Where's the spark ??

[Andrew Mawson]

"Ed Huntress" <huntres23@optonline.net> wrote in message
news:QRkJb.51529$Do6.12300146@news4.srv.hcvlny.cv.net...

"Brian Lawson" <lawsonb@mnsi.net> wrote in message
newseabvvsoevrmdqhmh1eogbfvr4gbu0be3j@4ax.com...
On Fri, 02 Jan 2004 01:48:55 -0500, Gary Coffman <ke4zv@bellsouth.net
wrote:

BIG SNIP OF GOOD STUFF

But this is all pretty much moot since EDM isn't done with
arcs through a gas. It is done with arcs through a liquid.
little snip

Gary

Hey Gary,

I think this last part of your reply is not really correct. Most EDM
I see is set up and started in free air. After location and some
other things are adjusted to suit the operator, then the work is
flooded, either by flooding the tank to immersion, or on smallish
parts or small burn area just with direct flow of die-electric
externally or through internal created flow ports. After the
die-electirc is applied, the rate of burn is then further adjusted to
the need, which seems to me to almost ALWAYS be MAX!!

Ed Huntress says that the arc starts without actual physical contact,
and I can't argue, but if so it sure isn't obvious. I always figured
that on die-sinkers the electrode (always carbon in my cases) does
touch and then retract ever so slightly, and now-a-days maintains the
arc through sophisticated controls on the newer CNC EDM's.

Happy New Year.

Brian Lawson,
Bothwell, Ontario.


The only reason they ever touch is to establish dimensional zeros, Brian.
And that's done with the EDM power supply turned off.

BTW, I've never seen an EDM started in air. The electrode has to be
submerged before you start.

Ed Huntress

Brian/ Ed,

My sinker EDM has a float switch that inhibits the volts if the electrolyte
isn't deep enough - no way can it arc in air. In the original design the
fluid was paraffin (??kerosene USA speak??) so to spark in air with it as a
film over the machine would be a fire hazard (I use a synthetic non-flamable
to a/ reduce the fire risk and b/ to not have the nasty smell)

Andrew Mawson
Bromley, Kent, UK

 

[Brian Lawson]

Hey Ed,

Next time you're up this way, com'on over and watch, and you tell me.
I'm just going by what I see happening in my buddies tool and die
shop. He has quite a variety,, maybe 8 EDMs, (mostly sinkers) from
24" X 36" tables and 100 gallon tanks and full manual operation, to
full CNC 2500mm X 1000mm tables with 4,000 litre plus filter tanks,
and 5 Axis CNC wire (can't guess at that size, but not huge). You
can't see where the burn is if it's "in" the oil already. Used mostly
to burn ribs in the tools. He's even got one "home-made" some years
ago that was made from a big old planer, and uses the gantry for
holding the hot rods. Used for big heavy tools.

He tried to give me a big old Elox (Spark-O-Matic??) power cabinet,
but it was right at the time I was having trouble getting all the
stuff I already had to move into a trailer, and it was 575VAC, which I
will sadly never have here. So I didn't take it, and I see it's gone
from there now.
As Gunner says.............Siiiigggghhh..............!!.

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXXXX
On Fri, 02 Jan 2004 20:52:00 GMT, "Ed Huntress"
<huntres23@optonline.net> wrote:

"Brian Lawson" <lawsonb@mnsi.net> wrote in message
newseabvvsoevrmdqhmh1eogbfvr4gbu0be3j@4ax.com...
On Fri, 02 Jan 2004 01:48:55 -0500, Gary Coffman <ke4zv@bellsouth.net
wrote:

BIG SNIP OF GOOD STUFF

But this is all pretty much moot since EDM isn't done with
arcs through a gas. It is done with arcs through a liquid.
little snip

Gary

Hey Gary,

I think this last part of your reply is not really correct. Most EDM
I see is set up and started in free air. After location and some
other things are adjusted to suit the operator, then the work is
flooded, either by flooding the tank to immersion, or on smallish
parts or small burn area just with direct flow of die-electric
externally or through internal created flow ports. After the
die-electirc is applied, the rate of burn is then further adjusted to
the need, which seems to me to almost ALWAYS be MAX!!

