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Thread: Having Fun Learning to Gas Weld with a new Cobra 2000 torch!

  1. #11
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    When I said that a regular torch can do what this one does, it's true, but for many this torch will be easier to set and adjust. I've seen the demos of the Cobra torch, and they are impressive, at least in the hands of those that demo.

    As for mig and tig - tig was invented in the 30s I believe, developed for welding aluminum parts for aircraft. At the time, chrome moly for lightweight aircraft frames was welded with gas welding. I don't know when mig was invented, but it wasn't widely used in industry until the 50s and 60s, maybe even later.

    As for gas welding on a quarter panel - well most shouldn't, but... I attended a metalworking seminar put on by the old customizer Gene Winfield. He, with help from the class, chopped the top on a 41 Chevy in 1-1/2 days. Only one joint was welded with a mig - the rest were done with an O/A torch. It was obvious that Gene was very good with the torch, he uses an ordinary style torch made by Smith. There were several welds across nearly the entire top. He welds 2-3 inches and while the weld is hot, he planishes the weld. Then he welds another 2-3" and planishes, repeating until the joint is complete. This pretty much removes the weld bead and minimizes the distortion. Little to no grinding, the area is metal finished with a hammer and dolly, alternated with filing and sanding. He even welded the A and B pillars with gas, saying he could get better penetration on the double thickness there (and I think he's right on that car).

    I've also seen gas welding of aluminum body panels for a race car where similar techniques were used - no way could you do something similar with tig IMO. Secret to gas welding sheet metal is 100% access to the back of the panel as you go. Hard to do that with a trifive quarter panel.
    Last edited by Rick_L; 02-25-2017 at 05:07 PM.

  2. #12
    Registered Member chevynut's Avatar
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    Quote Originally Posted by 55 Rescue Dog View Post
    I don't know for sure, but steel melts around 2000 degrees, and a O/A torch is around 6000 degrees with a wider HAZ, but the MIG/TIG arc is 11,000 degrees plus, making for a hot, harder weld.
    To me, that makes no sense. A wider HAZ means more distortion because the metal shrinks more. A hotter arc will melt the metal faster with a smaller HAZ. I'm betting that if you did a Rockwell hardness test on a MIG weld versus a TIG weld with the same kind of filler rod/wire, the welds would be essentially the same hardness. Once the metal melts it has to cool....and it cools at the same rate whether using MIG or TIG. Hardness is increased in metals when carbide forms and is locked in place from quenching or quick cooling. Slow cooling anneals it because the carbide disperses. I have always believed that the MIG bead "appears" to be harder because it's usually thicker than a TIG bead. I don't think the cooling rate is much different. Same with an acetylene torch...you can make a smaller, flatter bead if you want to.

    One thing I noticed with the low 4/4 psi pressures, the gas flow, and usage seems like just a slight whisper of gas, while putting out a lot of concentrated heat in the very small flame cone with the Cobra torch.
    I don't buy that either. The amount of heat you put into the metal is dependent on the size of the flame which depends on how much gas you burn. You can use a small tip on a regular torch and get a small flame too. The pressure has nothing to do with anything, because it's the flow rate of the gas and oxygen that determines how much gas you burn, not the pressure. The valves control the flowrate on either torch. And the more gas, the more heat you get versus time. The flame is the same temperature regardless of how big it is, but there's a higher rate of heat transfer with a larger flame.

    Very small puddle that you can move any direction.
    Which you can get with a regular torch with a small tip. Nothing magic about a Cobra Torch, imo. All it does is mix acetylene and oxygen and burn it. It might "feel" different because of the configuration, but put a small tip on your other torch and you can probably do the same stuff.
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  3. #13
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    Assuming all O/A torches operate and do the same things, is like assuming all carburetors and fuel injections accomplish the same thing, and wouldn't make a difference in efficiency. Good or bad the Cobra torch is not the same as a regular torch. It is a different design. Here is the best explanation of how it works I have found so far.
    http://www.cutlikeplasma.com/faqs.htm

  4. #14
    Registered Member BamaNomad's Avatar
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    I agree with RD on this.. there's no way you could adjust a regular oxy/ace down to the fine/sharp flame as with this unit. The cut/weld heat is intense (obviously) but it's held to such a tiny area, there was no warpage at all that I observed.

  5. #15
    Registered Member chevynut's Avatar
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    An acetylene flame is an acetylene flame. All you need is the same size orifice and the same gas flow rate and you'll get the same flame. There's no magic going on there.

    Glad you guys like it, but I'll stick with my MIG, TIG, and plasma cutter. The only thing I use my oxy-acetylene torch for is heating stuff to bend or shrink it.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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  6. #16
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    Quote Originally Posted by chevynut View Post
    An acetylene flame is an acetylene flame. All you need is the same size orifice and the same gas flow rate and you'll get the same flame. There's no magic going on there.

    Glad you guys like it, but I'll stick with my MIG, TIG, and plasma cutter. The only thing I use my oxy-acetylene torch for is heating stuff to bend or shrink it.
    That's awesome you have a TIG and plasma cutter. This is just a alternative addition to having just a MIG. I would love to justify having those too, but I think this is a another need to learn technique, that has some benefits of it's own, for a couple grand less or more.

  7. #17
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    I tried the Henrod many years ago for just a couple of minutes, and wasn't impressed at the time. I had been using o/a torch and wanted less heat distortion, so I bought a tig welder, and was immediately impressed.

    This floor patch was welded with o/a torch using 5# pressure with a small smith torch and low pressure regulators
    I don't do much welding but I always want to have the best equipment and do the best job I can with do in what ever I'm doing.







    This floor patch on the same car was done with a tig welder, and the difference in heat is obvious.







