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Old 04-25-2017, 05:25 PM
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miniman82 miniman82 is offline
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EDM possibilities for gun rebuilding

Electrical Discharge Machining (EDM) is a process of spark eroding a conductive material using pulses of high current into a workpiece, in our case the cathode retainers in electron gun assemblies. An electrode moves up and down on a ram, bathed in a dielectric oil, controlling the spark gap to the workpiece until a cut is effected. In the case of our electron guns we're not exactly cutting through a workpiece, merely eroding the cathode retainer to the point where we can remove it from the G1 cup without damaging anything.

The cost of an EDM machine is quite high, I found a very old one on Ebay for around $1,000 and newer computer controlled machines are much more- in the neighborhood of $3-6k for a used one.

While $1,000 is not outside the realm of possibility for something like the Charmilles D1S currently on the bay, I have no idea what the capabilities of it are or whether it would be appropriate for a process like gun rebuilding. That being the case I've done some digging, and apparently a DIY one can be built for a few hundred dollars using common shop tools and electronics.

http://www.homebuiltedmmachines.com/

The site above sells a book and bare PCB to build a pulse type power supply for a homebrew EDM, and the ram control can be implemented by stepper motor controlled by small computer (Arduino or similar). This would allow us to get the cathode retainers out of the guns without risk of breakage to the glass multiforms holding it together, a task that currently is completely manual in nature (hand grinding) and carries risk of gun breakage.

Please discuss, I need input as to how feasible a stepper controlled ram would be. The rest is pretty simple, as far as filtering of the dielectric oil and power supply end of it is concerned. Also if anyone has info on the D1S machine mentioned above, I would be interested to know if it's able to do precision work like this. We are talking thousandths of an inch, not just punching a hole in a piece of sheet metal here.
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Old 04-25-2017, 06:02 PM
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Notimetolooz Notimetolooz is offline
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I would think putting the gun in the oil would cause problems. I think you can also do something like EDM using water, but that might cause problem also.
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Old 04-25-2017, 06:13 PM
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miniman82 miniman82 is offline
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It's already been done actually, the oil is a very light grade and gets cleaned off in an ultrasonic bath.
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Old 04-30-2017, 04:14 PM
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miniman82 miniman82 is offline
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I received a pair of books from Ben Fleming, detailing how to build EDM machines at home. These are relatively low power machines, but would be suitable for cathode retainer removal. There are 2 flavors: capacitive and pulse. Either one should work fairly well, the difficult part would be building a ram with tight enough tolerance. Stepper based high accuracy ones can be found on the open market, but the cost is a little high.
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Old 04-30-2017, 07:26 PM
Chip Chester Chip Chester is offline
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If you want a little head start, keep an eye open for 'tap disintegrators' on CL and used machine sales places. Used to bust up taps that have broken and stuck in workpieces, but they're apparently baby EDM machines with somewhat looser precision. If you're retrofitting some position control, you could get most of what you need for not too much.

If you want the serviceability or challenge of starting from scratch, it's been done quite a few times. You might browse CNCZone for some play-by-play.

Chip
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Old 05-01-2017, 05:23 PM
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miniman82 miniman82 is offline
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Money would be better served by buying a real EDM and retrofitting it, a better power supply and tighter ram control are relatively easy. I saw one on ebay for only $1200, shipping is the problem.
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Old 05-01-2017, 07:45 PM
Chip Chester Chip Chester is offline
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I've been known to move a machine or two... Locally, though, not over in MD. Shipping can be handled if there's a knowledgeable person on the far end to oversee any needed disassembly/protection and crating. You can rent a truck and go fetch, but your standard smaller rental truck is not outfitted well for tying down hunks of cast iron. Ultimately, size matters.

Chip
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