Ed Huntress says that the arc starts without actual physical contact,
and I can't argue, but if so it sure isn't obvious. I always figured
that on die-sinkers the electrode (always carbon in my cases) does
touch and then retract ever so slightly, and now-a-days maintains the
arc through sophisticated controls on the newer CNC EDM's.

Happy New Year.

Brian Lawson,
Bothwell, Ontario.


The only reason they ever touch is to establish dimensional zeros, Brian.
And that's done with the EDM power supply turned off.

BTW, I've never seen an EDM started in air. The electrode has to be
submerged before you start.

Ed Huntress

 

[Brian Lawson]

Hey Andrew,

OK, I hear you. But read my most recent reply to Ed anyway. In my
case it's not an argument, it's just what I observe. And the CNC
machines I see have both a camera hooked to the PC, and a flame
sensor. The sensor allows "puffs" of open flame, but if it lasts more
than a second, something shuts down. The camera allows a variety of
uses, from observing the state of things from either at home or the
engineering office, to sending a real-time on-line video to the
manufacturer when a problem arises. It usually is pointed at the work
area, but can be swung to show the controller and its monitor.

I do know that there is a float switch as another accessory too,
mostly on the older ones, but it must be "switchable" because they
don't normally even use it. On the new machines, they are very fast
"flooders" to fill the tank, and the level is set by the height of the
hollow front screen (door), versus the old tanks that had a solid
front door that has to be lifted on and off and dogged shut.

Early on (10 or 12 years back) they had two fires in the old EDMs,
one of which did a lot of smoke damage in the middle of the night.
The other must have been arc fed, because it shut itself down as soon
as flames burned out the wiring on the dither head. Hasn't happened
for a number of years now though. I also recall that it took some
years to get the "I'm happy" dielectric. It's too expensive to
discard if you don't "like" it. You have to wait until its no longer
any good before trying something else. I could find out what they use
now, if anybody cares. They burn carbon exclusively, and it used to
turn the oil black in the older machines and with that oil. Now it
gets so well filtered that the oil is always as clear as water. I
don't know whether that's the oil, the filters, or a function of both.

Take care.

Happy New Year.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXXX
On Fri, 2 Jan 2004 21:55:08 +0000 (UTC), "Andrew Mawson"
<andrew@mawson.org.uk> wrote:

"Ed Huntress" <huntres23@optonline.net> wrote in message
news:QRkJb.51529$Do6.12300146@news4.srv.hcvlny.cv.net...
"Brian Lawson" <lawsonb@mnsi.net> wrote in message
newseabvvsoevrmdqhmh1eogbfvr4gbu0be3j@4ax.com...
On Fri, 02 Jan 2004 01:48:55 -0500, Gary Coffman <ke4zv@bellsouth.net
wrote:

BIG SNIP OF GOOD STUFF

But this is all pretty much moot since EDM isn't done with
arcs through a gas. It is done with arcs through a liquid.
little snip

Gary

Hey Gary,

I think this last part of your reply is not really correct. Most EDM
I see is set up and started in free air. After location and some
other things are adjusted to suit the operator, then the work is
flooded, either by flooding the tank to immersion, or on smallish
parts or small burn area just with direct flow of die-electric
externally or through internal created flow ports. After the
die-electirc is applied, the rate of burn is then further adjusted to
the need, which seems to me to almost ALWAYS be MAX!!

Ed Huntress says that the arc starts without actual physical contact,
and I can't argue, but if so it sure isn't obvious. I always figured
that on die-sinkers the electrode (always carbon in my cases) does
touch and then retract ever so slightly, and now-a-days maintains the
arc through sophisticated controls on the newer CNC EDM's.

Happy New Year.

Brian Lawson,
Bothwell, Ontario.


The only reason they ever touch is to establish dimensional zeros, Brian.
And that's done with the EDM power supply turned off.

BTW, I've never seen an EDM started in air. The electrode has to be
submerged before you start.

Ed Huntress




Brian/ Ed,

My sinker EDM has a float switch that inhibits the volts if the electrolyte
isn't deep enough - no way can it arc in air. In the original design the
fluid was paraffin (??kerosene USA speak??) so to spark in air with it as a
film over the machine would be a fire hazard (I use a synthetic non-flamable
to a/ reduce the fire risk and b/ to not have the nasty smell)

Andrew Mawson
Bromley, Kent, UK

 

[Ed Huntress]

"Brian Lawson" <lawsonb@mnsi.net> wrote in message
news61cvvkuanh0qm8peq0iac76rk4thavtvo@4ax.com...

