hard black plastic?

I wrote most of McGraw-Hill's old book on non-traditional machining; I was the EDM editor at _American Machinist_; and I was the US Marketing and Sales Manager at Sodick. I also gave Mitsubishi's East-coast lectures on wire EDM. When we were hard-pressed at Sodick, I took off my tie and made board-level repairs to power supplies.

But, as with many other things, my knowledge is becoming something that's mostly of historical interest. Hell, *I'm* becoming something that's mostly of historical interest. Just ask my wife. d8-)

As for the high voltage, that was just one way to do it. If you have patience, you can get sparks started with lower voltages, and the very earliest EDMs had only one capacitor bank, which compromised several things, including the time between sparks. But when Agie came up with sophisticated, heavily researched power supplies during the '70s, the Japanese, particularly, developed some work-arounds that kept them from violating Agie's patents (more or less -- but that's another story ) while competing with them on performance. Sodick's approach was common, although they did it particularly well for the time.

Early wirecut machines tended to have pretty high initiation voltages. I think the Andrew was in that category, but I'm not dead sure. It was a good machine and an excellent buy at the time. It also was one of the few American-made wirecut machines ever.

-- Ed Huntress

Mine measures -206 volts when you turn the burn on but haven't touched metal yet. I'm not sure I remember correctly, but I think he said up to 50,000 pulses per second while cutting. Apparently the voltage is very high for a few microseconds at the beginning of each pulse.

Karl

Yeah, that's the way. The high voltage ionizes a channel in the dielectric for the current to flow through, (the "spark"), which occurs at low voltage. I think the typical voltage during the spark is around 15 - 18 V.

-- Ed Huntress

Not porous per se, but has a high water absorption, 6 percent or more when saturated vs less than 1 percent for most polymers.

Hmm ... that must have been Ma Bell's 500 series phones. I know that the 300 series had aluminum (or brass) dials painted black. I don't know if I ever *saw* a 400 series phone.

But I have some of these still around here -- even some connected where I don't need to initiate calls. :-)

Enjoy, DoN.

What was Agie's approach? Those patents are expired now.

What was Sodick's approach?

Don,

I bought a booklet that is a copy of all WEDM patents though the '90s. Too technical for me. If you're interested, I'll mail it to you.

Karl

I'm hoping you just want a general description, and not design info, because I've long since forgotten it and some of the details were secrets that I never knew. But here's the general picture:

Let me first describe the EDM spark cycle as consisting of three stages. The first is ionization and polarization of molecules of liquid dielectric. As the molecules become polarized they start to flow; this flow, called "streamers," starts heating the liquid channel according to the IR drop. The second stage is low current flow via the streamers, within this "channel," that vaporizes the dielectric and then ionizes the resulting gas, forming the low-resistance channel that allows high current flow in the third stage. The third stage is an extremely high rate of current flow through the ionized gas, originating in a low-impedance circuit of the power supply. This is the spark that does the eroding of the workpiece.

Agie's breakthrough power supply design (it had a name -- Robo-something or other), which came along in the mid-'70s, used an oscillator to produce a string of pulses at the beginning of a spark cycle. These were fairly high voltage, high impedance pulses, but the voltage was much lower (IIRC) than that of other EDMs of the time. The pulses apparently were very quick and effective at starting the ionization of the liquid and they could be reliably controlled. The control is the key to producing a high duty cycle and, thus, high cutting rates at relatively low rates of current flow in the later stages of the cycle.

Sodick and others used an advanced version of earlier power supplies, without the oscillation. (That part was an Agie patent.) They had a separate circuit for each of the three stages: high voltage, high impedance; medium voltage, medium impedance; and low voltage, low impedance. They used transistors to switch each of the stages *off* at the appropriate time in the cycle. Timing circuits for each stage were adjustable, although not all of it was user-adjustable. Switching these circuits *on* was not actually under direct electronic control; the physics of the process determined when they turned on.

I think you can see in general how these circuits apply to the three stages of the cycle. With this system you had pretty good control, although the physics of the process limited what you could do electronically. Further advances in power supply design, since the '70s, could be thought of as efforts to "force" the physical dynamics to conform to desired timing, voltage, and current flow, and to do it with the highest possible predictability and reliability.

If you're interested in improving the RC relaxer circuits used in home-built EDMs, there is an earlier stage of commercial development that holds more promise and *could* be within the realm of a hobbyist who's knowledgeable about electronics. Before these power supplies I've described came along, the standard approach was an RC circuit with an electronic switch -- first tubes, then switching transistors -- to turn off the high current flow. This made EDM much faster, allowed finer finishes, and improved the process a great deal over the earlier, plain RC designs.

To understand how and why they worked is another story. I'll just point out the physics involved. The elemental RC power supply is a current source, a series charging resistor, a capacitor, and a second series resistor, which is the EDM channel itself, with the electrode and workpiece forming the terminals, and the channel being the resistor material. The key to the process is that the resistance of this "resistor" drops with changes in voltage, due to the ionization of the channel. It starts out behaving like a low-value capacitor, in other words, and then it becomes a resistor as current starts to flow.

