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  1. #1
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    Fillet brazing - managing heat

    Since my first braze joint, I've continued to practice and I even started my first frame. I've extensively used the resources of this forum in the process so if I missed a good thread on this topic, please redirect me.

    While practicing, I experimented with different torch set up and found that my personal preference was to use a slightly "bigger" holed tip to have a low velocity flame with high heat output. I feel like this enables me to put more heat faster in a given area where I am laying my fillet. I did however notice distortion on some of my joints (mainly HT bending after brazing the DT/TT to it) and that got me wondering about heat management while brazing.

    Fillet brazing - managing heat-pb200037.jpg

    How do you guys handle heat accumulation while brazing? Do you use heat sinks? More specifically, when fillet brazing, do you try to heat up the whole joint and mating tubes slowly and evenly all around and then drop you fillet or do you heat a small region as fast as possible, drop your brass and move on? Do you braze all around at once or 1/4, then the opposite 1/4, then the other two 1/4?

    I know torch and tip set ups have been discussed extensively, I'm more interested in the "how much" and "how fast" do you put heat in your joints to minimize distortion. Any input is appreciated. Thanks.

  2. #2
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    As a non-fillet guy (most of the time) I will just quickly chime in - you are not TIG welding here. You want the *entire joint* to be up to dull red/glowing temps before you start laying your fillet - you should only be using the flame (ideally) to keep the puddle molten and keep the joint from cooling too much, not using it to directly melt the rod.

    Heat sinks aren't usually used for fillet brazing as they make it really hard to get the joint up to temp in an reasonable amount of time.

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  3. #3
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    Some time back, I had some questions about distortion, so I asked Dave Kirk about it at NAHBS, and he suggested fluxing the front of the tube to tell when it was suitably warmed up. You should heat up the whole join or the brazed part will move differently than the rest of the tube and distort as things cool.

  4. #4
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    Watch the video discussed in this thread and check out how much Garro preheats the joint.

  5. #5
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    Yeah, I spend a good bit of time heating the whole joint up until the flux is "activated" sometimes a touch more - starting at the area I plan to end brazing and working my way back. Once all the flux is glassy I focus on where I'm going to braze and giv'r from there.

  6. #6
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    I concur with the others; you want to heat up the overall area before zeroing in on the fillet. I don't have a sequence for fillets, but learned this sequence at UBI for tacking:

    1. obtuse angle
    2. acute angle
    3. side
    4. other side

    The idea being that the acute angle will want to pull in even tighter, so start on the opposite side. I haven't tried this sequence for fillets, but maybe I should.

  7. #7
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    Thanks for all the good comments and tips. I will definitively whath the video from home (I get an "access denied" from work). I like the idea to flux the entire HT including opposite to the joint to help monitor the heat input. I will also try the suggested sequence for tacking and also laying the full fillet.

    So when are heat sinks really helpful?

    I was pretty clear that brazing required heating the whole joint until I came about this thread suggesting a timed race between TIG and fillet brazing. (did this race ever happen? I did not find a thread with the results!).

    I also found pictures of fillet braze where the flux looked like it only got activated only really close to the fillet, as opposed to the entire joint. Suggesting rapid and localized heat input.

    Name:  2013-11-22 10-06-17.jpg
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    Photo and brazing credit: Bohemian Bicycles

    Thats what got me wondering. I'm probably simply not (and will never be) at that level of brazing talent!

  8. #8
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    Quote Originally Posted by golden boy View Post

    The idea being that the acute angle will want to pull in even tighter, so start on the opposite side.
    I thought it was the other way around - that you wanted the joints to pull tighter. But at least it does make sense that you'd want to tack on the centerline first when possible.

  9. #9
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    As a follow up to the above, it's not so much about making the joint 'tighter' - but that your joint is hopefully tight enough to begin with that it has nowhere to move in that direction.

  10. #10
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    Keep in mind that I'm no pro. I have probably a little more time on the torch than you, but I'm no Steve Garro.

    It looks to me like you're getting things just a little bit too hot. It's that delicate balance of getting "everything up to temp" while keeping your HAZ as small as possible. It looks to me like you have slightly cooked the filler, based on the copper tones that I think I'm seeing in your pic, and your HAZ goes out quite a ways from the joint.

    Your joints also go out from the intersection quite a ways, but the filler is pretty thin and less like the more ideal fillet that is shown in the linked pic from Dave Bohm. I had this same problem for quite a while, until one day I turned my flame down a little bit. This gave me more time to add filler and control the filler better. It looks like when you dab the rod into the molten pool that it just kind of wicks away from where you directly placed it and crept farther up the tube or out to the sides (hence, the wider, thinner fillet). Reducing the heat of the flame will allow you to have a smaller molten pool that is more easily controlled, allowing you to build the fillet in the exact place that you want it, instead of having it wick away from the insertion point.

  11. #11
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    ^^ Francis, Thanks for your comments. They are very helpful. I can clearly see and understand what you are referning to when comparing my fillet to DaveB's. Looks like more brazing practice in the homeworks for me! There is only so much that reading on a subject can achive, this kind of feedback is truly appreciated.

  12. #12
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    Interesting comments, Francis. I think I will turn down the flame a bit on my next fillet. I happen to be in the midst of building frame #2.

    Frank, that is a very interesting photo from Dave Bohm. It does indeed appear that he kept the heat very localized, with good results. Not the way I learned at UBI, but there's always more than one way to skin a cat.

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    On the other hand, there are people who braze more in a 'welding' style, and even will argue that "bronze welding" and fillet brazing are different techniques.

    Ariel Atom bronze welding - YouTube

  14. #14
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    Quote Originally Posted by dr.welby View Post
    On the other hand, there are people who braze more in a 'welding' style, and even will argue that "bronze welding" and fillet brazing are different techniques.

    Ariel Atom bronze welding - YouTube
    Oh wow, that is cool! I've never seen that technique before. I like it! I'm going to have to read up on that.

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