Hey Ed,

Next time you're up this way, com'on over and watch, and you tell me.
I'm just going by what I see happening in my buddies tool and die
shop. He has quite a variety,, maybe 8 EDMs, (mostly sinkers) from
24" X 36" tables and 100 gallon tanks and full manual operation, to
full CNC 2500mm X 1000mm tables with 4,000 litre plus filter tanks,
and 5 Axis CNC wire (can't guess at that size, but not huge). You
can't see where the burn is if it's "in" the oil already. Used mostly
to burn ribs in the tools. He's even got one "home-made" some years
ago that was made from a big old planer, and uses the gantry for
holding the hot rods. Used for big heavy tools.

If he's starting the sparks before the electrode is covered, he's probably
using through-the-electrode flushing and the sparks are conducted through
the dielectric. Either way, he's inviting an arc and a fire. Even the
special dielectrics with flash-suppressants added can start a fire, the
results of which I've seen several times. It ain't pretty.

As someone else noted, Brian, modern EDMs have float switches or other
circuits to prevent exactly what you're describing. If somebody disables the
safety devices, he's on his own.

Believe me, that isn't the way it's normally done. I've been in at least 100
EDM shops over the years and I've sold around 30 of them, and given the
basic training.

Ed Huntress

 

[Don Klipstein]

In article <s83avvg8aoto0aa5nu0c750q75g3tippe3@4ax.com>, Gary Coffman wrote:

On 30 Dec 2003 23:09:40 -0800, billb@eskimo.com (William J. Beaty) wrote:
jalbers@bsu.edu (John Albers) wrote in message news:<800a9925.0312301242.d782a51@posting.google.com>...
I took a couple of old HP deskjet 40V DC wall warts and wired them in
series to produce 80V DC. I checked this with a DVOM and I am getting
80V DC output. According to various sources the break down voltage of
air is around 20V per .001 inch.

Nope. The breakdown for short gaps in air (Paschen's Law) is:

Volts = 30,000(cm) + 1,350

Paschen's Law only holds for uniform fields. In other words it
is valid when the electrodes have a large radius with respect
to the gap distance. But if one or both of the electrodes is
sharply pointed, or other shape which produces a nonuniform
field, ie is hollow, has complex features, etc, the field strength
is much higher at points in the gap than the simple potential
divided by distance between the electrodes would indicate.

That allows the arc to establish, then cascade, ie one electron
emitted from the cathode can knock loose more than one electron
in the gas, which then go on to knock loose more, etc. Secondary
electrons are also emitted when the stripped ions impact the
cathode. These actions cause the arc impedance to fall, so that
large currents can be carried between the electrodes at very
modest voltages.

Paschens Law "minimum voltage" appears to me likely to largely apply
even in the most favorable circumstances! If you are trying for a
"loophole" based on electrode geometry, it looks to me you spark through
cold air at maybe somewhere around 250-270 volts as opposed to the 320
volts mentioned in a prior post for spark-through of the air-gap
most-favored to spark through?

Pressure, density, temperature, and gas species also play
important roles. For example, Argon has a 3 times lower
breakdown potential than ordinary air.

Argon does indeed spark through in favorable circumstances less than 100
volts! Glow discharge (once established) with all conditions favorable
can drop around 60 volts, maybe close to 55 volts! You need establishment
of a "true arc" to drop much less than that!!!

That's why it is the
gas of choice for GTAW welding, ie arc starting and stability
are much better with Argon than with other shield gases.
Breakdown can occur as low as 137 V/cm in Argon at .5
torr.

Largely irrelevant for having the gap distance optimized for minimum
possible breakdown voltage of a given gas between "typical" or "good"
electrodes - supposedly somewhere in the 75-100 volt range for pure argon,
a little less (maybe as low as 75-80V) for a "Penning Mixture" of 99.5%
neon .5% argon, apparently around 300V for air!!!

As the arc begins to establish, temperature skyrockets.
This grossly lowers the needed ionization potential. That's
why the arc can sustain at much lower voltages than are
required to strike it.

But this is all pretty much moot since EDM isn't done with
arcs through a gas. It is done with arcs through a liquid.
or vapors thereof!!!

Paschen's Law doesn't apply at all in that case.

