Where's the spark ??

[John Albers]

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

 

[Ed Huntress]

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

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?

You need the capacitor to get a sufficient rate of current flow, through the
ionized channel, to sustain the channel and to do the work. The required
instantaneous current flow is very high -- far more than the internal
resistance of your transformers would allow.

Somewhere around here I have an article I wrote on EDM for American
Machinist, 25 years ago, that explains some of the things you're probably
looking for. It's 16 pages of magazine text and it's dated, but the basics
haven't changed. It was used by vo-techs and some EDM manufacturers as a
basic text on the subject years ago.

If I can find it, I'll scan it and send it to you. It's still copyrighted by
McGraw-Hill so I can't post it anywhere. Remind me in a few days if I
haven't gotten to it.

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

 

[Tom Gardner]

I'm sure you will receive many replies full of technical jargon and
unworkable calculus functions but the REAL answer is to be able to align the
bisecting planes of the electrodes tangent to both the absolute and implied
potential electrical tunneling effect with the true magnetic north pole of
the earth not true north. Of course, this is reversed if you live in the
southern hemisphere.


"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

 

[Paul R. Mays]

"Tom Gardner" <tom@ohiobrush.com> wrote in message
news:8cmIb.19720$P%1.18358026@newssvr28.news.prodigy.com...

I'm sure you will receive many replies full of technical jargon and
unworkable calculus functions but the REAL answer is to be able to align
the
bisecting planes of the electrodes tangent to both the absolute and
implied
potential electrical tunneling effect with the true magnetic north pole of
the earth not true north. Of course, this is reversed if you live in the
southern hemisphere.

Horsey Cocka Doo Doo....

"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

 

[PhysicsGenius]

You also need to make sure you have enough dilithium in your flux capacitor.

Tom Gardner wrote:

I'm sure you will receive many replies full of technical jargon and
unworkable calculus functions but the REAL answer is to be able to align the
bisecting planes of the electrodes tangent to both the absolute and implied
potential electrical tunneling effect with the true magnetic north pole of
the earth not true north. Of course, this is reversed if you live in the
southern hemisphere.


"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

 

[Martin Whybrow]

"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

John

If your set-up reaches a high enough field strength to ionise the air, the
current will almost certainly flow through the wood, it's not a good
insulator. Substitute plastic, preferably not black (some black plastics are
loaded with carbon) for the wood. Add a capacitor because the transformers
have way too much resistance and inductance to get a healthy spark.
Martin
--
martin<dot here>whybrow<at here>ntlworld<dot here>com

 

[Paul Victor Birke]

John Albers wrote:

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.

Dear John

I believe air at standard pressure and temp kicks over at ~ 28.5 kV/inch

So with 80 volts you need 80/28500 or 0.003 " . So you should have no
problem at 0.001"

Paul

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

 

[Chris Oates]

"Paul Victor Birke" <nonlinear@rogers.com> wrote in message
news:3FF209E0.5000008@rogers.com...

I believe air at standard pressure and temp kicks over at ~ 28.5 kV/inch

So with 80 volts you need 80/28500 or 0.003 " . So you should have no
problem at 0.001"

Spark erosion machining with 2 HP Printer
wallwarts, I think some 'industry' purchasing
departments will be kicking themselves.

 

[Tweetldee]

And since he's using pine wood against "brash" shim stock, that needs to be
dilithium phosphate.
--
Tweetldee
Tweetldee at att dot net (Just subsitute the appropriate characters in the
address)

Never take a laxative and a sleeping pill at the same time!!
"PhysicsGenius" <physics_seeker@yahoo.com> wrote in message
news:nzmIb.1222$nK2.419@nwrddc01.gnilink.net...

You also need to make sure you have enough dilithium in your flux
capacitor.

Tom Gardner wrote:
I'm sure you will receive many replies full of technical jargon and
unworkable calculus functions but the REAL answer is to be able to align
the
bisecting planes of the electrodes tangent to both the absolute and
implied
potential electrical tunneling effect with the true magnetic north pole
of
the earth not true north. Of course, this is reversed if you live in
the
southern hemisphere.


"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

 

[Jon Elson]

John Albers wrote:

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?


Probably not. My experience is you'll never get a fat spark from a TV anode

power supply over 1". More like 1/8" or so. You may get some
ionization, but
not enough to create a full conduction channel in air. So, I think you
need a LOT
more voltage per inch, maybe about 70 KV/inch, to get full breakdown.

Anyway, sparks in air just make noise and light, but don't do much to metal.
Running EDM processes under a liquid seems to greatly enhance the metal
removal effect. You can use water or hydrocarbon fluids, although the
latter
do pose a fire hazard. I have had good results using about 30 V at an amp.

