1) When attaching tube to metal plate, what plate are you using? For cages made from brake line? For cages made from brass?
I've used cold rolled steel, galvanized steel, stainless steel, brass. All are pretty much brazed the same way with small tweeks here and there. Make sure it's all clean by using alcohol or acetone to clean both pieces of metal. You don't really have to clean off the metal but you should if you're looking for the best joint. After that, it's all just practice. You will have to learn to evenly heat both metals at the same time so they're both the same temperatures which can be a challenge sometimes. Different thickness metal obviously heats up at different rates but also, the melting point of metals are also different, so you'll have to be super careful when brazing to thin brass. You WILL melt the thin brass before you melt the steel and that's a pain when you have a nice piece already completed on the brass side. That said, as a general rule, keep the flame on the thicker side of the metal more than the thinner metal. It'll make sense after you try to heat up different thickness metals at the same time with the same amount of flame on both parts. As far as metal goes, it depends on if it's structural or just cosmetic plate as well as if I'm looking for that area to dent easily or not. But anything from 16-22 gauge.
2) Once you have the pieces cut and ready to braze, how to keep them aligned through the brazing process? Welding magnets, a vise, spring clamps?
Yes. I use my vise, magnets, vise grip pliers, and third hand the most. Masking tape also works well if you think you can braze faster than the tape will catch fire. I also have a spool of 0.032" safety wire that I use to hold pieces together in tight spots when I can't use a magnet, pliers, or vice grips. I find that medical tools work great if you can get a set somewhere. Small little medial clamps are just awesome and stand up well to high heat (I'm pretty sure they're stainless steel).
3) What work surface do you use? Metal table, iron plate, some sort of special stone? Is there a special mat or table that works best?
I just work off of a plywood bench. If you can avoid brazing on a sheet of metal, do it. You're trying to heat up a bit of metal and it'll be much, much faster if it's just suspended in the air and doesn't have a sheet of metal to conduct all the heat to. Not to mention, it wont stick as easily if you're not touching something. So, a vise works great, just clamp the metal as far away from the joint as possible and as secure as you can. I will often set the part up on a flat surface, making reference marks around the joint so when it's suspended, I can still tell that it is aligned. Just remember, this is brazing, not welding. You're heating up metal at a relatively slow rate and will take forever if you increase the effective metal size by conducting your heat away from the joint.
4) What material is lighter for building cages, brake line or brass tube?
Brass tube depending on the wall thickness. Obviously, 1/4" wall tube will be heavier than 3/16" brake line. But also, judge your material on strength. I mix my cages between brake line, solid rod, and brass of all different wall thickness depending on if it's a structural part or a cosmetic part.
5) How do you cap the ends of the tube? Metal plate, body filler, leave open?
For brake line, I use 1/8" solid rod brazed in the end. Works great. For larger, just find some scrap solid rod and cap the end.
Also, as a tip, keep all your scrap. Metal is expensive and you never know when you'll need a tiny piece of scrap sheet or rod. I've gone through my scrap box too many times to number and found bits that just work great.
