Brazing works by melting a lower melting point filler metal between two pieces of parent metal. Typically the heat for this operation comes from an oxyfuel torch.
Any Oxyfuel torch is one of the oldest welding technologies (aside from a forge, which is much older). It works by burning Acetylene in Oxygen. Acetylene is a common fuel that you can buy in compressed gas cylinders at your local welding supply shop. That acetylene cylinder is deadly serious. Acetylene is so flammable that it can spontaneously detonate at pressures above 15psi. We need to store a lot of it, so it’s pumped into a cylinder that is filled with a calcium silicate sponge (like a synthetic pumice stone) that is then filled with acetone. The Acetylene is dissolved into the acetone, allowing it to be stored at pressures higher than 15psi.
Why is that important? If you draw off the acetylene too fast, some of the acetone will come out with it and damage all of the seals that keep the acetylene inside your regulators, hoses, and torch. That’s bad news, considering that you have an open flame on the end of the torch. If you smell a garlic smell or the flame of your torch is purple, shut down your system IMMEDIATELY.
The other component of the system is a tank of oxygen. Oxygen isn’t flammable itself, but it makes everything else flammable. Either of these things leaking is a bad situation.
If you’re buying an oxyfuel rig to make bikes, do not cheap out on any part of the rig. Buy a new, brand name torch setup with new tips, hoses and regulators. Install them on cylinders that you buy from a welding supply house that you go to in person. Buy the nicest cart you can stand to hold your cylinders. If you keep your oxyfuel rig in nice shape, it’s very safe. Just about every automotive garage has one, and they almost never explode. Maybe don’t keep it in your basement.
To start up your oxyfuel setup, open the acetylene side and light the flame. There will be a sooty flame coming out of the torch. Open the Oxygen cylinder and adjust the oxygen regulator until you have a neutral flame. You will have a neutral flame when the inner and secondary cones of the torch flame converge.
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When you are done using your torch, shut off the oxygen first, then the acetylene. Close the valves on both cylinders. Drain the system by opening the torch valves. When the pressure on both sides of both regulators reads zero, loosen both regulator screws.
Brazing requires shaded eye protection, of at least shade 3. For some silver soldering, sunglasses are enough. Protect your vision with the appropriate shaded eyewear.
When torch brazing, both parts are coated with flux. Brazing flux is an acidic paste that is tuned to melt at the right temperature for the filler metal to become liquid, and it’s acidity etches the surface of both metals to be joined so that the filler metal molecules can penetrate the surface of the parent metal. It’s important to use fluxes that are made for your filler material. I have experience using gasflux products, and have been happy with their performance. Cycle Design is another popular brazing supply company. I’m told their stuff works well.
In practice, here’s how it goes: Make sure everything is clean before applying flux. You liberally apply flux to all the areas you’d like to braze together. If they have a close fit like a sleeve or a lug, apply flux to both parts separately and assemble them wet. Once they’re assembled, put on your shaded eye protection and light the torch. Adjust the torch to a neutral flame, and work it over the joint until the flux becomes clear and liquid. At that point, the joint is nearing the brazing temperature. Once the parent metal is hot enough, touch it with the filler metal. The filler metal should melt freely and smoothly wet out over the surface of the parent metal. There should not be any fuming or sizzling. If the filler makes any sizzling or popping noises, the joint is too hot. Let it cool off, clean up the burned flux, and start over. This will happen a lot as you’re learning to braze.
If you’re brazing a sleeve or a lug, you can pull the filler metal through the joint with heat and capillary action. The front of the filler metal will follow the heat of the torch. With practice, you will be able to pull molten filler metal from one side of a joint to the other to know that you have a completely brazed joint. If there are any globs of filler metal, those are areas where the joint was not hot enough or did not have enough flux.
Once the joint has cooled, there will be a clear crust of hardened flux over your brazed joint. This flux can be difficult to remove without hot water. I like to keep an electric tea kettle in the shop and pour the boiling water over the brazed joint. It heats and dissolves the flux, which is the only way to go vs trying to mechanically remove it.