Hi Nicklas:
I am not seeing any response on this forum regarding the erosion pulse
frequency.
Wikipedia mentions on their Electrical_discharge_machining page:
> Several hundred thousand sparks occur per second...
and in Ben Fleming's book on pulse type EDM, he shows frequencies that range
from 2.5kHz up to 40kHz for finishing burns.
It's not yet clear to me that the servo response needs to be fast enough
to respond to every spark. It might be helpful here if I described some of
the basics of the process, based on personal experience and more so based on
others' info, sources unknown.
The EDM process removes metal by superheating it with electrical sparks, and
the collapse of said spark which per some authors blows out part of the metal.
The rapidly cooled workpiece surface becomes very hard and the surface finish
is not particularly integral. This is commonly called a re-cast layer.
The sparks while underway have very little resistance as measured in ohms.
The electrical supply must be stopped periodically (square wave or other) so
the present arc extinguishes and the next spark initiates at the next closest
point between electrode and workpiece. That spark path could include bits of
metal (called swarf) that have been blown out previously, resulting in a longer
arc path and unstable erosion. Flushing is important to minimize or eliminate
these alternate spark paths.
The electrode advances (sinks) in a manner that is nearly the same as a bridge
support caisson being sunk into a riverbed. The workers remove whatever is in
the way to allow the caisson to descend through the mud, gravel and boulders
down to and into the bedrock. See:
https://en.wikipedia.org/wiki/Caisson_(engineering)
<end of process particulars>
Each spark or pulse will vary from the one following and the one preceding
it. My experience suggests that a little averaging is not a bad thing, in
order to keep the process from hunting constantly. This could be mathematical
averaging of erosion gap voltages or it might be a lower servo response
frequency or other solution. Also backlash and machine elasticity needs to be
considered. Even with a 40kHz servo response, the electrode to workpiece gap
is not likely to change at 40kHz.
TJ pointed reminded us that EDM is a very slow process. Whether it is 99%
of optimal or 99.99% of optimal won't change the burn times by much. If the
cost of 40kHz servo response on four axes is excessive and 1kHz servo response
is reasonable then I say we start with 1kHz on all four axes and let experience
inform us on how right or wrong that choice was. It's possible that someone
with hole popper work could run at 1 and 40kHz and report if there is a
noticeable benefit at 40kHz.
--------------------------------------------
On Thu, 3/3/16, Nicklas Karlsson <[email protected]> wrote:
Subject: Re: [Emc-developers] EDM gap control (Control parameters)
To: "Pete_Gruendeman" <[email protected]>, "EMC developers"
<[email protected]>
Date: Thursday, March 3, 2016, 12:01 PM
> ... 40kHz servo
response is very responsive but not necessary if the spark
frequency is less than that.
> Pete
Gruendeman
I have a choice
between 40kHz servo thread inside driver or linuxcnc 1kHz
servo thread. For z motion only choice is simple but for
motion of several axis/joints for example wire EDM?
Nicklas Karlsson
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