EDM

[Eric R. Skov]

I recently built an EDM based on the series of articles by Robert Langlois in HSM. It works very well. It has bored many holes in broken HSS bits and even tungsten carbide. I am new to this type of machine. I would like information and tips on using it. Also information on building the modulator circuit mentioned in part VI such as what is the freq. range this should run at, type and source of the power transistors and any other related information that might be available. Eric

[Thrud]

Can't help you much as I missed those articles and have not ordered the book yet.
Commercial units run in the 30-80KHz range if I am not mistaken. A quick call to a dealer could confirm this for you.

For a hobby unit HF may not be feasable - a lower modulation rate may work just fine resulting in a slower machining rate. Experiment.

Power MOSFET devices can handle more current, switch faster, and are less prone to thermal runaway than bipolar transistors. IRC and Motorola have Data books on their devices and have sample circuits listed for different applications.

[msrm]

EDM is cool. "EDM Digest" is a magazine
for professional EDMers. I'll get the
website info in a day or so that might help
you. Do you use dielectric fluid? If you
don't know what dielectric fluid is then you
probably don't use it. Just curious. I know
at work ventilation of the fumes is a big
deal. I was told that when wire burning
aluminum it gives off very bad fumes. Be
careful with the fumes when burning.
Burning is the term used to describe the EDM
process in our tool room, regardless if it is
wire or ram edm. Thrud I like your answers.

[Thrud]

I would like them better if I was not brain dead and could remember...

what were we talking about?

[Eric R. Skov]

Thanks for the replies. The EDM I built produces about 80VDC @ up to 8Amps. Probably pretty small by commercial standards. It has fried transistors rated 100v and 20 Amps so I'm doing something wrong. I'm using the dielectric fluid recommended by Mr. Langlois- 5% antifreeze (ethelyne glycol), 5% soluable cutting oil and 90% water. It seems to work well. I live in a rural area and have done all the burning out doors. I have noticed the fumes are very irritating when I do get a whiff of them. So far I have only tried copper and brass electrodes on HS steel and on tungsten carbide. The copper or brass electodes work well on steel, but not carbide. I tried a tungsten electrode on carbide and this worked well. Eric

[Bill Neufeld]

I see that the transistors you are using are rated at 100 volts and your supply is at 80 volts. This may not be enough margin to operate the transistors safely and they burn out. There may be switching transients greater than 100 volts. Try going to transistors with a higher voltage rating or mosfets as suggested by Thrud.There is a good basic article in MEW #80 on using mosfets. IR and motorola have web sites where you can get information on semi conductors.
Be safe
Bill

[msrm]

One website you can go to for EDM info of
a professional manner is www.edmtoday.org (http://www.edmtoday.org)
I typed in edm publications for a search
and got about 9000 items. good luck

[Eric R. Skov]

Thanks for The replies. I figured going to a professional web site on EDMs would be like nailing a couple of skate wheels to a board and the going to a Ford or GM web site
for skateboard improvements. But it was actually quite interesting. I didn't know graphite was a commonly used electrode material. In considering electodes, I had wondered pencil leads would work for small holes. Now I'll have to try it. As for transistors, I think I'll need much higher rated versions for them to work in the modulator. Probably transients of both voltage and current are much higher than rated values. For example one transistor switchs power from a capacitor filter bank 10,800 uF to a second capacitor bank of 1 to 10 uF. Probably a lot of current here for a brief moment. The second transistor discharges the smaller capacitor bank to the work at maybe 30 to 80K per second. Its beyond me what is actully going on, so I'll just try the larger hammer approach until I get it to work. There is a 35A 600V bridge on the output that senses the voltage-current to tell the stepper motor which way to go. It has suvived, so thats the specs I'll look for in my next try. Eric

[Bill Neufeld]

What you may also want to do is install a small value (.1 - 1 ohm) high wattage resistor to limit current levels that the transistor switches, this may also lower the transient levels you may have.

Be safe
Bill

[Eric R. Skov]

Bill,
You are right about the limiting resistor. As you may not be aware, the original EDM uses a current limiting resistor at the heart of the circuit. The modulator circuit, which is to replace the resistor, was of a more genenal nature without any componant values. I think I will place the modulator in seris to start. Actually, where I think I need help is in the power switching portion of this circuit. I could not get the oscillator portion to operate as described so I designed my own which does work, but I'm not sure if there may be problems in the power switching portion that I'm not aware of. I've ordered two MOSFETs rated 500V @ 43Amps, 450 Watt. It will be interesting to see how they fair. Eric