Originally Posted by
randomgear
While I haven't tried it, I fully expect that trying to fillet braze a seatstay cluster with propane (and probably MAPP) would cause the adjacent brazed joints at the top and seat tubes to melt.
I wont have issues with melting any brazed joints as the frame is mostly welded, other than the seat stays, tips, bidon mounts which are brazed.
Originally Posted by
Andrew R Stewart
The practice of a welded steel frame having a second heat cycle of a bronze or silver fillet added onto the joints isn't new. GT did just that years ago with some of their Excell tubed CO built bikes back in the 1990s. I have my old GT sales rep's frame hanging in my garage with a crack with that process. I was never comfy with their rational for the added fillet. Either the weld is the right way and done well or not.
As a repair method I also question the method. The crack would need to be just so, the repair fillet just so. I haven't done much 45% silver work so am unsure of it's merits for this stuff.
What I am getting more and more comfy with is classic fillet in bronze. Having a broad and low temp flame is the opposite of what I would want to have for a well controlled fillet build up. When I do my fillet work I slightly turn down the flame but tighten it up with a tad more OX to narrow the point. Then I play the flame on the previously flowed/sweated joint in a small area till the previous bronze is just shinning up and start adding filler in dabs with the pin point flame coming in and out to control the build up. I would think a broader and cooler flame would cause more area to be heated before the right temp is gotten to. This makes the filler run out and up the joint, not build up in a small spot.
To do this with propane or MAPP sounds like bringing a shotgun to a target range. You might cover the bullseye but... Andy.
The welds on the frame are nice enough, however, there is barely any filler at the fastback seat stays, which is probably why they cracked. Minimal surface area with minimal filler is not a good combination.
Mangled ST top. This will probably need a proper clamp brazed onto the top.
It would be nice to fillet the welds, but I guess it isnt necessary. I plan on adding on some gussets so I could just use body filler to smooth out the welded joints. But I would still like to have a go with repairing the cracked seat stay joint, and add on a seat post clamp. Im taking it I need some sort of way to localise the heat to get the tubes up to working temp quickly. Will wet rags/heatsinks be of any help?
56% silver would be nice to use as it melts at a lower tempurature, but it is expensive. 45% is cheap enough to have a go. Is something like this what Id be looking at? What sort of diameter would be best?
BOC ProSilver 45T Cd Free Brazing Alloy | BOC Australia
Or these which are the same but are flux coated.
BOC SilverCoat 45T Cd Free Brazing Alloy: 2 Rod Pack | BOC Australia