Where's the spark ??

In sci.physics.electromag Pipper <mail@any.com> wrote:

Can anyone tell me what use you would put "spark erosion" to that couldn't
be done by any other way?
I'm puzzled about it's advantages.

One is the ability to cut small, odd shaped holes, i.e. square, star,
triangle, etc. with sharp corners.

The neatest business card I've ever seen was from a guy representing an
EDM outfit. The card was EDMed into thin sheet steel and in addition to
the normal stuff you find on a business card was a bunch of intricate
artwork.


--
Jim Pennino

Remove -spam-sux to reply.

 

[Paul Victor Birke]

Not sure things can't be done other ways but years ago one of my manu.
engineers cut a magnet with it into some unusual shape I wanted at the
time. So operations on magnets are >>gentle<< so as not to cause
demagnetization.

Slow but smooth!

Paul

Pipper wrote:

"John Albers" <jalbers@bsu.edu> wrote in message
news:800a9925.0312301242.d782a51@posting.google.com...

I am thinking about building an EDM (electric discharge machine) which
remove tiny bits of metal by sparks. I have found plans and
schematics on the web and in various magazines like Home Machinsts
Workshop. I could just blindly copy their designs, but I wanted to
understand how they work first. Most are build around a relaxation
oscillator circuit which charges a capacitor and the capacitor
discarges across a spark gap. Most of the EDM machines that I have
plans for work with voltages around 80-100 volts. Expreimenting helps
me understand things better, so I tried the following experiment:

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. I have a block of wood (pine) with a
notch in it holding two machine screws facing each other to create a
spark gap. Using a piece of .001 inch brash shim stock, I adjusted the
gap to be .001 inch. I am thinking that if 80V is supposed to jump a
.004 inch gap, then 80V should have no problem jumping a .001 inch
gap.

I am not getting a spark. Should I be getting a spark with this set
up?

I tried using
flat ends --|*|--
pointed ends -->*<--
rounded ends --)*(--
with no luck.

Also, I have questions few about the relaxiation oscilator circuits
used for EDM. Why do they need a capacitor. The voltage across the
capacitor is the same voltage as the power supply. Therefore the
power supply wired directly to the spark gap should create a spark.
Or is there a difference in trying to create a single spark and trying
to create a cont. arc?

Any help would be greatly appreciated. Thank You



Can anyone tell me what use you would put "spark erosion" to that couldn't
be done by any other way?
I'm puzzled about it's advantages.

 

[Ed Huntress]

"Paul Victor Birke" <nonlinear@rogers.com> wrote in message
news:3FF32FA0.2080004@rogers.com...

Not sure things can't be done other ways but years ago one of my manu.
engineers cut a magnet with it into some unusual shape I wanted at the
time. So operations on magnets are >>gentle<< so as not to cause
demagnetization.

Slow but smooth!

Cutting magnets was an early production use for EDM. Before digital watches,
Timex cut the little kidney-shaped alnico magnets for their electronic
watches on an Eltee-Pulsitron ram-type EDM. The electrodes were pieces of
copper tubing squashed into the kidney shape and plunged straight into the
alnico. They ganged something like 30 or 40 electrodes into a tool block.

Ed Huntress

 

[John Fields]

On Wed, 31 Dec 2003 16:17:18 -0000, "Pipper" <mail@any.com> wrote:

Can anyone tell me what use you would put "spark erosion" to that couldn't
be done by any other way?
I'm puzzled about it's advantages.

---
No dimensional changes because of mechanical stress or heat of
machining.

--
John Fields

 

[Paul Victor Birke]

Ed Huntress wrote:

"Paul Victor Birke" <nonlinear@rogers.com> wrote in message
news:3FF32FA0.2080004@rogers.com...

Not sure things can't be done other ways but years ago one of my manu.
engineers cut a magnet with it into some unusual shape I wanted at the
time. So operations on magnets are >>gentle<< so as not to cause
demagnetization.

Slow but smooth!


Cutting magnets was an early production use for EDM. Before digital watches,
Timex cut the little kidney-shaped alnico magnets for their electronic
watches on an Eltee-Pulsitron ram-type EDM. The electrodes were pieces of
copper tubing squashed into the kidney shape and plunged straight into the
alnico. They ganged something like 30 or 40 electrodes into a tool block.

Ed Huntress

Kewl !!


thanks Ed

 

[William J. Beaty]

"Tim Williams" <tmoranwms@charter.net> wrote in message news:<vv4vt0akkhi22e@corp.supernews.com>...

