Lost art of flame straigthening

Flame straightening is one of those old-timer skills everybody has heard of but few have had the opportunity to witness. The ability to straighten bent plates and shafts is almost magical in its simplicity. It’s definitely a valuable skill if you intend to spend any time around a welding shop, unless you relish the thought of beating a stubborn shaft or plate into submission with a mallet. This technique has been used to straighten everything from the hulls of ships to samurai swords.

Metalworkers are continually confronted with the negative aspects of joining metals by welding. Whenever metal is heated, it expands and not always in a favorable way. Most often it has a negative impact on the end product. The results are manifested as warpage, distortion and stress. If you have spent any time in a welding shop, you have seen some of these types of distortion, like welds pulling, flame-cut strips and shapes twisting and bowing, and oil-canned plates.

Flame straightening is an easy-to-learn technique for correcting bent plates and shafts.

Every welded structure will try to seek the position of least-stored stress. A good illustration is when you tightly crumple up a sheet of paper and drop it on a desk. The paper expands slightly and finally settles in the position of least-internal stress. When welding on a metal structure, this is happening continuously. The weldment is undergoing a constant rearrangement to find this state of least stress.

Welding sequence, clamping and fixturing can help, but can never eliminate the effects of expanding hot metal. When you stress-relieve a weldment or part, you are allowing remaining locked-in stress to dissipate via heating or vibration.

Flame straightening uses the same principles that cause warpage or distortion to make positive corrections. The best way to imagine how this process works is something like reverse welding. An example is using the effects of controlled distortion to induce a large camber. These kinds of results can be accomplished with the humble cutting torch and the knowledge of where to apply the technique.