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| Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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Where's the spark ??
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 |
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#2
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Where's the spark ??
"John Albers" wrote in message
m... 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) |
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#3
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Where's the spark ??
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" wrote in message m... 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 |
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#4
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Where's the spark ??
"Tom Gardner" wrote in message gy.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" wrote in message m... 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 |
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#5
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Where's the spark ??
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" wrote in message m... 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 |
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#6
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Where's the spark ??
"John Albers" wrote in message m... 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 -- martindot herewhybrowat herentlworlddot herecom |
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#7
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Where's the spark ??
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 |
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#8
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Where's the spark ??
"Paul Victor Birke" wrote in message ... 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. |
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#9
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Where's the spark ??
In article , 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 ) |
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#10
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Where's the spark ??
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" wrote in message ... 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" wrote in message m... 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 |
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#11
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Where's the spark ??
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 |
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#12
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Where's the spark ??
In article , 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 ) |
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#13
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Where's the spark ??
Don Klipstein wrote: In article , 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 ) 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 |
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#14
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Where's the spark ??
(John Albers) wrote in message om...
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. |
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#15
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Where's the spark ??
"William J. Beaty" wrote in message
om... (John Albers) wrote in message om... 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), 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 |
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#17
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Where's the spark ??
"Martin Whybrow" wrote in message
... 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 |
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#18
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Where's the spark ??
"William J. Beaty" wrote in message
om... 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 |
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#19
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Where's the spark ??
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... |
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#20
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Where's the spark ??
In sci.physics.electromag Pipper 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. |
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#21
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Where's the spark ??
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" wrote in message m... 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. |
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Where's the spark ??
"Paul Victor Birke" wrote in message
... 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 |
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#23
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Where's the spark ??
On Wed, 31 Dec 2003 16:17:18 -0000, "Pipper" 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 |
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#24
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Where's the spark ??
Ed Huntress wrote: "Paul Victor Birke" wrote in message ... 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 |
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#25
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Where's the spark ??
"Tim Williams" wrote in message ...
"William J. Beaty" wrote in message om... 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. |
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#26
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Where's the spark ??
"William J. Beaty" wrote in message
om... "Tim Williams" wrote in message ... "William J. Beaty" wrote in message om... 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 |
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#27
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Where's the spark ??
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" wrote in message om... "Tim Williams" wrote in message ... "William J. Beaty" wrote in message e.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 |
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#28
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Where's the spark ??
"Paul Victor Birke" wrote in message
... 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 |
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#29
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Where's the spark ??
Ed Huntress wrote:
"William J. Beaty" wrote in message om... "Tim Williams" wrote in message ... "William J. Beaty" wrote in message e.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 -------------------------------------------------------------------- |
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Where's the spark ??
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" wrote in message ... 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 |
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"Bert Hickman" wrote in message
... 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 |
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Where's the spark ??
"Paul Victor Birke" wrote in message
... 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/pub...PaperNo_89.pdf Happy New Year To you too, Paul. -- Ed Huntress (remove "3" from email address for email reply) |
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Gary Coffman wrote: On 30 Dec 2003 23:09:40 -0800, (William J. Beaty) wrote: (John Albers) wrote in message om... 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 |
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Where's the spark ??
Gary Coffman wrote: On 30 Dec 2003 23:09:40 -0800, (William J. Beaty) wrote: (John Albers) wrote in message om... 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 |
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Where's the spark ??
Gary Coffman wrote: On 30 Dec 2003 23:09:40 -0800, (William J. Beaty) wrote: (John Albers) wrote in message om... 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 |
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Where's the spark ??
Gary Coffman wrote: On 30 Dec 2003 23:09:40 -0800, (William J. Beaty) wrote: (John Albers) wrote in message om... 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 |
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Where's the spark ??
On Fri, 02 Jan 2004 01:48:55 -0500, Gary Coffman
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. |
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"Brian Lawson" wrote in message
news  On Fri, 02 Jan 2004 01:48:55 -0500, Gary Coffman 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 |
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#40
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Where's the spark ??
"Ed Huntress" wrote in message . net... "Brian Lawson" wrote in message news On Fri, 02 Jan 2004 01:48:55 -0500, Gary Coffman 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 |
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