"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0312302331.4c2ca0f2@posting.google.com...
I wonder where this 300V (or 1350V from Jim Lux' page) actually
comes from?

Nowhere. Tell me, how is stick welding performed?

Um... You don't know? And you just IGNORE the breakdown
equation without comment? OK, I'd like to hear your reasoning
for why that "Paschen rule" isn't important. We can send your
discovery to Jim Lux and he can add it to the High Voltage Handbook.




Stick welding has nothing to do with the electrostatic breakdown
of normal air which the OP was asking about. To start a welding
arc you have to TOUCH the stick to the workpiece. Once the arc has
formed the physics (and the voltages involved) are entirely different,
and you can pull the electrodes apart to a fairly large distance.
The physics is then that of e-fields within conductive plasma
which keep the plasma "alight." Seen a "jacob's ladder?" The
spark can only leap across an inch or so, yet once it has appeared
it can grow to many inches in length. The original question was
about that spark-leaping phenomenon, not about the length such a
spark can grow after it has started.

 

[Ed Huntress]

"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0401011211.171e1732@posting.google.com...

"Tim Williams" <tmoranwms@charter.net> wrote in message
news:<vv4vt0akkhi22e@corp.supernews.com>...
"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0312302331.4c2ca0f2@posting.google.com...
I wonder where this 300V (or 1350V from Jim Lux' page) actually
comes from?

Nowhere. Tell me, how is stick welding performed?

Um... You don't know? And you just IGNORE the breakdown
equation without comment? OK, I'd like to hear your reasoning
for why that "Paschen rule" isn't important. We can send your
discovery to Jim Lux and he can add it to the High Voltage Handbook.

The rules and handbooks you're talking about are things with which I'm not
familiar, but it sounds like this is being made more complicated than it is.

The dielectric strength of air is around 3 x 10^6 V/m. That means that you
can create a spark with something like 90V at a gap of 0.0012 inches.

That's within the gap range of a typical EDM. Gaps run from perhaps 0.0002
in. for fine-finishing to maybe 0.005 in. or a little more for roughing.

It doesn't take a very high voltage to initiate a spark if you have a
sensitive servo mechanism to maintain a close gap.

Ed Huntress

 

[Paul Victor Birke]

Dear Ed
I at least think you should look at > Pachen's Law<< which talks about
gas breakdown as function of pressure and E (is there something
else-another variable I forget?).

I did not know the long form till Bill advised here.

Books are important even if you don't have them, n'est pas?

Paul



Ed Huntress wrote:

"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0401011211.171e1732@posting.google.com...

"Tim Williams" <tmoranwms@charter.net> wrote in message

news:<vv4vt0akkhi22e@corp.supernews.com>...

"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0312302331.4c2ca0f2@posting.google.com...

I wonder where this 300V (or 1350V from Jim Lux' page) actually
comes from?

Nowhere. Tell me, how is stick welding performed?

Um... You don't know? And you just IGNORE the breakdown
equation without comment? OK, I'd like to hear your reasoning
for why that "Paschen rule" isn't important. We can send your
discovery to Jim Lux and he can add it to the High Voltage Handbook.




The rules and handbooks you're talking about are things with which I'm not
familiar, but it sounds like this is being made more complicated than it is.

The dielectric strength of air is around 3 x 10^6 V/m. That means that you
can create a spark with something like 90V at a gap of 0.0012 inches.

That's within the gap range of a typical EDM. Gaps run from perhaps 0.0002
in. for fine-finishing to maybe 0.005 in. or a little more for roughing.

It doesn't take a very high voltage to initiate a spark if you have a
sensitive servo mechanism to maintain a close gap.

Ed Huntress

 

[Ed Huntress]

"Paul Victor Birke" <nonlinear@rogers.com> wrote in message
news:3FF4A280.9070207@rogers.com...

Dear Ed
I at least think you should look at > Pachen's Law<< which talks about
gas breakdown as function of pressure and E (is there something
else-another variable I forget?).

I'm afraid it's 'way down on a very long list, Paul. I know EDM, and at one
time I probably was writing more about it than anyone else in North America.
I've worked for two EDM companies (Sodick and Mitsubishi) and I've
researched EDM power supply designs, including their history.