You need a capacitor to deliver a burst of current, and then to let the
spark
die out as the cap recharges from the power supply. A light bulb can do
pretty well for that.
See my el-cheapo EDM at http://pico-systems.com/edm.html

Jon

 

[Don Klipstein]

In article <800a9925.0312301242.d782a51@posting.google.com>, John Albers wrote:

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 believe it takes around 200, maybe 300 volts to break through air no
matter how little, and where the voltage and/or electrode geometry do not
favor corona and where the electric field within the spark gap is evenly
distributed, it takes about 75 volts beyond that 200-300 volt figure per
..001 inch (3 volts per micrometer), give or take a little depending on
temperature and air pressure.
I suspect EDM requires actual contact or some sort of assistance
("ignition pulses", UV, photoelectric effect, whatever) to spark at 80
volts.

- Don Klipstein (don@misty.com)

 

[Don Klipstein]

In article <3FF21177.1080508@artsci.wustl.edu>, Jon Elson wrote:

John Albers wrote:
SNIP
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 setup?

Probably not. My experience is you'll never get a fat spark from a TV
anode power supply over 1". More like 1/8" or so. You may get some
ionization, but not enough to create a full conduction channel in air.

That's a function of current. A milliamp or two of "average current"
will typically, in my experience, get the voltage drop down to
somewhere around 15-20 KV per inch. 30 milliamps will get the voltage
drop down to somewhere around 6-7 KV per inch. A few amps plus some fumes
of metal vapor will get the voltage drop to a few hundred volts per inch.

So, I think you need a LOT more voltage per inch, maybe about 70 KV/inch,
SNIP

Varies from approx. 1 to 3 KV per millimeter, or 25 to 75 KV per inch,
depending on how the electrode geometry affects evenness of the electric
field between the electrodes. Can be less volts/distance at higher
voltages over 40 or 50 KV or so.
I have heard 20 KV/inch before as some sort of a "1-size-fits-all".

You will usually need a couple hundred volts or so to spark through air
no matter how small the gap is because of the cathode fall of a "glow
discharge".
Voltage drop in a spark gap will be less once the temperature of the air
rises. That couple hundred volts "cathode fall" will "collapse" to a much
lower voltage once an arc process on the cathode forms.

- Don Klipstein (don@misty.com)

 

[Paul Victor Birke]

Don Klipstein wrote:

In article <800a9925.0312301242.d782a51@posting.google.com>, John Albers wrote:

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 believe it takes around 200, maybe 300 volts to break through air no
matter how little, and where the voltage and/or electrode geometry do not
favor corona and where the electric field within the spark gap is evenly
distributed, it takes about 75 volts beyond that 200-300 volt figure per
.001 inch (3 volts per micrometer), give or take a little depending on
temperature and air pressure.
I suspect EDM requires actual contact or some sort of assistance
("ignition pulses", UV, photoelectric effect, whatever) to spark at 80
volts.

- Don Klipstein (don@misty.com)

Interesting Don, it usually takes some minimum in oil also, I did not
know there was a lower I would say Weibull limit (called a location
parameter and usually >0 for many things!) here but it sounds correct.
There must be some empirical evidence here but this sounds like the
problem why it won't work for 80 volts no matter what.

Needs more of those 40 volts sources, say 10.
Paul

 

[William J. Beaty]

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

....in other words, you get no spark at all for voltages below 1.35KV,
and then the voltage rises like this:

Gap (millimeters) Voltage
0 1400
0.2 1900
0.4 2600
0.6 3200
0.8 3800
1.0 4400




For even better equations see:

HIGH VOLTAGE HANDBOOK
http://home.earthlink.net/~jimlux/hv/hvmain.htm

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?

Nope. As I understand it, in EDM the electrodes actually touch
together. Since they're not perfectly smooth, they touch in one
or two tiny spots. The discharge then blasts a crater in both
electrodes, the capacitor charges back up, and a spring pushes the
electrodes back into contact for a repeat performance.

And isn't this always done immersed in an insulating liquid? I'd
think you'd want to prevent the parts from welding themselves
together. A liquid would cool and quench out the arc, and also
prevent new arcs until the two electrodes were pushing together
hard enough to squeeze out the liquid from between the highest
solid points.

 

[Ed Huntress]

"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0312302309.708bc710@posting.google.com...

