Re: EDM spark control system

Larry -

I've been doing a little research (I'm otherwise clueless), and
here's what I've found:

It seems the sinker EDM CNC controls try to maintain a constant
Gap Setpoint, based on voltage required to generate a spark. When
the voltage required to produce a gap is too high, the gap is
closed a bit. Many commercial power systems supply in the range
of 25-30 A (some to 300 A) and are capable of 96V no load (some to
100-260V). One model has preset-able ON times of 1-9uS and OFF
times of 8-50uS. There are several fancy methods of dealing with
the "bad" sparks that damage the electrode, and shorting, such as
searching for a high-frequency component. Also, many systems have
orbiting and "jumping" mechanisms to improve dielectric flushing,
etc. It seems there are also several power "step" settings to
control erosion rate and surface finish. Most sources agree that
there needs to be very fine control of gap and feedrate for good
performance. That's about as far as I've gotten. Anyone feel
free to "refine" my perspective.

Take a look at these links, in order of interest to me:
http://www.uni-magdeburg.de/iget/publikat/2000/Article_Wollenberg.
pdf
http://www.unl.edu/nmrc/Diesinking/Equipment/equip.html
http://www.edmsolutions.com/optimize_how.html
http://www.planetee.com/planetee/servlet/DisplayDocument?ArticleID
=2163
http://www.ipnews.com/archives/edm/july2001/makino.htm

HTH,

Carlos Guillermo
VERVE Engineering & Design

-----Original Message-----
From: Larry Edington [mailto:ledington@...]
Sent: Friday, July 27, 2001 6:11 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: EDM spark control system

Since I know nothing about EDM, but would like to build one as
well, I shall
ask some newbie questions.

How would a chopper style drive that lets the current ramp up then
shuts the
driver off at a preset current level work?

An additional 'pause time' setting could then be added which would
ensure X
amount of time passed between sparks.

Second thought. How about a standard high frequency PWM drive?
Varying PWM
frequency and duty cycle would provide the spark on and off time
variations.

If either would work, I've already got both designs done that are
controlling 120V DC motors with LCD displays for operator inputs.
The LCD's
connect to the control board via RS-485 links. These are
optoisolated Mosfet
drives.

It would be trivial to change the boards to add more Mosfets to
provide for
higher current. I'm using a 200V 47A Mosfet for the 15A locked
rotor current
120 VDC motors now.

I could also modify the code to provide for a simple variable on
and off
time.

Am I correct in assuming that the EDM controller should assert a
'feed hold'
signal to the CNC controller when firing pulses to the electrode
then when
current no longer flows after X number of pulses it then releases
the feed
hold signal while continuing to fire pulses to allow the CNC
controller to
move the electrode into the work piece?

This I could also add to my existing code as it would be a trivial
change.
Right now, I look for a stalled motor over time to shut down the
drive and
turn on an alarm buzzer and blink an LED. The logic would just be
reversed
on the EDM.

Since I missed the start of this discussion, could someone repeat
the
voltages and currents required for EDM?

If my assumptions are correct, I've already got the basis for an
EDM
running.

Is Kerosene still the most common dielectric fluid for EDM?

later,
Larry E.

> > It looks like the "electronic switch" would be a power

transistor

> > (MOSFET or bipolar) that would have to tolerate fairly high

current

> > pulses. Perhaps much higher than the "1/4 to 1/2 amp" average
> > current. Once that parameter is nailed down, it's easy to

generate

> > whatever pulse rate and duration you want.
>
> Since this is essentially a short circuit during the spark, a

MOSFET would

> be the best choice, as it resists secondary breakdown. i would

guess

> about 4 of a 25 A 100 V transistor in parallel would be a good

start.

> I already scribbled down a circuit with a current sense resistor

(although

> a transformer-type current pickup would also work) a 556 dual

timer

> and a MOSFET gate driver. It would have pots to adjust on and

off

> time, and could also output a signal to the CNC telling it when

to proceed

> and when to stop the feed.
>
> I could enter this as a schematic and post it to the drop box.

I'm a bit

> too busy with commercial work to do any construction and testing

of it

> right now, but would be glad to help anyone who wanted to build

it.



Addresses:
FAQ: http://www.ktmarketing.com/faq.html
FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/

Post messages: CAD_CAM_EDM_DRO@yahoogroups.com
Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com,
wanliker@...
Moderator: jmelson@... timg@...
[Moderator]
URL to this page: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
bill,
List Manager



Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/