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  1. #41
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    Default Re: Welding on a vehicle

    This subject is a lot more complex than one thinks.

    If one connects electrodes to a cars body and carefully and slowly crank up the current, you will get volt drops developing, pretty much predictable and measurable, and probably with no ill effects.

    Unfortunately we don't weld like that, there are aggressive and severe start of arc and end of arc surges, and even a continuous weld is actually not a steady current but a series of high speed arc, recognized by that crackling sound.

    Every time a current is induced, or current collapses, we generate back EMF. A 20V source can generate back EMF of hundreds or even thousands of volts.

    Think about a cars spark coil. You saturate the coil with current, when the points open the current collapses, and magic, 12V becomes 15K Volts. Every electrical path has the potential (sorry) to generate back EMF when faced with a changing current. dI/dT can be a devastating master to the untrained.

    Combine with high back EMF there is also magnet induction. currents flowing are electromagnets. Little EMP generators if you will. Try bringing an AM radio near an arcing electrode and see how much noise there is. That noise can be induced into cabling.

    -

    @ Marius, that reference to Earth somebody used, applies 1000% to DC in a car body. In this case the car body is both Earth and the conductor, which in and of itself should sound an alarm bell.
    Last edited by Fluffy; 2021/09/22 at 10:20 PM.
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  3. #42
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    Default Re: Welding on a vehicle

    I would disconnect at least the battery.
    The ECU is not so easy.

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  4. #43
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    Default Re: Welding on a vehicle

    While they are going about that I'll only say protect your paint work.
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    Default Re: Welding on a vehicle

    What about holding the workpiece with a bare hand when welding ?
    Kobus

  6. #45
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    Default Re: Welding on a vehicle

    Quote Originally Posted by KobusDJ View Post
    What about holding the workpiece with a bare hand when welding ?
    Soos bo gestel, die stroom is lui en volg die pad van minste weerstand - die metaal. Maar staan nou kaalvoet in 'n plas water en weld met 'n nat hand, dan gaan mens beslis 'n paar rukke kry.
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  8. #46
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    Default Re: Welding on a vehicle

    Quote Originally Posted by Fluffy View Post
    This subject is a lot more complex than one thinks.

    If one connects electrodes to a cars body and carefully and slowly crank up the current, you will get volt drops developing, pretty much predictable and measurable, and probably with no ill effects.

    Unfortunately we don't weld like that, there are aggressive and severe start of arc and end of arc surges, and even a continuous weld is actually not a steady current but a series of high speed arc, recognized by that crackling sound.

    Every time a current is induced, or current collapses, we generate back EMF. A 20V source can generate back EMF of hundreds or even thousands of volts.

    Think about a cars spark coil. You saturate the coil with current, when the points open the current collapses, and magic, 12V becomes 15K Volts. Every electrical path has the potential (sorry) to generate back EMF when faced with a changing current. dI/dT can be a devastating master to the untrained.

    Combine with high back EMF there is also magnet induction. currents flowing are electromagnets. Little EMP generators if you will. Try bringing an AM radio near an arcing electrode and see how much noise there is. That noise can be induced into cabling.

    -

    @ Marius, that reference to Earth somebody used, applies 1000% to DC in a car body. In this case the car body is both Earth and the conductor, which in and of itself should sound an alarm bell.
    Thanks. Good points about the emf.

    I understand the "earth".

  9. #47
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    Default Re: Welding on a vehicle

    Quote Originally Posted by MariusFourie View Post
    Thanks. Good points about the emf.

    I understand the "earth".
    Look, I think one needs to be pragmatic. Depends where and what you are welding.

    Also important is where you attach the clamp, obviously as near as possible to the weld makes sense.

    Similarly welding a bracket to a rear bumper, is hardly as dangerous as welding a support for the starter motor.
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  10. #48
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    Default Re: Welding on a vehicle

    Quote Originally Posted by KobusDJ View Post
    What about holding the workpiece with a bare hand when welding ?
    If you have earthed the work piece with earth clamp and hold the work piece,you won't get shocked. If you touch the earth clamp with bare foot and then hold the work piece in your hand and decide to arc the on the work piece to see pretty sparks,you will form part of the circuit and get a good electric belt. Best advice is wear welding gloves.
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  12. #49
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    Default Re: Welding on a vehicle

    Quote Originally Posted by RossJ View Post
    My Disco 1 has a crack on the bullbar which looks like an easy weld - its on the flat plate upright connecting it to the chassis

    I can get to it easily but am wondering if welding it while its still on the vehicle is a wise thing to do

    Its not too much of a pita to take off and put back

    comments before i get the darwin award plse

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  14. #50
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    Default Re: Welding on a vehicle

    Just a silly note. All welders produce an arc to melt the electrode. CO2,Argon etc gas used in these welders is there to prevent oxidation of the weld. Welding Rods have flux on them to do the same thing. Not the expert.
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  16. #51
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    Default Re: Welding on a vehicle

    Sam - We do owe each other a very long overdue beer or 12 - will catch up soon

    Right so it seemed like welding it in-situ with battery disconnected was the way to go

    till

    i read Fluffy's post

    now i am going to take it off and weld it at least 3 blocks away from the landy - maybe in the next town just to be safe from the flux capacitor syndrome

    thks all

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    Default Re: Welding on a vehicle

    Quote Originally Posted by RossJ View Post
    Sam - We do owe each other a very long overdue beer or 12 - will catch up soon