The storage capacitor is charged through the charging resistor until the voltage on the capacitor is high enough to start the ionization process at the electrode-workpiece interface. Then the cap discharges through the channel, eroding the workpiece in the process. The whole process depends on getting all the values -- charging resistance, capacitor size, and electrode-workpiece gap -- in the proper balance for the process to proceed.

The limitation of this simple circuit results from the resistance of the cutting channel immediately dropping to a very low value. If the value of the charging resistor is too low (and you want it to be as low as practical, because its value determines cutting speed -- lower value means faster cutting), and if the channel resistance becomes too low from high current flows, the spark doesn't stop; it becomes a continuous arc, and you've just turned your EDM into a welding machine that wrecks the workpiece and the electrode. This is the dreaded "arc" that was the bane of EDMs before fancier transistor-controlled power supplies came along. A simple hobbyist machine must be run at very low cutting rates to prevent this from happening. That means that the charging resistor must have a higher value than you might prefer, and it also places limits on the size of the storage capacitor, because a big one will allow the EDM channel's resistance to drop through the floor.

I hope you're following all this. I haven't tried to explain it for years. If you're with me, you can see how a switching transistor (or vacuum tube) in the circuit can be used to great benefit if it switches off the current flow under direct control, using something as simple as a 555 timing circuit that's triggered by the high current pulse at the start of the cut. It allows low charging-resistor values, larger storage capacitors, and high current flow rates, and it keeps the sparks from turning into arcs.

In theory. In practice, it works pretty well, but not perfectly. The famous old Elox EDMs (vacuum tubes) and the iconic Charmilles D20 (transistors) of the 1960s were practical and effective machines that made EDM the practical process that it is today.

Two more things: Notice that I'm not distinguishing between wirecut and ram-type EDM power supplies. The differences are in the values of the circuits more than in the concepts. Wire EDMs can be a little more brutal on the power supply end because electrode wear is not a problem. You toss the wire after it makes one pass.

The other thing to note is that EDM has developed from experience, not from theory. The companies involved threw things at the wall until something stuck, and then tried to figure out later why it worked. It's led to some screwy theories that seem to contradict each other over time. The Swiss were particularly good at buggering the story up. The point being, don't get too hung on the theories. And face the fact that EDM has been developed by people who had well-equipped labs and lots of financial incentive, not by hobbyists. It really doesn't lend itself to easy, low-cost experimentation.

Oh, I should mention that I haven't looked at the hobby designs for at least five or more years. so someone may have hobby-level designs incorporating transistors. If so, I wouldn't try to re-invent them, if my object was to build a homemade EDM that worked and actually cut things.

Good luck, if that's what you have in mind.

-- Ed Huntress

I got a second "parts machine" with my EDM. The spark supply boards in this one are an earlier generation design and not compatible with my machine. They would be perfect for a homebuilt unit. there are sixteen boards for 32 power levels (each board can be 1/2 on). Free for the asking.

Karl

If it is of no use to you I would like a look at it. Reading patents can be like swimming thru peanut butter but I do have some experience.

Thanks!

Thanks for taking the time to write this, Ed.

I've thought about doing a "hobby" EDM for years but I don't really need one. I now have a friend with a real EDM! I was able to follow your clearly-written treatise with no difficulty.

I would definitely "re-invent" hobby level design for myself. Some stuff found on the web is quite good but some of it is abysmally bad design tinkered into existance. I do have some degrees in EE and a bit of experience in design and as a research puke. I also have a benchstock of suitable transistors --like 800-volt 50-amp MOSFET's courtesy of my son the silicon peddler.

In a sort of EDM experiment some years ago, I was surprised to discover how high the conductivity was of "distilled" water from Wal-Mart. I was also surprised at how much the inductance of leads affected rise time of immersed sparks as observed on a scope. I had my apparatus on the mill so I could use the quill feed (with dial indicator) as a calibrated part of my experiment.

Good. If I lose that, I'll have to look for other work.

If you do give it a try, you'll find that your experience with stray inductance tends to get multiplied in an actual machine. It's a real problem, and some of the older machines won't work at all if you ground the machine chassis. This leads to some surprising jolts. You may find that you have to tune out the inductance by adding some experimentally derived capacitance.

FWIW, I think that water dielectric is more problematic for a homebrew machine than oil dielectric is. The water is quickly re-ionized and needs some kind of attention, like a deionizing filter.

-- Ed Huntress

Phenolic plastics were the first, and still the commonest, thermosetting plastics (they take heat well).

The earliest, inventor Baekeland used with lots of asbestos as reinforcement and filler. That is not a currently available product.

Bakelite (Union Carbide), and Formica (General Electric) and Micarta (Westinghouse) are all tradenames for phenolic plastics, and nowadays use either wood-fiber (paper) or for high performance, linen reinforcement/filler.