In the ED micromachining, we must assume that the pulse energy must be getting smaller. However, the limit of the arc-curve shows us that we must not fall below a certain current threshold. How do you solve this problem?
Your remarks apply to ECM, but not for EDM. The latter only from a certain current spark discharge is produced. There is always the danger of a glow discharge, which leads to the wire breaking during the WEDM and the SEDM to evaporation of the dielectric in the gap for lower currents. Clean theroretisch is all erkläarbar, but where this limit is handy if you, for example, The tool uses a copper electrode and the workpiece iron.
Ok, that is practical solutions, which generally apply to the EDM. But they say nothing to that minimum erosion current must be achieved in order to prevent corona discharges.
My interest now is to find out what they have gained knowledge for other researchers, where this limit the erosion current lies and / or whether there is this limit at all. After the u i characteristic for discharges there must be this limit, but it depends on the materials used, the dielectric, geometric specifications (?) ....
Objective of the pulse generation is always to generate a spark discharge, no arc, no glow and no other abnormalities like.
Dear Dr. Schulze,
The state of the art specifies that in micro-EDM, low discharge energies are used approximately in the range of 10-9 – 10-5 J to remove small volumes of material of around 0.05 – 500 mm3, even using single discharges. So, the problem is to assure this very low level of energy, keeping under control its comprising parameters: discharge voltage, discharge current and pulse duration. For example, even a relative high discharge current of 0.8 A could be used with a pulse duration of 1 microsecond at superior limit of energy level. At inferior energy limit, using very low current and pulse duration creates major difficulties of maintaining the discharges and also parasite effects provided by supplying electric circuits. Hence the inferior limit of the current is strongly related to concrete micro-machining conditions, mainly the feed system assuring a very narrow frontal gap at a level of few micrometers and the filtration of dielectric.
Go for very small value of current available with the pulse generator of the micro EDM. Observe some trials for your erosion current . Then fix the current value and the pulse frequency for the required erosion rate.
Thanks for the lively discussion.
I think Daniel has already made a correct observation that the "true" pulse energies are usually falsified by the parasitic elements (C, L) in the working gap and in the supply line. The only correct determination of the pulse energy is the measurement of current and voltage waveform. Then let rearwardly the parasitic elements determine approximately.
We assume a threshold current of 800 mA and a spark burning voltage of 25 V, then I can reach about 1 μ J pulse energy, pulse duration 50 ns. In this pulse duration range can be also assumed that the full pulse duration becomes effective. Due to the rising / falling of u (t) - and i (t) procedure of sessions, the pulse energy halved yet. But Daniel also indicates that correctly and pulse energies can effect up in the nJ. How can this be explained?
That is correct, that the static pulse energy sources to have no pulse durations below 500 ns. The relaxation sources (RC, RL, RCD "generators") can there be much lower pulse durations.
Your data to C1 would be interested to know. How to get to 55.1 nJ?
I start from the stated capacity, shall be considered minimal WC1 = 150 V * 150 V * 0.1 nF / 2 * = 17 uF without considering additional parasitic capacitances. I start from the pulse parameters, it would at 120 ns and an assumed peak current of 1 A spark burning voltage of 142 V can be calculated, which is of course far too high. Assuming a needle pulse, the burning voltage would be doubled. I correct this statement on a common burning voltage of 30 V, a peak current of 8 A would rest, which is quite possible.
Have you ever measured these parameters during normal handling?
My next question is called the discharge frequency, which, as it stands simply results from the reciprocal of the pulse duration, which does not correspond to reality. Now there are two solutions that are not 120 ns their erosion duration, but their entire pulse duration, ie, Erosion duration plus pause duration. Normally, the process energy sources are structured so that a group of 6 to 12 discharges occur pulses without break and then a longer interval duration is switched. Then, however, the discharge frequency is significantly lower.
I have machined graphite material with a lowest current value of 0.25 amp, and able to machine 5mm thick graphite. It may be difficult to apply less than this value, depending on machine set ups.
This is the sense of my question. Who made machining with currents less than 800 mA. The next question is then, of course, there is the related current and voltage waveforms, so that you can define clearly designate which was current. Is it the default value of a technology (??), it is the overshoot value of the current or the average current, I can accept for the calculation of the pulse energy.
Prof. Rao, as you said current of 0.25 A is determined? Measured or calculated according to predetermined values? The follow-up question is then, namely, the limit current at lower electrode intervals may be considerably smaller than for spark discharges at 100 to 200 micron gap.
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