Merely does not apply for maintenance of an arc or "an established
repeatedly-sparking discharge". (Most brief/intermittent high current
sparks utilize the "cold cathode arc" process for getting electrons off
the cathode into vapor/air. There is another "arc" cathode process known
as "thermionic arc". A few arcs have significant extent of both processes
- known to shorten the life of underpowered/"dimmed" fluorescent lamps if
extra measures are not taken! - see
http://www.misty.com/~don/dschtech.html)

Gary

- Don Klipstein (don@misty.com)

 

[Brian Lawson]

Hey again Ed,

Certainly I'm not advocating anyone doing anything unsafe. I was
merely commenting on the early basis of this thread, that the starting
arc does not HAVE TO be a high voltage to bridge a gap, but can be
started by surface-to-surface contact. And these guys probably never
did take anything other than anecdotal training. At least I don't
recall anyone ever "going away" to a training course. But they do
operate a number of them 24/7, with great results.

I have a bunch of their used carbons. (In fact, I brought a bunch of
them to NAMES as "freebies" one year. Over 100 pounds was gone by
Saturday noon!!) Very very few had the "oil holes". Seems to me they
use the ram stroke on a timed basis to pulse "pump flush" the debris
away.
And I hope I have not been misleading. The burn grade and rate with
and without dielectric is not comparable, and all the "work" is done
flooded. Just the set-up is done "dry".

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXX
On Sat, 03 Jan 2004 01:05:46 GMT, "Ed Huntress"
<huntres23@optonline.net> wrote:

"Brian Lawson" <lawsonb@mnsi.net> wrote in message
news61cvvkuanh0qm8peq0iac76rk4thavtvo@4ax.com...
Hey Ed,

Next time you're up this way, com'on over and watch, and you tell me.
I'm just going by what I see happening in my buddies tool and die
shop. He has quite a variety,, maybe 8 EDMs, (mostly sinkers) from
24" X 36" tables and 100 gallon tanks and full manual operation, to
full CNC 2500mm X 1000mm tables with 4,000 litre plus filter tanks,
and 5 Axis CNC wire (can't guess at that size, but not huge). You
can't see where the burn is if it's "in" the oil already. Used mostly
to burn ribs in the tools. He's even got one "home-made" some years
ago that was made from a big old planer, and uses the gantry for
holding the hot rods. Used for big heavy tools.

If he's starting the sparks before the electrode is covered, he's probably
using through-the-electrode flushing and the sparks are conducted through
the dielectric. Either way, he's inviting an arc and a fire. Even the
special dielectrics with flash-suppressants added can start a fire, the
results of which I've seen several times. It ain't pretty.

As someone else noted, Brian, modern EDMs have float switches or other
circuits to prevent exactly what you're describing. If somebody disables the
safety devices, he's on his own.

Believe me, that isn't the way it's normally done. I've been in at least 100
EDM shops over the years and I've sold around 30 of them, and given the
basic training.

Ed Huntress

 

[Paul Victor Birke]

************************************************************************
Dear Brian and All

If you look into books on contact theory, one could realize if you made
yourself very small and was caught between the electrode and the work
surface you would be staring at two contours that looked like the
Rockies and its inverse (near mirror image) coming together. Sort of
two fractal contours coming together but of different makeup for sure
since the electrode and work may be of different makeup and local
surface preparation.

Things at this microscopic level is really the environment for the
contact. Certainly one can have sharp peaks looking at one another.

Maybe this microscopic interpretation might suggest a lower voltage
again in my thinking about high local stresses.

However, by posting this maybe someone can see something else here that
can explain the relatively low starting voltage for this arcane process.

BTW contact theory seems arcane and dealt with years ago

Paul Birke, EE in Guelph ON

all the best, Brian

**********************************************************************************

Brian Lawson wrote:

Hey again Ed,

Certainly I'm not advocating anyone doing anything unsafe. I was
merely commenting on the early basis of this thread, that the starting
arc does not HAVE TO be a high voltage to bridge a gap, but can be
started by surface-to-surface contact. And these guys probably never
did take anything other than anecdotal training. At least I don't
recall anyone ever "going away" to a training course. But they do
operate a number of them 24/7, with great results.

