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  1. #1
    Ipso Facto
    Reputation: bikecycology's Avatar
    Join Date
    Apr 2010

    Flux coated rods

    I've dug around a little but didn't find anything really to the point on my question.

    Is there any reason that frames couldn't be successfully brazed with flux on the rod as opposed to using paste?

  2. #2
    mtbr member
    Reputation: adarn's Avatar
    Join Date
    Aug 2009
    Just buy some gas flux stuff from HJ. It's not expensive

  3. #3
    mtbr member
    Join Date
    Mar 2011
    I tried that in the 70's. You certainly can braze a joint with that stuff. The problem is the type and amount of flux. It is usually the type that does not dissolve well in hot water after brazing. You will be chipping what looks like glass from the whole joint. Be sure to wear safety glasses when doing this. If you have too much flux in a joint when you are brazing a filleted joint flux inclusion (bubbles of flux and contamination that cause voids in the brass fillet) are common and large ones have to be rebrazed or filled with silver.

    As adarn said get some rod and flux from Henry James or Cycle Design and it will be much easier.

  4. #4
    mtbr member
    Join Date
    Dec 2012
    I like to have flux inside the joint, as well as on the outside. I don't know if any of the flux on the rod will make its way inside. Just get the Gasflux or Cycle Design stuff.

  5. #5
    Reputation: G-reg's Avatar
    Join Date
    Jan 2004
    You've got your answer already, but I'll pile on.

    I have some rolling around the garage from my first frame, it's just the only thing the local welding store had. And I still melt them for practice, but that's it. I'm not positive what the intended use is for that flux, but it doesn't really do anything you want it to do for frame building purposes. The glass like substance that is left over is truly evil. It's seriously physics bending hateful stuff. It doesn't soak off. It's got some flex in it so it doesn't really chip off, unless it has a chance to fly into your eye. Don't even think about using a file on it as you'll ruin the file. Good luck wearing it down with emery.
    Slowly slipping to retrogrouchyness

  6. #6
    mtbr member
    Join Date
    Nov 2010
    I believe the coated brass rods are designed to be used for a process called "braze welding" which would make decent fillets, but might not wick into a good joint. The coating is, I believe, a mix of flouro- and Chloro-silicates (glass) which flow over the metal, etch them to an extent and keep air away until the brass hardens. They are also easier to use for a couple of reasons: they protect over a wider range of temps, and hold out longer, so are more forgiving. Further, the brass rod is a fairly brittle, hard alloy, mostly copper and a lot of zinc. They work acceptably well for repairing iron castings and even auto body work where clean-up is done with large grinders

    Other metals added in, like Tin, even cadmium, extend the heat range the braze flows and allow it to wick into joints well. They add ductility and flexibility; joints can be measurably tougher. The fluxes for more demanding work contain flourine and boron salts that degrade quickly at high temps, but don't leave oxides within the melt puddle.

    Disclaimer: I've never built a new bike frame. But I have repaired a few, as well as a number of other things. I have had a tub of Harris 600 flux for years: its water soluble so you can put it on as a paste and scrub it off again. Plus, my nearest welding supply shop carries Harris products. I have a feeling it isn't quite as good as the henry james stuff, but its what I have.

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