Just to add a bit of detail to what Andy said, for fillet brazing practice materials I would use a 2 foot length of 1 ¼” as a foundation. For short stubs mitered to fit on the end of the long piece, I would get 1” OD tubing. This difference in size makes it easier to fillet around the ears of the miter. The long length allows you to constantly move it without burning your hands while you are brazing so the angles of the 2 walls of the tubing are always equidistant from straight up so the brass doesn’t flow away from the crease. .049” wall thickness is good to start with because as Andy says it slows down your reaction time but you will want to switch to lighter 035” and get that right before brazing a real frame. And probably you will want to use only a 1/8th inch OD difference between tubes to practice brazing around the ears – that is the most challenging part of the circle.

I’ll give a couple of other tips but remember that the way I pay my bills is by teaching short brazing and longer framebuilding classes so it isn’t smart for me to give away for free all my knowledge of how I make money. For example I’ve observed almost all rookies make similar beginning mistakes that interfere with their success. Knowing what they are and how to avoid them really speeds up their learning curve. How well someone does has a lot to do with his or her hand skills. The most talented catch on right away and the worst never do. For a normal person with proper instruction it should only take 2 or 3 practice joints to braze an acceptable fillet.

Fundamentally you are controlling the position of the melting brass with your flame by keeping its temperature between its liquid and solid state. Most experienced brazers do this by flicking the flame on and off the joint like flipping a light switch. The bronze has to be hot enough to wet out and flow into the inside of the tubes and begin to stack up. But it can’t be so hot it starts to run away from where you want it to stay.

Flicking the flame on and off is the way I fillet braze but it isn’t the only way. I had a student from England that had attended the Bicycle Academy in the UK. He took a few days to learn how we brazed fillet brazed transportation frames in our shop in Ukraine. He has dedicated himself to be as good a framebuilder as possible. He demonstrated to me how the Bicycle Academy found a way for beginners to get an acceptable result. He started with a very small tip that produced an almost tiny flame. Because this greatly slowed down the melting process it doesn’t demand great hand coordination to keep the brass in the putty state. Heat control is based on moving the joint rather than the flame when the right amount of brass has accumulated.

Learning to fillet braze is somewhat like learning to cross country ski. One can do an okay job early in the learning process but it can take lots of time with a torch to be really skilled.