Hi:
TomP presented on-time and off-time values from the Poco manual which
point to a range of 8.3 kHz to 50kHz, or cycles per second. It's still not
clear how responsive the servo motion control needs to be. As far as arc
detection and shutdown, I suspect that can be accomplished outside the motion
loop, so it can respond much more quickly than the motion control.
Question: Are arcs detected by the same circuit as erosion gap voltage
monitoring or by another means? For readers who haven't seen a DC arc in the
tank, they are as bright as staring at the Sun. Poor or no flushing is what
caused these arcs for me. Once detected, one could shut down the power supply
for some fixed amount of time with an off-time of a second or two, whatever it
takes to be sure the arc is cold and won't re-initiate in the same spot.
Speculation: Once an arc is detected the pulse generating code can
increase the off-time by 20% (a guess) to make sure arcing doesn't continue to
be a problem and then after X minutes of no arcing, progressively scale back to
the starting off-time value.
Pete Gruendeman
--------------------------------------------
On Sun, 3/6/16, TJoseph Powderly <[email protected]> wrote:
Subject: Re: [Emc-developers] EDM gap control (Control parameters)
To: "Pete_Gruendeman" <[email protected]>, "EMC developers"
<[email protected]>
Date: Sunday, March 6, 2016, 8:25 PM
On 03/07/2016 02:13 AM, Pete_Gruendeman wrotry
spark?re:
> 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.
respond to every spark? no, not the
servo, maybe the anti arc
fast as you can...
yes ;-) gotta run ttul8r
> 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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