I have a bunch of their used carbons. (In fact, I brought a bunch of
them to NAMES as "freebies" one year. Over 100 pounds was gone by
Saturday noon!!) Very very few had the "oil holes". Seems to me they
use the ram stroke on a timed basis to pulse "pump flush" the debris
away.
And I hope I have not been misleading. The burn grade and rate with
and without dielectric is not comparable, and all the "work" is done
flooded. Just the set-up is done "dry".

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXX
On Sat, 03 Jan 2004 01:05:46 GMT, "Ed Huntress"
huntres23@optonline.net> wrote:


"Brian Lawson" <lawsonb@mnsi.net> wrote in message
news61cvvkuanh0qm8peq0iac76rk4thavtvo@4ax.com...

Hey Ed,

Next time you're up this way, com'on over and watch, and you tell me.
I'm just going by what I see happening in my buddies tool and die
shop. He has quite a variety,, maybe 8 EDMs, (mostly sinkers) from
24" X 36" tables and 100 gallon tanks and full manual operation, to
full CNC 2500mm X 1000mm tables with 4,000 litre plus filter tanks,
and 5 Axis CNC wire (can't guess at that size, but not huge). You
can't see where the burn is if it's "in" the oil already. Used mostly
to burn ribs in the tools. He's even got one "home-made" some years
ago that was made from a big old planer, and uses the gantry for
holding the hot rods. Used for big heavy tools.

If he's starting the sparks before the electrode is covered, he's probably
using through-the-electrode flushing and the sparks are conducted through
the dielectric. Either way, he's inviting an arc and a fire. Even the
special dielectrics with flash-suppressants added can start a fire, the
results of which I've seen several times. It ain't pretty.

As someone else noted, Brian, modern EDMs have float switches or other
circuits to prevent exactly what you're describing. If somebody disables the
safety devices, he's on his own.

Believe me, that isn't the way it's normally done. I've been in at least 100
EDM shops over the years and I've sold around 30 of them, and given the
basic training.

Ed Huntress

 

[Bert Hickman]

Paul and all,

I suspect you may be correct for systems operating at lower voltages,
particularly under dielectric fluids. All EDM systems rely on energy
storage capacitors which are repetitively disruptively discharged to
erode material. Although the process typically uses dielectric breakdown
in either a working gas or fluid, is it possible that lower voltage
systems rely on intermittent physical contact?

As Paul indicated, contact theory does indicate that a high current
flowing through a small contact area creates magnetic constricting
forces which tend to further reduce the contact area, leading to the
contact instability and opening of the contact. This can be a serious
problem for contacts having high current density, and it is a serious
problem for utilities where switchgear needs to stay "closed" in the
presence of high fault currents. (References: Francis Moon,
"Magneto-Solid Mechanics", John Wiley & Sons, ISBN 0471885363, pp
142-146, or Holm, R., "Electric Contacts, Theory and Application",
Spinger-Verlag, 1967).

If the EDM machine's storage capacitor is previously charged and the
electrode brought into brief contact with the work piece through a small
contact asperity (the small projecting area of actual contact), it may
be possible that the small contacting region is blown away by the
combination of material heating/evaporation, magnetic forces from the
high current capacitor discharge, and the spark's shock wave. As long as
the time delay was sufficient to permit voltage recharge before the time
of the next contact, it would seem that the process could continue in
this fashion as well. I can also see Ed's point - if the capacitor
doesn't have sufficient time to recharge before the next contact, then
the process would stop and the electrode may indeed weld.

-- Bert --
--
--------------------------------------------------------------------
We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
Lichtenberg Figures (electrical discharges in acrylic), & Scarce OOP
Technical Books. Stoneridge Engineering -- http://www.teslamania.com
--------------------------------------------------------------------

Paul Victor Birke wrote:

************************************************************************
Dear Brian and All

If you look into books on contact theory, one could realize if you made
yourself very small and was caught between the electrode and the work
surface you would be staring at two contours that looked like the
Rockies and its inverse (near mirror image) coming together. Sort of
two fractal contours coming together but of different makeup for sure
since the electrode and work may be of different makeup and local
surface preparation.

Things at this microscopic level is really the environment for the
contact. Certainly one can have sharp peaks looking at one another.

Maybe this microscopic interpretation might suggest a lower voltage
again in my thinking about high local stresses.

However, by posting this maybe someone can see something else here that
can explain the relatively low starting voltage for this arcane process.

