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Thread: A DIY plasma cutter

  1. #1
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    Default A DIY plasma cutter

    I saw this video posted on another site. Thought some of you might find it of interest. It is fairly crude, but it seems to do a decent job. All there is that I can find is this you-tube video about it, but the guy shows his hand drawn schematics at the end. I am guessing the majority of the cost was purchasing an off the shelf torch. I would still rather have this home built critter than a chinese POS cutter...

    Link--DIY Plasma

    Later,
    Jason

  2. #2
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    He wont post his schematics and what he does says about it makes me think its a death trap. Bucket of water for a resistor? Oh boy..

    You need about 200v OCV. I am going to assume he is just rectifying line current (Probably 240v) and running that though the "resistor" and inductor. And a small hf unit to start.

    This violates all sorts of codes for a plasma cutting machine. Very unsafe to use.

    EDIT: He has the schematic in the video. I was right. He is just rectifying mains. DO NOT BUILD THIS!
    Last edited by macona; 11-09-2008 at 03:53 PM.

  3. #3
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    Macona, you are a tech/engineer for one of the big US brands, aren't you? Out of curriosity, how is it normally done? Obviously no water resistor, I am sure current control is solid state. Transformer isolated from the mains, right?

    To clarify, I am not going to build it. I own a powermax 1000g. This falls in the same catagory of the "make an xray machine" thread that popped up a few years back. But if someone were to want to try, can you give any pointers, or is it just something that a HSM should not attempt, like staring into the sun, DIY x-rays, and surgery?

    Later,
    Jason

  4. #4
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    Default Say what?

    No diy surgery? Up here in Canada it's about the only way to get it done in good time.

  5. #5
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    On the oldest and simplest machines they used a current limited transformer. The voltage out of the transformer depends on the torch used. Most torches like to see about 100v loaded and about 200v OCV, some machines have OCV as high as 300v. Output of that is usually through a contactor and then to rectifier. From there to the torch through the output inductor. Depending on wether the machine is a blow back or a HF start there may also be a HF coupling coil in line. Positive goes to ground, is split in to two with a contactor and a big power resistor to the tip. Negative goes to the electrode.

    In a HF system you hit the button, gas flow starts, contactors pulls in and HF comes on after a couple second delay which starts the pilot arc. Some machines will have a sensor on the pilot wire going to the tip to shut off the HF once the pilot is lit. The torch is brought to the work and then "transfer" occurs where the arc travels from the electrode through the tip and into the work. Once current flow is detected through the ground lead the pilot contactor opens and takes the tip out of circuit. Then your cutting.

    When you let go all the contactors open and a timer keeps the air on for a cool down period.

    In a blow back system there is no HF. The electrode is a plunger that is spring loaded into contact with the tip at rest. When you hit the trigger power is delivered across the tip and electrode and then the air comes on a couple seconds later. The pressure pushes the electrode away from the tip and initiates the pilot arc. After that operation is the same.

    Here is a miller zip-cut from the mid-80's. About as basic as you can get. Schematic is on page 21.

    http://www.millerwelds.com/om/o284c_mil.pdf

  6. #6
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    Quote Originally Posted by Dunc
    No diy surgery? Up here in Canada it's about the only way to get it done in good time.
    I have kind of gathered that from Torkers posts

    Macona, thank you for the detailed explanation. I have always wondered what sort of voodoo went on in my machine Do you happen to know if the Hypertherm Powermax I have is HF or blow back? I was under the impression that it was HF start, but I searched for this info some time ago, for no reason but to satisfy my curiosity, and I could not find any details.

    As for the home built rig in question, is there any hope of making it safe, or is it just a lost cause? Is it to be considered unsafe due to the lack of disconnects, mains isolation, and the huge bucket of kill me water? Obviously, the people something like this is going to appeal to are the curious, the tinkerers and, well, the average HSM... If there is no way to make it safely, then we'll leave it at that, but in my experience, we HSM guys will build a lot of stuff that is dangerous, or outright deadly, even if we shouldn't.

    Obviously, answering these questions may well be a conflict of interest for you, and that is understandable, we can let it die. I was simply impressed at the ingenuity and the outcome.

    Thanks,
    Jason

  7. #7
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    You need an isolation transformer of some sort. By the time you do that you can buy a chinese HF inverter that will be safer and work better than the one you are building.

    I believe the Max40 was the last small machine out of hypertherm that has HF start. All of the big guys like the Max 200 on up have HF start though. When you get to those high of currents having your connections held together by spring tension is not so hot of an idea.

    Pretty much everyone has gone to the blowback system. Hypertherm, Miller (Uses hypertherm torches), Lincoln, Thermal Dynamics. Esab is about the only holdout on HF start.

    And whatever you do, never, never buy their (Esab) small machine with a built in compressor. Damn think kept shocking me with the HF when I was trying to cut a hole in a panel. And then it barely worked!

  8. #8
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    It isn't particularly more dangerous than many things we do. It does come with a lot of baggage but once understood it works fine. In my mind, flying an airplane is dangerous and yet I've done it hundreds of times and enjoyed it immensely. I don't recommend it for everyone, but for those of us who have mastered it there's nothing to compare. This plama cutter is in that class. It's only a matter of mastery.

    FWIW, water resistors have been used for a very long time to control current in circuits that have a low impedance source voltage. They're efficient, cheap, and easily replaceable. Another good ol' boy testing with water: http://www.youtube.com/watch?v=fSyNaFy25eE

  9. #9
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    The main issue is the lack of isolation. but the ground clamp and the torch are at 120v refereced to ground. That means conecting that ground clamp makes the work electrically hot. Get yourself between that and some sort of ground and you are toast. Also there is no current limiting on the ground side so if your work does come into contact its a direct short to ground blowing the mains breaker. He dosnt even have fuses on this thing!

  10. #10
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    I have to agree with Macona, this is a death trap, and all the disclaimers in the world will not save him from a even half a$$ lawyer. If you read all the comments he does tell people that his machine will kill you. He seems to quit answering questions. After a couple of pages of the same questions from people that have no clue or idea. And that any contact between you and the equipment will result in death. That is a clear admission of prior knowledge of the deadly results. So save up and get something that will work and won't kill you.
    After all this is the internet, Just because its out there, doesn't make it good, safe or useful.
    Last edited by PTSideshow; 11-10-2008 at 12:56 PM.
    Glen
    Been there, probably broke it, doing that!
    I am not a lawyer, and never played one on TV!
    All the usual and standard disclaimers apply. Do not try this at home, use only as directed, No warranties express or implied, for the intended use or the suggested uses, Wear safety glasses, closed course, professionals only

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