I broke a 4-40 tap in my filing machine casting. It was for the top cap, which is there to hold packing as a seal for the file rod, so not that big of a deal since there are five good screws. But it feels like a berry seed in my tooth. Aggravating.
I got Langlois' book on building a hobbyist edm, but have a few other thoughts. I have a very nice tig welder, Miller Dynasty 200DX. In tig mode, it'll go down to 1A DC or 5A AC, plus a pulser on dc up to
250 Hz and lots of control over ac frequency and wave form. Seems I might be able to use it in lift start mode for edm.
I'd still need to build the carriage, stepper drive, controls, etc. I have a Beaglebone Black which could be applied to that job. I can get a reasonable stepper for $11 from the salvage place in town, plus some linear bearings and a bellows coupler for not much.
It's an interesting idea. Basic EDMs deliver current from a big capacitor, however, so the energy in that spark is a lot greater than you think.
We had a tap buster (a proto-EDM) in my shop back in the '70s. I think we paid $25 for it, used. The power supply was a simple RC circuit and it was the size of a lunch box. It had a hand-cranked feedscrew rather than a servo. It worked very well. I spark-eroded a stud out of the head of my Honda motorcycle with it (a frequent need in those days
-- Honda's bolts and studs were made of frozen shit), and eroded several taps out of pieces we were working on. The hand feed made it kind of slow, but the real electric servo systems in EDMs are kind of special. Steppers are FAR easier to implement for this job than servo motors, which is why Sodick used them into the 1980s. If you want to know why, I'll explain.
What I don't know is if a welder power supply will do it. You might want to take a look at this description of early EDMs from Poco Graphite, which is close to being accurate:
I think you could try it out by hand, to see if you get the results you want. Take a screw and try repeatedly lift-starting on the end of it. See if it actually eats the screw quickly enough. If so, it will be worth taking the next step.
BTW, tap busters did not use liquid dielectric, with few exceptions. They just work in air. Be careful; hot globs can spit out of the hole.
There are some DIY edm plans on the internet. You might look at some of th em for ideas.
Many years ago, somewhere around 1969 a friend and I kludged up a edm. We used his drill press for the feed by setting the depth nuts and then leaned on the handle to feed the tool a very small amount. As I remember we used a good sized maybe 500 VA transformer and I think a voltage doubler. A l ight bulb in series with the supply had low resistanece when things were wo rking and a higher resistance when we had the tool shorting to the work. A kludge , but it worked and we could put square holes into tool steel.
We were charging the cap to about 300 volts. So a bunch higher voltage tha n your welder would supply, but the plans on the internet all use lower vol tage. So in answer to your qoestion , I do not know.
Thanks all. Lloyd, yeah I'm retired and I think it'd be educational, plus I already spent an hour or two picking and pecking at it.
Good ideas, Ed. I might combine that with Dan's drill press idea for a test. I'll try the welder at much lower voltage. If that doesn't work, then I'll ge further into Lanlois' design using some big caps.
Here's another bit of inside info that may be good to know about -- not that you're trying to build a real EDM, but it may be useful to know how they do it.
When they're using a hydrocarbon dielectric (kerosene, for all intents), the voltage required to initiate a spark ranges from 90V to
300V. Keep in mind that their parameters are for producing a highly accurate hole with minimum damage to the workpiece.
Around 1980, Agie had the world by the tail with a patented power supply that initiated a spark by using a *string* of lower-voltage pulses at high frequency. The voltage probably was around 90V. The Japanese avoided the patent on their export machines. They didn't worry about it on the machines they sold in Japan.
Anyway, Sodick, for example, broke the process into three steps, without the patented pulse circuit. There is more than one way to skin that cat. First, a high-impedance 300V circuit polarized the channel between the electrode and the workpiece, producing "stringers" of ionized dielectric that reduced resistance for the second step.
The second step was a medium-impedance circuit at 150V. This one began to turn the liquid dielectric into a low-resistance channel that delivered enough amperage to turn the dielectric into ionized gas. As it heated up, it became a plasma.
The final step was the money step. This one operated more like a welder, at least in terms of delivering power. This was a very low-impedance circuit (nearly a short circuit, with a big capacitor supplying the energy) at around 15V - 30V. This step in the power delivery used the plasma channel to deliver an extremely concentrated spark, which was delivered to the workpiece at a rate of thousands of amperes per square inch. The actual average amperage delivered through the whole cycle might be 30A.
Anyway, that's how they did it then. I don't know how they do it now. But it's useful to know that real EDMs employ *sparks*, rather than an
*arc*, because an arc is difficult to shut off and it can damage the workpiece.
For a tap buster, that's less of a problem. You'll get sparks, and also some arcs. You shut off the arcs the same way you stop it in arc welding -- by pulling the electrode away from the work enough to break the arc.
In real EDM, the retraction is only a few thousandths of an inch, at most. That's not enough to stop an arc, so there is a lot of power-supply and servo design directed at producing sparks but not arcs. Again, that will be less of an issue for busting taps, so you don't need all of that complication.
But it's good to know what's going on physically in those machines.
Some practice runs first, using similar materials of little value would probably be helpful...
When I was young and foolish I used to cut/burn out problem items similar to that with an acetylene torch. Nowadays I think I'm too cautious and worry too much about screwing up the good parts some how. Some practice first would be really helpful but it's against my nature to use up consumables in that manner...
Yeah, I'm nowhere near as steady on my best day as the guy in the video. I'll practice. A 4-40 tap is one tiny target.
I remember an old gunsmithing trick to remove a busted tap in a receiver; hit the tap with the oa torch, the tap will heat up much more quickly than the receiver. Then hit it with just oxy. One short burst and a shower of sparks... But I don't have the nerve to try that.
Don't let Jody fool you. He takes advantage of any sort of prop/bracing technique to steady himself that he can think of. He quite often will grab hold of the electrode (welding gloves on) while stick (SMAW) welding to keep it steady.
I've never had the opportunity to try TIG welding but would try resting the cup on something convenient for trying to build up your broken tap.
Yeah, that's the kind of stuff I remember doing as a dumb teenager. Bolt broke off in a casting, just blow it out of there with the torch. Never gave the consequences of screwing up a second thought... Of course I used to ride my motorcycle down the road back then at night too. Full throttle (~65mph), no headlight... if I stared straight ahead my peripheral vision would register the sides of the road. I tried to stay in the middle somewhere. I added a second headlight to my Honda Magna and would really rather not ride at night if at all possible :-)
This was a new Greenfield, so I can't blame the tap, and it was well in the hole, just my wobble. Also couldn't use my bench block. Maybe a hand tapping machine would be a better use of my time than an edm. We'll see how the welding goes after I practice.
I used to have a very limited aluminum production item with 14 4-40 holes that I tapped. After I spent a few hours mechanically removing broken taps, I switched it over to 6-32. My life was easier. Also soon after that I bought a HSS tap locally and that was the best tap I ever used in aluminum. I have limited metal working experience, just got lucky when I needed a tap. Don't really know what it was, just labelled HSS. Mikek
It's a shame they're going to the dump/scrap yard, but at $1k to ship plus a lot of space to set it up, it's no big wonder. Are smaller units built? I'll bet tiny EDMs find new homes a lot more easily, especially with hobbyists like us.