Where's the spark ??

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

Reply to
Andrew Mawson
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Hey Ed,

Next time you're up this way, com'on over and watch, and you tell me. I'm just going by what I see happening in my buddies tool and die shop. He has quite a variety,, maybe 8 EDMs, (mostly sinkers) from

24" X 36" tables and 100 gallon tanks and full manual operation, to full CNC 2500mm X 1000mm tables with 4,000 litre plus filter tanks, and 5 Axis CNC wire (can't guess at that size, but not huge). You can't see where the burn is if it's "in" the oil already. Used mostly to burn ribs in the tools. He's even got one "home-made" some years ago that was made from a big old planer, and uses the gantry for holding the hot rods. Used for big heavy tools.

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

Take care.

Brian Laws>> >>

Reply to
Brian Lawson

Hey Andrew,

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

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

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

Take care.

Happy New Year.

Brian Laws>

Reply to
Brian Lawson

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

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

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

Ed Huntress

Reply to
Ed Huntress

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

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

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

or vapors thereof!!!

Merely does not apply for maintenance of an arc or "an established repeatedly-sparking discharge". (Most brief/intermittent high current sparks utilize the "cold cathode arc" process for getting electrons off the cathode into vapor/air. There is another "arc" cathode process known as "thermionic arc". A few arcs have significant extent of both processes

- known to shorten the life of underpowered/"dimmed" fluorescent lamps if extra measures are not taken! - see

formatting link

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

Hey again Ed,

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

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

Take care.

Brian Laws>> Hey Ed,

Reply to
Brian Lawson
************************************************************************ Dear Brian and All

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

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

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

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

BTW contact theory seems arcane and dealt with years ago

Paul Birke, EE in Guelph ON

all the best, Brian

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

Brian Laws> Hey again Ed,

Reply to
Paul Victor Birke

Paul and all,

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

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

142-146, or Holm, R., "Electric Contacts, Theory and Application", Spinger-Verlag, 1967).

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

-- Bert --

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Reply to
Bert Hickman

Interesting and enlightening.

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

Yes and the point is there are many more especially and new work area is processed underneath the electrode.

. very well said!

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

good stuff Bert--looks close!!

all the best

Paul

Reply to
Paul Victor Birke

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

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

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

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

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

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

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

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

Ed Huntress

Reply to
Ed Huntress

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

Gary

Reply to
Gary Coffman

Yeah. An' stuff like that.

Ed Huntress

Reply to
Ed Huntress

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