My interest is practical, not scientific. In this case, it seems to have
left me in a less confused state. <g>

Be assured, EDMs, particularly modern ones, run at open-circuit voltages
well below those suggested by some here. The one I'm most familiar with had
a maximum open-circuit voltage of 350V. That was 20 years ago. But there
have been EDMs that run well with a maximum open-circuit voltage of 90V.

Ed Huntress

 

[Bert Hickman]

Ed Huntress wrote:

"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0401011211.171e1732@posting.google.com...

"Tim Williams" <tmoranwms@charter.net> wrote in message

news:<vv4vt0akkhi22e@corp.supernews.com>...

"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0312302331.4c2ca0f2@posting.google.com...

I wonder where this 300V (or 1350V from Jim Lux' page) actually
comes from?

Nowhere. Tell me, how is stick welding performed?

Um... You don't know? And you just IGNORE the breakdown
equation without comment? OK, I'd like to hear your reasoning
for why that "Paschen rule" isn't important. We can send your
discovery to Jim Lux and he can add it to the High Voltage Handbook.




The rules and handbooks you're talking about are things with which I'm not
familiar, but it sounds like this is being made more complicated than it is.

The dielectric strength of air is around 3 x 10^6 V/m. That means that you
can create a spark with something like 90V at a gap of 0.0012 inches.

That's within the gap range of a typical EDM. Gaps run from perhaps 0.0002
in. for fine-finishing to maybe 0.005 in. or a little more for roughing.

It doesn't take a very high voltage to initiate a spark if you have a
sensitive servo mechanism to maintain a close gap.

Ed Huntress

Bill Beaty is correct. The minimum breakdown voltage for air is at STP
is around 300 volts. Dielectric breakdown of air is a complex process.
Free electrons in the gap must be sufficiently accelerated by the
electrical field so that they can create additional electrons when they
collide with neutral air molecules. Furthermore, the rate that new free
electrons are created must be greater than the rate that existing free
electrons are being lost through recombination - the average lifetime of
a free electron is only about 11 nanoseconds in air at STP. If the rate
of free electron creation exceeds the rate of loss, then "avalanche
breakdown" of the air occurs and a spark jumps the gap.

If you are to the right of the minimum point on the Paschen Curve,
decreasing the gap size will continue to decrease the breakdown voltage
in a fairly linear manner since there is still sufficient distance for
electric field to accelerate electrons in the gap sufficiently to
trigger avalanche breakdown. However, once you reach the Paschen minimum
(about 320 volts for a 1 mil air gap for air at STP), any further
decreases in gap distance will require a HIGHER electric field to break
down the air. Because the gap is so short, a higher E-field is needed so
that electrons can reach the velocity needed to trigger avalanche
breakdown before disappearing into the positive electrode. The commonly
accepted Paschen minimum sparkover voltage for air is 320 volts at a
distance of about 1 mil. It can be more or less for other gases.

However, much of the above is moot since most EDM'ing is done under a
dielectric fluid using actual mechanical contact and high pulse currents
to "blast" away the small areas that make contact as the working
tool/wire is slowly advanced into the work piece.

-- 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]

No problem Ed

I only know Paschen's Law because it is in an obscure book at work on
insulating oil by Frank Clark.

Otherwise, (I guess now with net it is easy to find, here it is
http://home.earthlink.net/~jimlux/hv/paschen.htm


but otherwise many EEs have not heard of it.

Your experience would count for a lot.


all the best
Paul

Ed Huntress wrote:

"Paul Victor Birke" <nonlinear@rogers.com> wrote in message
news:3FF4A280.9070207@rogers.com...

Dear Ed
I at least think you should look at > Pachen's Law<< which talks about
gas breakdown as function of pressure and E (is there something
else-another variable I forget?).


I'm afraid it's 'way down on a very long list, Paul. I know EDM, and at one
time I probably was writing more about it than anyone else in North America.
I've worked for two EDM companies (Sodick and Mitsubishi) and I've
researched EDM power supply designs, including their history.

My interest is practical, not scientific. In this case, it seems to have
left me in a less confused state. <g

Be assured, EDMs, particularly modern ones, run at open-circuit voltages
well below those suggested by some here. The one I'm most familiar with had
a maximum open-circuit voltage of 350V. That was 20 years ago. But there
have been EDMs that run well with a maximum open-circuit voltage of 90V.

Most interesting, tending to make Paschen's Law somehow not practical
here or over-ridden !!

Happy New Year

 

[Ed Huntress]

"Bert Hickman" <bert.hickman@delete_this.aquila.net> wrote in message
news:3FF4B271.4050609@delete_this.aquila.net...

