Fillet brazing sequence question for a newb

[bikecycology]

Hello forum,

I'm working on getting a frame built and I will be starting with the seat tube to bb shell fillet. My question has to do with the junction once the down tube is introduced. Does it make sense to just tin the seat tube to the bb and then miter the down tube in and build my fillet around all of it or should I do a full fillet at each tube? I believe I've heard both ways done. I was hoping to learn the pros and cons of either.

Thanks, Wil

[Andrew R Stewart]

The few filleted shells I've done were tinned/flowed only for the ST then fully filleted when the DT was in place (and after the DT was tacked and the frame aligned). Andy..

[duanedr]

The few filleted shells I've done were tinned/flowed only for the ST then fully filleted when the DT was in place (and after the DT was tacked and the frame aligned). Andy..

Having fully brazed and filleted the seat tube and then added the downtube, I can say that what Andrew suggests seems like a better way. fitting the DT/BB miter around the fillet (or filing away the fillet) is tedious and time consuming and if you don't get it right, the DT/ST angle will likely change on you so you end up with a tighter ST/DT angle than planned. Don't ask how I know. :-)

I suppose the pro of fully filleting the ST to the BB first is that you get a good fillet under the downtube and it feels like you're making progress when you complete the ST/BB joint. It's false progress...

[Eric Estlund]

Heavy tack (ie- short fillet):


Load tubes/ tack:


Braze (once- I do chain stays and BB tubes in the same pass):


Different strokes for different builders, but this is my current approach.

[christo]

Eric, can i ask for more details on the pictures above ? More specifically, when you do your heavy tack on the BB/DT/ST, do you pre-heat the entire BB or do you only apply heat locally ? And do you re-apply flux after tacking and before final fillets ?
Thank you

Christophe

[Eric Estlund]

I may warm the BB shell, but I'm not bringing the whole thing up to brazing temp (certainly not all of the tubes, too). I'm just tacking at that stage.

Before tacking I load the insides up the tubes and do a small, thin "skim coat" on the outside for the tack. When I am free from the fixture and am ready to braze I add another thin-ish (but more substantial, and wider reaching) coat of flux.

[christo]

Eric, this is very helpful.
Many thanks for sharing your experience