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Framebuilders Thinking about a custom frame? Lugged vs Fillet Brazed. Different Frame materials? Newvex or Pacenti Lugs? why get a custom Road, Mountain, or Track Frame? Got a question about framebuilding? Lets discuss framebuilding at it's finest.

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Old 02-10-06, 12:43 PM   #1
AfterThisNap
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welding near brazing. Easy question.

So I have this bianchi road frame with tweaked dropouts that I want to put horizontal track ends onto.
In my life before I moved north, I welded race truck/jeep chassis, so I'm pretty handy with a welder when it comes to general fabrication. Back at home I would have just milled out some of my own ends, cut off the old ones, and TIGed those puppies on there.
Now I'm away from any sort of real shop so at best I'll only have access to an angle grinder and a flux-core wire feed unit. I may get to use a MIG, but it's not "my baby" so I'll be unfamiliar with the amperage and speed settings.

The plan is to cut out the current dropouts but leave the factory "tabs" brazed into the frame with about 1/2" of protursion from the tubes. Then I will cut the flat pre-fabbed track ends to fit the tabs. Grind/weld. Adjust/bend with frame alignment tools as necessary.
I think braze filler melts from 4-800 degrees F. I'll be heating the metal 1/2" away from these joints to 3-4 times that temperature. Will the heat from the welder permanently destroy the factory brass brazing, or will the filler soften then reanneal to it's original integrity?

I thought about filling the seat and chainstays with water before doing this to act as a heatsink. In theory the interior of the tube won't reach above 212 degreees F, so I'm hoping that the brazed joints won't get hot enough to exceed the melting temp of the filler. The heat affected zone of the weld will lose heat fast too though, but If I post-heat it over a torch or stove then I think that the crystalization of the grain structure will be minimized. Plus, it's a bike, not a truck, so the forces will never be high enough to matter.
Obviously,I''ve never brazed before.

Thoughts? Opinions?
I know it seems like a lot of work for an old frame, but it has some sentimental value. Besides, I really have the urge to do some metalwork. Even if it's fisher-price level fab.
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Old 02-11-06, 01:05 AM   #2
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I don't much like the sound of that. The heat affected zone is usually about 1", of course it rarely get's much more than blue outside of the weld area, which is probably low end for the braze being used.

A wire feed flux core mig is a messy thing without a lot of capability to make a neet little weld in a tiny tab of metal. It can look good but be bad. One the other hand, the negative is not enough heat into the weld to fully penetrate, or boil off the braze. I have never done the particular alteration you are suggesting, but I would guess the weld will be crap while the braze will probably stay put.

I wouldn't put water into the stay It could be pretty dangerous to have a source of trapped steam near some electricity. And what you are reallyw orried about is sufficient heat to move the braze in the direction of the heat, which is where you are welding not up in stay.
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Old 02-11-06, 02:43 AM   #3
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interesting. so melted braze filler moves in the direction of the heat? I have a couple of years of welding experience (zero brazing) so making a strong/neat weld in a small area on a small tab isn't a concern for me. Assuming that I do funk up the brazing, but get the track ends welded straight, how would I go about repairing the brazed area? Do I have to melt them out and start over, or can I add a little filler material on top?
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Old 02-12-06, 11:46 AM   #4
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I'm not a welder, but I had the rear drop out crack at one of the triangles on my steel road bike. I too was concerned about melting the brazing during repair. Took it to a welder, told him about my concerns, he cut through the crack with a grinder and filled in the crack with an arc welder. worked perfectly. The burned paint wasn't even near the brazed joints. Just a thought.
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Old 02-12-06, 02:24 PM   #5
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What I would do is get a similar sized piece of metal and weld it together. note the tranformation colours. straw is about 400 degress, dark blue is about 900 degrees. Note how far they spread out from the weld. Unless the dark part (Or the hotter part nearer the weld that doesn't discolor because it is shielded by the gas) is far enough up the piece you are welding to be in the braze, you don't even need to think about it.

Second, The braze wants to move towards the heat source, but that is usually only with flux present in the area between the braze and the heat, so I doubt it will travel out in your situation.

Unless you tip the frame in such a direction that the braze could fall into the strut you will probably see any problems and know if you have them. Then if worst comes to worst you can fix as SR suggests.

Look at this thread, sounds similar to your situation:

http://www.bikeforums.net/showthread...17#post2166017
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Old 02-12-06, 09:56 PM   #6
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Quote:
Originally Posted by San Rensho
I'm not a welder, but I had the rear drop out crack at one of the triangles on my steel road bike. I too was concerned about melting the brazing during repair. Took it to a welder, told him about my concerns, he cut through the crack with a grinder and filled in the crack with an arc welder. worked perfectly. The burned paint wasn't even near the brazed joints. Just a thought.
That would be the usual process(not necessarily that type welding) .
maybe drill a small hole on each side of the "crack" also..............hmmmmm.....
STD had a "thought"!!
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Old 02-13-06, 03:50 PM   #7
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I would guess it depends on how near the braze joint is to the weld and how long the weld is (how much heat buildup) what the danger of melting the braze would be.
You can also pick up a heat sink compound from your local Welding Distributor. It comes in a caulk tube or tub, just apply it to the area you need to protect. It works.
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