Another oxy welding question..

Here's my question: when is donor steel/wire necessary? As it is, I have a hard time getting the two pieces to puddle at the same time: adding a third piece to the mix would seem to make it even more difficult to create a solid weld.

Also, how close -- generally speaking -- should the torch be to the material? I've been holding it very close, with the neutral flame "core" almost touching the materials.

Thanks much...

NO, no, no. You only want to heat the base metal hot enough to melt the bronze. If you melt the base metal, you are too hot unless the base is also bronze or brass. Overheating is typically the problem when brazing while too cold is generally the problem with welding. Overheating will cause the bronze to contaminate the steel or, in the case of cast iron, will cause it to form brittle and hard cast iron. That is a guaranteed break, right at the edge of the joint.

Brazing and welding are two totally different operations and require different techniques, materials and skills. Brazing will require a flux or a flux coated rod. The bronze will melt at a temperature about 500 degrees less than the base metal. Brazing is more similar to electronic component soldering. You flux the joint, which chemically cleans the work and allows it to "tin", in which the bronze will flow out over a wide area, called "wetting". Once tinned, the braze can be built up to a thicker section.

Yes, torch distance is a form of heat control, but that movement range is limited to about 1/4". The weld area must be inside the flame envelope to prevent exposure to the atmosphere and contamination/oxidation. If you cannot control the torch within this range, change tips or crank up the gas. When brazing this is not such a consideration because the base metal will not be actually melted and the flux will maintain a proper atmosphere for the bronze. You can move the torch back a lot further in this situation to control the heat.

1/8" steel filler rod is normally used for welding .032 tubing or sheet with OA. Thinner rod requires too high a filler feed rate and can result in a thin weld section, a point of weakness. The rod is also an important heat control tool. Adding filler or just holding the rod over a hot spot will cool that area down.

You are correct to get in close. Don't let the cone touch, but it is OK to get it close. The further away you are, the more the heat will spread out, and you will also have trouble getting enough heat to weld. If you are having trouble getting the material hot and keeping it hot, crank up the gas. If you are wide open with the torch set properly and the reccommended pressures on the regs, go up to the next larger tip. If the torch is popping and crackling excessively, go down a size and crank the gas up to increase velocity in the tip, which will prevent the backfiring.

Adding filler rod actually makes the two puddles unite easily. The filler will melt long before the parent metal. For practice work, use old coathangers as filler. Once you can weld with a coathanger, then go buy some good high quality copper coated mild steel welding rods and you will find it even easier.

When welding very thin walled material, turn up the acetylene just a tad (1 1/2 reducing flame, feather about half the length of the inner cone). This adds carbon to the outer layer of the metal and makes it melt just a touch easier. Helps keep from blowing through.. old aircraft welding trick.