    As far as planishing a tig weld, this quarter panel was taken off the car for good access and finished without any filler.



    There was actually two long welds



    The next pictures were wiped with reducer to add gloss.







    That took a lot of planishing with no cracking, and I would not want to try it with a mig.

    I'm not a metal shaper and seldom visit those sites, but that is the place to get the correct answers.
    Here are some very knowledgeable people discussing mig weld hardness.
    http://www.allmetalshaping.com/showthread.php?t=1287

  8. #18
    Registered Member chevynut's Avatar
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    Quote Originally Posted by 55 Rescue Dog View Post
    That's awesome you have a TIG and plasma cutter. This is just a alternative addition to having just a MIG. I would love to justify having those too, but I think this is a another need to learn technique, that has some benefits of it's own, for a couple grand less or more.
    I guess my point is I can't see why anyone would spend $500 on an acetylene torch when you can buy a decent MIG welder for $600 or so. Even a more versatile TIG can be had for not too much more, especially if you buy used.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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  9. #19
    Registered Member chevynut's Avatar
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    Quote Originally Posted by LEE T View Post
    That took a lot of planishing with no cracking, and I would not want to try it with a mig.

    I'm not a metal shaper and seldom visit those sites, but that is the place to get the correct answers.
    Here are some very knowledgeable people discussing mig weld hardness.
    http://www.allmetalshaping.com/showthread.php?t=1287
    Thanks for the great pics Lee. That TIG welding is pretty impressive. My problem with TIG welding sheet metal has been cleanliness. It's hard to clean a used panel and not get "volcanoing" from my experience. In fact, for some reason I have trouble trying to use 1/16" TIG rod at all and not get the "volcanoing" typical of a contaminated weld. I don't know why, unless my rod is bad. I typically use 3/32" and it works well on 1/8" steel.

    I read some of the stuff on that site you linked, and to me the most convincing argument for a harder MIG weld is the CO2 in the gas. If it adds carbon to the metal, it will be come harder. Everything else is essentially the same as TIG welding, just a different process and technique.

    And this guy on that link is totally FOS imo:

    "Mig welds are "harder" because the temps mig run at superheat the metal, effectivley above the boiling point, commonly around 10,000 degrees. Add to that the superfast air quench, and that results in a very large grain structure.

    It has nada to do with the gas used, or the wire, or the deoxidizers in the wire. Arc welding is not nearly as hot, although deeper penetration is achieved.

    Tig welding gets it well above melting temperatures, but well below the boiling point, and gas welding brings it generaly to just a few hundred degrees above melting. Also, the cooling rate is slower for each process listed.

    Marty "

    MIG and TIG are both electric arc welds, and I don't see why MIG would be any hotter than TIG at the same settings. Also, when you ANNEAL (soften) metals you get grain growth, so the "very large grains" he says makes a MIG weld harder is BS. Cooling rate is not substantially different either...why would it be? In fact, a larger bead would tend to cool slower.

    And his comment that "arc welding is not nearly as hot" as MIG is also BS. How do you get "deeper penetration" then? TIG welding temperature is around 11,000 degrees F and MIG temperature is similar since they're both arc welding processes. The boiling point of steel is around 6,000F.

    Here's some pretty good stuff on metallurgy and how it relates to welding:

    http://www.gowelding.com/met/carbon.htm
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  10. #20
    Registered Member chevynut's Avatar
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    Ok this is the best explanation I've seen for why a MIG weld is harder than a TIG weld. Watch the video...

    "The biggest factor influencing the weld integrity is the polarity used in the Mig welding process, electrode positive, this means that the electrons flow out of the base metal toward the torch and concentrate the majority of the heat there. This polarity is required by the Mig process to melt the constant addition of filler wire from the torch, but it leaves the base metal colder.

    By contrast, the Tig process uses the electrode negative polarity, where the electrons flow from the torch to the base metal, concentrating the heat in the base metal and mimicing the function of the oxy-fuel torch process.

    The biggest functional difference is that the Tig and oxy-fuel processes allow you to melt the base metal and filler metal together completely as you progress, whereas the Mig process progresses regardless of weld fusion. The Mig process heats the base metal to a lesser extent, sometimes barely melting the edge of the work pieces, but it melts the filler wire completely until a ball of fluid molten metal falls to the workpiece.

    Since the base metal is much colder than the molten ball of filler, the filler is quenched on contact until the base metal reaches sufficient heat to allow proper fusion to occur. Since most sheet metal welding is done in short bursts the base metal doesn't reach sufficient temperatures for proper fusion.

    In this video you can see the short circuit welding action and it is plainly evident that the filler is radically heated while the base metal remains relatively cold and the filler piles up on top of it.

    http://www.youtube.com/watch?v=UFFSJ...eature=related

    It is possible to get more complete fusion with Mig, but it involves running a short wire stickout and a long arc to generate sufficient heat, but the risks are blowouts, burn back to the contactor tip, and drastically increased spatter due to the longer distance the molten metal has to travel to the base metal.

    There are certain processes that allow you finer control over Mig welding parameters, such as those developed by Fronius;


    http://www.youtube.com/watch?v=_WrhW...eature=related

    This process is called CMT (Cold Metal Transfer) and is especially usefull for things like additive manufacturing and cladding where they boast low weld dilution. This is an exaggerated example of the above assertion, low weld dilution means that there is very little base metal melting and mixing with the filler metal. This is important if you want to build up lots of weld metal without heating the base metal very much, or welding dissimilar metals like steel and aluminum where the dilution would cause reactions, but it is the opposite of what we are looking for in welding sheet metal with complete fusion and resulting ductile joint.

    Cheers,
    Daniel "
    Last edited by chevynut; 02-27-2017 at 09:49 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
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    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

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