BTW contact theory seems arcane and dealt with years ago

Paul Birke, EE in Guelph ON

all the best, Brian

**********************************************************************************


Brian Lawson wrote:

Hey again Ed,

Certainly I'm not advocating anyone doing anything unsafe. I was
merely commenting on the early basis of this thread, that the starting
arc does not HAVE TO be a high voltage to bridge a gap, but can be
started by surface-to-surface contact. And these guys probably never
did take anything other than anecdotal training. At least I don't
recall anyone ever "going away" to a training course. But they do
operate a number of them 24/7, with great results.

I have a bunch of their used carbons. (In fact, I brought a bunch of
them to NAMES as "freebies" one year. Over 100 pounds was gone by
Saturday noon!!) Very very few had the "oil holes". Seems to me they
use the ram stroke on a timed basis to pulse "pump flush" the debris
away. And I hope I have not been misleading. The burn grade and rate
with
and without dielectric is not comparable, and all the "work" is done
flooded. Just the set-up is done "dry".

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXX
On Sat, 03 Jan 2004 01:05:46 GMT, "Ed Huntress"
huntres23@optonline.net> wrote:


"Brian Lawson" <lawsonb@mnsi.net> wrote in message
news61cvvkuanh0qm8peq0iac76rk4thavtvo@4ax.com...

Hey Ed,

Next time you're up this way, com'on over and watch, and you tell me.
I'm just going by what I see happening in my buddies tool and die
shop. He has quite a variety,, maybe 8 EDMs, (mostly sinkers) from
24" X 36" tables and 100 gallon tanks and full manual operation, to
full CNC 2500mm X 1000mm tables with 4,000 litre plus filter tanks,
and 5 Axis CNC wire (can't guess at that size, but not huge). You
can't see where the burn is if it's "in" the oil already. Used mostly
to burn ribs in the tools. He's even got one "home-made" some years
ago that was made from a big old planer, and uses the gantry for
holding the hot rods. Used for big heavy tools.


If he's starting the sparks before the electrode is covered, he's
probably
using through-the-electrode flushing and the sparks are conducted
through
the dielectric. Either way, he's inviting an arc and a fire. Even the
special dielectrics with flash-suppressants added can start a fire, the
results of which I've seen several times. It ain't pretty.

As someone else noted, Brian, modern EDMs have float switches or other
circuits to prevent exactly what you're describing. If somebody
disables the
safety devices, he's on his own.

Believe me, that isn't the way it's normally done. I've been in at
least 100
EDM shops over the years and I've sold around 30 of them, and given the
basic training.

Ed Huntress

 

[Paul Victor Birke]

Bert Hickman wrote:

Paul and all,

I suspect you may be correct for systems operating at lower voltages,
particularly under dielectric fluids. All EDM systems rely on energy
storage capacitors which are repetitively disruptively discharged to
erode material. Although the process typically uses dielectric breakdown
in either a working gas or fluid, is it possible that lower voltage
systems rely on intermittent physical contact?

As Paul indicated, contact theory does indicate that a high current
flowing through a small contact area creates magnetic constricting
forces which tend to further reduce the contact area, leading to the
contact instability and opening of the contact.

Interesting and enlightening.

This can be a serious
problem for contacts having high current density, and it is a serious
problem for utilities where switchgear needs to stay "closed" in the
presence of high fault currents. (References: Francis Moon,
"Magneto-Solid Mechanics", John Wiley & Sons, ISBN 0471885363, pp
142-146, or Holm, R., "Electric Contacts, Theory and Application",
Spinger-Verlag, 1967).

It is this last book I believe I had remarked upon-sounds familiar!.

If the EDM machine's storage capacitor is previously charged and the
electrode brought into brief contact with the work piece through a small
contact asperity (the small projecting area of actual contact), it may
be possible that the small contacting region is blown away by the
combination of material heating/evaporation, magnetic forces from the
high current capacitor discharge, and the spark's shock wave.
Yes and the point is there are many more especially and new work area is

processed underneath the electrode.

As long as
the time delay was sufficient to permit voltage recharge before the time
of the next contact, it would seem that the process could continue in
this fashion as well

..
very well said!