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

...in other words, you get no spark at all for voltages below 1.35KV,
and then the voltage rises like this:

Gap (millimeters) Voltage
0 1400
0.2 1900
0.4 2600
0.6 3200
0.8 3800
1.0 4400

That's all very interesting, but tap busters (primitive EDM machines that
use air dielectric, and that are used for removing broken taps and drill
bits in workpieces) typically have an open-circuit voltage ranging from 90V
to 120V. They'll spark without contact, although, given the servo mechanisms
on those things (often a hand crank <g&gt, it's hard to tell when you've
actually contacted the work.

FWIW, more sophisticated EDM machines, which use oil for dielectric
(kerosene with flash suppressants) have an open-circuit voltage that ranges
from around 100V to 350V. Those aren't the power-delivering cricuits. The
power supplies are cascade arrangements with high-impedance, high-voltage
circuits to polarize the gap; medium-impedence, medium-voltage (~90V)
circuits to ionize the channel and to initiate discharge; and low-impedence,
low-voltage (~15V) circuits to deliver the amperage that does the real work.

In those machines, the electrode never contacts the work. They have a servo
mechanism that maintains a gap on the order of 0.0002 - 0.003 in.

Ed Huntress

 

[William J. Beaty]

don@manx.misty.com (Don Klipstein) wrote in message news:<slrnbv446p.5jq.don@manx.misty.com>...

I believe it takes around 200, maybe 300 volts to break through
air no matter how little, and where the voltage and/or electrode
geometry do not favor corona and where the electric field within
the spark gap is evenly distributed, it takes about 75 volts beyond
that 200-300 volt figure per .001 inch (3 volts per micrometer),
give or take a little depending on temperature and air pressure.

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

Since the rest of the Paschen equation is simply the e-field needed
for air breakdown, the mysterious threshold voltage sounds like some
sort of microscopic "screening field" at the surface of the metal,
with half the threshold mystery-voltage appearing at each surface
(so 1350V would actually be 675V at each metal surface.) Since
any polarized air molecules near the metal surface will be attracted
to the metal by image effect, maybe the image effect somehow acts as
a proportionally-growing voltage barrier, and the air out between
the electrodes never even SEES any e-field until the 1350V threshold
is exceeded? Imagine something like a Helmholtz Double Layer
appearing against the electrodes, but in air rather than in
electrolyte?

If my above speculation isn't right, then there must be some sort
of mechanism which prevents the air in the gap from simply breaking
down when the usual value for e-field is reached. Why won't
the spark appear at a field strength of 30KV/cm? Or in other words,
what strange thing happens in the region within 1350/3000/2 = .23mm
from each metal surface which both prevents any air breakdown there,
and also prevents the air farther away from that region from breaking
down when 30KV/cm is reached?


(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://amasci.com
EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci

 

[Tim Williams]

"Martin Whybrow" <a_hole_in_the_ground@ntlworld.com> wrote in message
news:JvnIb.13519$FN.13309@newsfep4-winn.server.ntli.net...

If your set-up reaches a high enough field strength to ionise the air, the
current will almost certainly flow through the wood, it's not a good
insulator.

Eh? Wood was being used for an insulator for voltages well above this, well
before you were born! It won't be bothered by 80V.
The ionization comes from concentrating the field between the screws.

Add a capacitor because the transformers
have way too much resistance and inductance to get a healthy spark.

No, although healthier transformers is probably a good idea anyway. You
need an RC circuit because current kills the metal, and you get much higher
peak currents when you discharge a capacitor in microseconds than
continuously releasing the same energy (which BTW won't work for EDM...
you'll be arcwelding instead .

Tim

--
"That's for the courts to decide." - Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms

 

[Tim Williams]

"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?

Tim

--
"That's for the courts to decide." - Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms

 

[Reto]

It usually takes some minimum in oil also
Yes, defenitely EDM is normally not done in air but in oil

(die-sinker) or (deionized) water (wire edm).
Pay attention if you use oil, it could begin to burn (the spark must
absolutely not happen near the oil-surface but deep submerged). I
would suggest to use water to begin becaus it's harmless...
It is possible to erode also with 80V, but you need to get closer...
(~20 micrometers?).
If you don't want to use more power-supplys you could also build a
step-up converter. Since the spark-gap-voltage is about 20-25V
(depends on electrode and machined material),the best would be to use
a source lower than that voltage.
Diode
-----------Inductor--------->| -------------------
__|__ | __|__ | electrode
_____DC source _|/switch _____ C *
| (below 24V) |\ | _____Workpiece
---------------------------------------------------

The switch can be a NPN-bipolar transistor or a mosfet.
If you searches the web for a step-up converte circuit you'll surely
get a more compensible picture as my bad ascii-art...

 

[Pipper]

"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.