Home Made Plasma Cutter

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Old 10-01-04, 02:50 PM
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Home Made Plasma Cutter

I probably should have searched the threads first but I'm here now. Anyone ever tried to make or know of anyone who made a DIY plasma cutter? It doesn't seem too difficult to me but what do I know? It seems to be similar to arc-gouging of the '50's 7 '60's. The trick, I think, is to make the plasma with a high intensity acr and force air into the molten puddle to blow it away. I've heard of someone who made a TIG with an automotive alternator and 5-HP motor.

Any takers?

DLee
 
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Old 10-24-04, 04:39 AM
tesseract
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Hey NHengineer. I did a little research myself on how plasma cutters work and this is what I found: The OCV is, conveniently, about what you'd get if you rectified and capacitor-filtered a 240V line (or full-wave doubled a 120V line), or about 370VDC. The sustained arc voltage is around 90VDC - which is about double that in stick welding. The big voltage drop comes from inductive reactance and resistance in series with the torch electrode. The arc is initiated by a high voltage pulse - a capacitor dumped via pushbutton into an automotive ignition coil would be a good and cheap way to do this. So far, so good, and none of it particularly complicated or difficult to achieve (though at 60Hz that series inductor is going to be mighty heavy). But then there's the torch head! It's probably hard to beat the cost of a commercially made unit, especially since the consumable parts are available off-the-shelf, which was what finally dissuaded me from rolling my own. Also, I hadn't yet figured out how to make the pilot arc for cutting expanded metal, and that was basically the last straw that convinced me to buy a commercial unit (used on eBay: a Lincoln ProCut 25). Ironically, I now have a partial schematic of the Lincoln that makes reverse-engineering it much simpler

Have at it, and I, for one, am interested in what you may find!
 
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Old 10-24-04, 01:34 PM
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Originally Posted by tesseract
Hey NHengineer. I did a little research myself on how plasma cutters work and this is what I found: The OCV is, conveniently, about what you'd get if you rectified and capacitor-filtered a 240V line (or full-wave doubled a 120V line), or about 370VDC. The sustained arc voltage is around 90VDC - which is about double that in stick welding. The big voltage drop comes from inductive reactance and resistance in series with the torch electrode. The arc is initiated by a high voltage pulse - a capacitor dumped via pushbutton into an automotive ignition coil would be a good and cheap way to do this. So far, so good, and none of it particularly complicated or difficult to achieve (though at 60Hz that series inductor is going to be mighty heavy).
Hello Tesseract,

If I can figure out how, I'll attach a schematic for a DIY TIG HiFreq made from stuff like an oil burner transformer, sewing machine foot pedal, PC power supply transformer core, etc. It was designed by Chris McKinnon on YAHOO's Welding Projects group. I'm about to make a DIY TIG and my next project will be a DIY pasma cutter. Do you think it's possible to use the same Hi-Freq circuit?

NHEngineer

PS:
OK, I can't figure out how to do it. If you're interested, write to me (link below I think) and I'll send it to you.

[EMAIL[/ EMAIL Address removed by moderator
 

Last edited by Ed Imeduc; 10-24-04 at 04:17 PM.
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Old 10-24-04, 02:56 PM
tesseract
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There's no need for a HF arc sustaining circuit in plasma cutting because DC is used - you only use HF in TIG when the arc output is low frequency AC. I think HF arc sustainment has largely been supplanted - by what, I don't know! - in TIG these days, anyway. Probably the result of the HF wreaking havoc with computers, radios, etc...

In plasma cutting, high voltage is used so that one doesn't have to "scratch start" the arc. The pilot arc feature maintains an arc at the torch - usually at half of the machine's rated amperage - that is then transferred to the workpiece when the torch is brought close enough to it. I imagine this is done by duty cycle control in inverter based plasma cutters, but I've no idea how it's managed in one's that derive their arc output straight from the AC line.

Once again, though, I concluded that if I had to roll my own torch head to make it economical then it likely wouldn't be economical. That said, the 25A maximum output of the Lincoln ProCut I bought is just barely adequate for my uses. I'd be much happier with about 35-40A of output, but no way can I afford (justify) the cost of such a commercially made unit!
 
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Old 11-22-04, 07:44 PM
ehrichweiss
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Originally Posted by nhengineer
Hello Tesseract,

If I can figure out how, I'll attach a schematic for a DIY TIG HiFreq made from stuff like an oil burner transformer, sewing machine foot pedal, PC power supply transformer core, etc. It was designed by Chris McKinnon on YAHOO's Welding Projects group. I'm about to make a DIY TIG and my next project will be a DIY pasma cutter. Do you think it's possible to use the same Hi-Freq circuit?

NHEngineer

PS:
OK, I can't figure out how to do it. If you're interested, write to me (link below I think) and I'll send it to you.

[EMAIL[/ EMAIL Address removed by moderator
Hi,

I'm interested in this. I'm guessing that we need to find a way to exchange emails without going through the system here since your email address was removed. Any ideas? I can't tell if there are "Private message" functions here yet...
 
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Old 11-23-04, 05:28 AM
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Originally Posted by ehrichweiss
Hi, I'm interested in this. I'm guessing that we need to find a way to exchange emails without going through the system here since your email address was removed. Any ideas? I can't tell if there are "Private message" functions here yet...
Go to xxxxxxxxxxxxxxxxx Search for junkyard projects. Leave a post there and I'll send the schematic I have. I guess this group doesn't allow links.

Regards,
David Lee
 

Last edited by twelvepole; 12-18-04 at 12:40 PM. Reason: Directing discussion off DIY.com forums
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