    Right so it seemed like welding it in-situ with battery disconnected was the way to go

    till

    i read Fluffy's post

    now i am going to take it off and weld it at least 3 blocks away from the landy - maybe in the next town just to be safe from the flux capacitor syndrome

    thks all
    And don’t forget about the neighbours cars and the bunnies.
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  19. #53
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    Default Re: Welding on a vehicle

    Quote Originally Posted by RossJ View Post
    Sam - We do owe each other a very long overdue beer or 12 - will catch up soon

    Right so it seemed like welding it in-situ with battery disconnected was the way to go

    till

    i read Fluffy's post

    now i am going to take it off and weld it at least 3 blocks away from the landy - maybe in the next town just to be safe from the flux capacitor syndrome

    thks all
    This is funny!
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    Default Re: Welding on a vehicle

    So the clever boffs, tell me. How does it help disconnecting the battery negative? Everything in the cars is connected to negative via chassis connection points, but you don't remove those. Only the battery. Exactly how does that protect the ecu and other “sensitive electronics”, as they are not disconnected?
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  22. #55
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    Default Re: Welding on a vehicle

    Quote Originally Posted by hunter26 View Post
    Just a silly note. All welders produce an arc to melt the electrode. CO2,Argon etc gas used in these welders is there to prevent oxidation of the weld. Welding Rods have flux on them to do the same thing. Not the expert.
    correct, but your arc welding rod is typically 1.6mm and up, while the mig wire is only 0.6mm or 0.8mm so the energy in the arc is very different
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  23. #56
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    Default Re: Welding on a vehicle

    Quote Originally Posted by hunter26 View Post
    If you have earthed the work piece with earth clamp and hold the work piece,you won't get shocked. If you touch the earth clamp with bare foot and then hold the work piece in your hand and decide to arc the on the work piece to see pretty sparks,you will form part of the circuit and get a good electric belt. Best advice is wear welding gloves.
    Nah wear shoes it will give a little more protection from falling sparks

    Quote Originally Posted by OomD View Post
    So the clever boffs, tell me. How does it help disconnecting the battery negative? Everything in the cars is connected to negative via chassis connection points, but you don't remove those. Only the battery. Exactly how does that protect the ecu and other “sensitive electronics”, as they are not disconnected?
    Tank you, someone else seeing what I see. the whole system is still connected to the earth.negative line and only the battery is out ofthe equation emf and all tha woo da daa is all in the circuit

    Risking another can of worms, same goes for not putting a modern battery on the concrete directly but on a piece of wood
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  24. #57
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    Default Re: Welding on a vehicle

    Maybe disconnect lead in battery prevents current from running through negative chassis through battery into positive of battery out of battery positive into electric components connected to positive then through components back to negative (breath), hence electric components become fuses. Bet no one thought of that
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  26. #58
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    Default Re: Welding on a vehicle

    Drill a hole at the end of the crack before welding. That will help to stop the crack propagating.
    Last edited by garygee; 2021/09/27 at 07:50 AM.

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  28. #59
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    Default Re: Welding on a vehicle

    Quote Originally Posted by hunter26 View Post
    Maybe disconnect lead in battery prevents current from running through negative chassis through battery into positive of battery out of battery positive into electric components connected to positive then through components back to negative (breath), hence electric components become fuses. Bet no one thought of that
    Ding ding ding we have a winner!

    Yes, you disconnect the negative (or the positive) of the battery because whatever voltage appears at the negative of the battery also appears at the +ve, plus 12V of the battery itself. So if you have a 1Kv spike reaching the negative of the battery then the positive goes to 1012v. It's like connecting to 12v batteries in serie, you get 24v out. So now you are welding and the most negative voltage on the vehicle is no longer at the battery negative, due to the arcs and spike and the multitude of current paths it follows. On the positive of your battery, where you electronic is also connected, you see the same spikes. So your ECU, for example, can see many many volts across it's power input, depending on where the ECU's negative is connected to earth (relative to the location you're welding). And it can thus blow.

    The big cause of all this is due to all the GND connections on a vehicle not going directly to the battery negative, but being connected to the chassis at the closest convenient spot... and it is this same chassis you're welding on.

    Hope this helps!
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    Default Re: Welding on a vehicle

    Some common problems needs it explained before we find a solution.

    Quote Originally Posted by OomD View Post
    So the clever boffs, tell me. How does it help disconnecting the battery negative? Everything in the cars is connected to negative via chassis connection points, but you don't remove those. Only the battery. Exactly how does that protect the ecu and other “sensitive electronics”, as they are not disconnected?
    Quote Originally Posted by OomD View Post
    Ding ding ding we have a winner!

    Yes, you disconnect the negative (or the positive) of the battery because whatever voltage appears at the negative of the battery also appears at the +ve, plus 12V of the battery itself. So if you have a 1Kv spike reaching the negative of the battery then the positive goes to 1012v. It's like connecting to 12v batteries in serie, you get 24v out. So now you are welding and the most negative voltage on the vehicle is no longer at the battery negative, due to the arcs and spike and the multitude of current paths it follows. On the positive of your battery, where you electronic is also connected, you see the same spikes. So your ECU, for example, can see many many volts across it's power input, depending on where the ECU's negative is connected to earth (relative to the location you're welding). And it can thus blow.

    The big cause of all this is due to all the GND connections on a vehicle not going directly to the battery negative, but being connected to the chassis at the closest convenient spot... and it is this same chassis you're welding on.

    Hope this helps!

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