However, much of the above is moot since most EDM'ing is done under a
dielectric fluid using actual mechanical contact and high pulse currents
to "blast" away the small areas that make contact as the working
tool/wire is slowly advanced into the work piece.

NO physical contact, Bert. Ever. Physical contact and a short circuit, or
even an arc that lasts more than a few milliseconds, usually will destroy
the workpiece and/or the electrode. As I said, servomechanisms are employed
to keep the gap somewhere in the range of 0.0002 in. - 0.005 in., more or
less, depending upon the other operating conditions. Fine finishes are
obtained at gaps of less than a thousandth of an inch.

EDM as we know it today is indeed run in a liquid dielectric, but the
precursors to EDMs, called "tap busters," used air. These machines are still
made. I don't know who's in the business today but two of them that were
around at least through the '80s were Electro-Arc and Camman. Maybe there is
some info about them on the web that would tell you their operating
voltages.

In my shop we had a crude air-dielectric tap buster sold by Do-All. It had a
240V transformer, but it's not a good example because it had no automatic
servo. It often *did* achieve contact, but it didn't matter, because we
weren't trying to machine accurately, as one does with an EDM machine today.

I just looked at the parameters published for some currently-marketed
ram-type EDMs, and I see that open-circuit voltages fall into the range of
50V - 300V, which is about where they have been since the beginning. These
machines use a light mineral oil for dielectric.

Also, wirecut EDMs, which use deionized water for dielectric, seem to run
around 90V open-circuit.

Ed Huntress

 

[Ed Huntress]

"Paul Victor Birke" <nonlinear@rogers.com> wrote in message
news:3FF4B26D.3040503@rogers.com...

Be assured, EDMs, particularly modern ones, run at open-circuit voltages
well below those suggested by some here. The one I'm most familiar with
had
a maximum open-circuit voltage of 350V. That was 20 years ago. But there
have been EDMs that run well with a maximum open-circuit voltage of 90V.

Most interesting, tending to make Paschen's Law somehow not practical
here or over-ridden !!

I suspect it's just some additional phenomena that we aren't accounting for
here. If you're interested, here's a technical paper about EDM pre-spark
phenomena that may clear it up. I believe it concerns liquid dielectrics,
and they used a 600V one-shot power supply to create the observed events,
but it may suggest to you what's happening with air dielectric:

http://www.uni-magdeburg.de/iget/publikat/2001/ISEM_XIII_PaperNo_89.pdf

Happy New Year

To you too, Paul.

--
Ed Huntress
(remove "3" from email address for email reply)

 

[Gary Coffman]

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.

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

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

Gary

 

[Paul Victor Birke]

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.

Ratios here go up to about 4:1 for actual E over uniform for a point
source- like a needle perpendular to a flat surface but a bit away from it.


^
|
________|_______


A protruding somewhat rounded source is about 3.

So as Gary says you get a corresponding reduction in V.

Lets try my 4

V =~ 1350/4= 338 volts (not bad eh!)

but as Gary points out Paschen's Lay only true when gas present.

Paul

 

[Paul Victor Birke]

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.

Ratios here go up to about 4:1 for actual E over uniform for a point
source- like a needle perpendular to a flat surface but a bit away from it.


^
|
________|_______


A protruding somewhat rounded source is about 3.

So as Gary says you get a corresponding reduction in V.

Lets try my 4

V =~ 1350/4= 338 volts (not bad eh!)

but as Gary points out Paschen's Lay only true when gas present.

Paul

 

[Paul Victor Birke]

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.

Ratios here go up to about 4:1 for actual E over uniform for a point
source- like a needle perpendular to a flat surface but a bit away from it.


^
|
________|_______


A protruding somewhat rounded source is about 3.

So as Gary says you get a corresponding reduction in V.

Lets try my 4

V =~ 1350/4= 338 volts (not bad eh!)

but as Gary points out Paschen's Lay only true when gas present.

Paul

 

[Paul Victor Birke]

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.

Ratios here go up to about 4:1 for actual E over uniform for a point
source- like a needle perpendular to a flat surface but a bit away from it.


^
|
________|_______


A protruding somewhat rounded source is about 3.

So as Gary says you get a corresponding reduction in V.

Lets try my 4

V =~ 1350/4= 338 volts (not bad eh!)

but as Gary points out Paschen's Law only true when gas present.

Paul

 

[Brian Lawson]

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.

 

[Ed Huntress]

"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