I can also see Ed's point - if the capacitor

doesn't have sufficient time to recharge before the next contact, then
the process would stop and the electrode may indeed weld.

good stuff Bert--looks close!!

all the best

Paul

 

[Ed Huntress]

"Brian Lawson" <lawsonb@mnsi.net> wrote in message
news:fcddvvs7kui0h5f0cud81fpo92i0lhkjq3@4ax.com...

Hey again Ed,

Certainly I'm not advocating anyone doing anything unsafe. I was
merely commenting on the early basis of this thread, that the starting
arc does not HAVE TO be a high voltage to bridge a gap, but can be
started by surface-to-surface contact.

Ok. I'm not trying to be argumentative here, but I see that the "contact"
issue is being perpetuated throughout this thread, and I think it should be
cleared up.

There is no contact in EDMing. None. Nada. Not at the beginning, not at the
end, and not in between. If you have contact, that represents a failure of
your machine, your programming, your power supply, or your technique. It is
NEVER acceptable to have ANY contact between work and electrode in EDM.
Ever. Or never. <g>

The process of establishing a spark, or at least the current understanding
of it, is represented well in that technical paper PDF for which I supplied
the URL in an earlier post. It is a good examination of the creation of an
ionized channel in a liquid dielectric. I've read dozens of these papers
over the years, since I first started researching EDM in the mid-'70s, and
this one is the most sophisticated I've seen. It's based on observed
phenomena rather than on theory.

If you accidently get contact in a wirecut EDM, you will break the wire,
because it will weld to the work once it shorts, or it will soften at the
weld spot and break under wire tension. If you accidently get contact in a
ram- or sinker-type EDM, you will either weld the electrode to the work (it
happens) or you will start an arc that will melt out a spot in your
workpiece. If you're in the late roughing stages, approaching the final
dimensions of the cavity you're EDMing, you will ruin the work.

It's also possible to get an arc without contact. With old EDMs, this was a
frequent problem. It happens under certain conditions when the power supply
that feeds the discharge capacitor(s) has a low impedance and is capable of
suppling sufficient amperage to sustain the ionized channel, effectively
bypassing the capacitor. When that happens, your fancy EDM turns itself into
a crummy arc welder. This, too, will melt a pocket out of the work and ruin
it.

In normal EDMing, the electrode approaches the work slowly, with full
open-circuit voltage appearing between the work and the electrode until a
spark occurs. Once one does, the servo mechanism takes over. This was
originally a solenoid-type electromagnetic servo; later a hydraulic one;
then a ballscrew-driven, motor-powered one; and now, in a few EDMs of the
latest design (Sodick, for the most part), a linear-motor-powered servo. The
servo is very quick and can dither in increments on the order of one
ten-thousandth of an inch (2 microns or so). It avoids contact at all
cost -- it will retract the electrode significantly if it detects the
sustained current of an incipient arc -- unless it fails to do its job.

Now, I can't think of any way to make this clearer. In real-world EDMing,
there is no contact. Making contact is regarded with the same horror as
breaking an endmill when you're pocketing with a milling machine because you
accelerated too fast out of a corner. It can happen because of bad
programming, but there are several fail-safes built into the power
supply/servo system that *should* keep you out of trouble. In the latest
machines, they're pretty effective. In the old ones, you got arcs from time
to time, and, rarely, actual contact, which often resulted in an electrode
stuck to the work with a little weld spot.

I don't wish to interfere with the scientific speculation that's going on
here about how it all works, but that's what actually happens with real
EDMs.

Ed Huntress

 

[Gary Coffman]

On Sat, 3 Jan 2004 07:56:24 +0000 (UTC), don@manx.misty.com (Don Klipstein) wrote:

Paschens Law "minimum voltage" appears to me likely to largely apply
even in the most favorable circumstances!

According to the paper Ed posted, Schottky barrier tunneling occurs
at the electrode spacings used in EDM. You can't use classical physics
rules in situations where quantum mechanics dominates.

Gary

 

[Ed Huntress]

"Gary Coffman" <ke4zv@bellsouth.net> wrote in message
news:32qgvvc42bdakku3udmrl60giq6g5vc4h2@4ax.com...

On Sat, 3 Jan 2004 07:56:24 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:
Paschens Law "minimum voltage" appears to me likely to largely apply
even in the most favorable circumstances!

According to the paper Ed posted, Schottky barrier tunneling occurs
at the electrode spacings used in EDM.

Yeah. An' stuff like that. <g>

Ed Huntress