiling isothermal (at classic EDM) to melting isothermal (at EDM+US) as it is emphasized by the results of finite element analysis. The preliminary experimental data analyzed in the paper are obtained in case of random overlapping at ultrasonic frequency of 20 kHz. They demonstrate an increase of machining rate with more than 300% at EDM+US in comparison with classic EDM in the same working conditions.

Research concerning ultrasonic chains with torsional vibrations for different industrial applications

“…As it can be noticed, the increase of machining rate at EDM+US (torsional vibrations) compared to usual EDM is at least 200% with higher values at longer pulse time. This is based on higher probability to overlap the pulse time on cumulative microjets stage, the collective implosion of the gas bubbles from the gap at the final of dielectric liquid stretching semiperiod [15]. The roughness decreased due to ultrasonic assistance with at least 20%.…”

Some aspects of finite element modelling of ultrasonically aided micro-EDM of CoCr alloys

“…4. At µEDM+US with t i =8 µs, t 0 =8 µs, the overlapping probability of pulse time on CMS, decreases very much, under 20% [6]. So, classic thermal removal like in classic µEDM occurs dominantly, and the temperature distribution after pulse end is presented in fig.…”

“…5, validating the model. Over this thermal removal mechanism is superimposed, the ultrasonic removal due to shock wave produced by CMS parallel to machined surface [6]. In fig.…”

“…The Van Dijck and Snoeys's overheating model was considered for EDM component modelling. In this case, boiling is the main mean to remove overheated material after pulse end when pressure drops suddenly; the gas bubble formed around plasma channel lasts very long after the end of pulse, so the material melted by discharge is already solidified [6]. Constant 3473 K temperature on EDM spot, thermal isolation on area covered by gas bubble, and convection cooling on workpiece periphery in contact with dielectric liquid with 313 K were setup as boundary conditions.…”

“…Ultrasonically aided micro-electrical discharge machining (µEDM+US) is an appropriate response in terms of machining rate, precision and surface quality, supported by relative low costs [6]. This hybrid process was applied for other advanced materials difficult to machine by cutting technologies [7,8].…”