Hey all,
I am attempting to build myself a bike frame using Columbus SL tubing and lugs. I've started putting the sub-assemblies together, and my lack of brazing skills is quickly becoming apparent. I had trouble regulating the heat in the first couple of joints and the flux got pretty burnt. I also suspect that I haven't got good penetration in some areas of the joint, but the silver will no longer flow in, probably because the flux is toast. Is there any way of saving this (i.e. by soaking off the burnt flux, adding more, and trying again), or is it game over? Do I have a chance, or should I just heat it up, take it apart, and try again?

Thanks!
Neil

Soak off the flux, clean the joint to really try to see what your shorelines look like, reapply flux, heat it up again until your old silver flows, then feed in more silver if you need it. Most of the time when I have minor gaps, this is all I need to do to make things right.

cool, thanks MB, I'll give it a go.

Now, when you say you had a hard time regulating heat, does that mean you think you were too hot or not hot enough? Keep in mind that you want to focus your heat primarily on the lug until you're nearing a good temperature indication from the flux (the flux I use bubbles, then "glows", then goes almost clear when it's at temp), then bring the tube up to temperature. If the lug is hot and the tube is cool (relatively), it won't work, and vice versa. Also, you're focusing your flame on where you want the silver to go, and work systematically and methodically.

my guess is that you ruined it. It's a lot cheaper to try to braze a sleeved joint made out of 4130 tubes.

You could yank the tubes, clean up the lugs, and buy new tubes. That's assuming you didn't fubar all of them

another thing you can do to practice is to get some brass and braze together random scraps of junk. I am always making myself new tools that are brazed together, there are plenty of examples on flickr

Details: I'm pretty sure I got it too hot for too long. I did start on the BB shell, and only after it was hot did I move on to the tube. Initially, the silver flowed well, and penetrated through most of the joint. However, toward the end it didn't seem to want to go through in one particular spot, and the flux on the tube took on a black appearance. I took this to be burnt.

Part 2: I took the advice to soak it, add flux and try again, and it seems to have worked. I focused my heat more, and tried to work more quickly. The silver flowed into the gap left behind the first time, and appears to have come through the inside of the bb shell. Unless I took the temper out of the tube, i think it will be solid.

Hindsight: I did buy some cheaper, practice lugs, but in my enthusiasm to get into this, I jumped right in with the better stuff. Ah well, live and learn. I think I'll stop now, put the frame aside and do some practice joints with the cheaper parts for a while. I'll finish the frame off later, hope it holds up for a little while, and chalk this one up to experience. I probably won't invest a lot in paint!

Oh, and thank for the advice unterhausen. I like your suggestion. I could probably put together a few little jigs and things, and get a better handle on brazing at the same time!

Cheers,
Neil

doesn't sound as bad as your original post did. Do you see filler on the inside of the BB?

I do see filler around most of the tube, particularly around the back, between the chainstays (where I understand it is often missed). The initial gap was on the side, and as I said, it seemed to fill in when I cleaned it up and tried again. I'll clean off the flux and tidy up the shore-line to get a better look at things before I go any further.