A Technique For Edge-To-Surface Welds

[larry cottrill]

I was asked to describe my technique for welding a sheet metal edge onto an underlying sheet metal surface, so as to end up with a continuous weld. (An example of the finished weld is shown in the photo below - an intake shell welded to a conical combustion chamber wall.) My welding is done with oxyacetylene; however, someone who is a good MIG or TIG weldor can probably work out a similar method.

First, I clamp the parts together with fairly gentle pressure using an appropriate clamping device. Once I'm sure the alignment is right, I make a few tack welds along the seam - one near the left edge, one right in the middle, one near the right edge. These are conventional tack welds made in the normal manner. Then I remove the clamp and make sure the pieces are together reasonably snug along both sides. Slight gapping (less than 1 mm) is of no consequence.

The trickiest welding is at the exposed edges at the left and right ends. Using a tiny flame, I start out behind the flared edge a little bit, make a puddle in the chamber cone wall and build weld up into the exposed edge - working slowly, like sculpturing. I could have started on the exposed edge and worked down into the chamber wall, but I think it's better to get the heat into the underlying sheet first. This should end up like a sort of oversize tack weld, but more scupltured.

I now increase the flame size somewhat - still very small, but with a barely perceptible 'roar'. I work right handed, so I start at the right edge - at the "oversize tack weld" I've just described. (So, as I'm seeing it, I will be working along the seam from right to left.) I create a molten puddle on the front side of that weld, and immediately come in from the left with the welding rod and gently shove it into the puddle (the rod gets slightly pre-heated as it comes in toward the weld puddle). To do this, the rod is brought in low and straight along the seam to be welded, but not quite down on it - up away from the metal surfaces at maybe a 3 or 4 degree angle. I continue to push gently INTO the puddle while melting the rod and blending it in. So, I'm NEVER LIFTING THE ROD OUT OF THE PUDDLE - that's the trick. As the rod melts in, I swish the puddle along, with the rod disappearing into the puddle from left to right just a little faster than the puddle moves along from right to left! Note that this 'pushing' of the rod is just the gentlest pressure - just enough to feed the process. When I get to the next tack weld, I just blend the puddle into it as I pull the rod out. Then, start the same thing again on the other side of that tack weld.

This technique sounds weird, but it's easier to do than to describe it, and makes the weld go very quickly. Of course, with the torch I aim a little more toward the chamber cone than the intake wall, to try to get the temps next to the weld about equal. But having the rod end in there taking up some of the heat helps with that, as well. Very simple and effective technique. For this to work right, the welding rod should be about twice as thick as the sheets you're welding - that gives just the right heat distribution and the right amount of reinforcement in the finished weld. For the example shown, I used 1/16-inch welding rod for the light sheet metal used (22 gauge). I do not slant the torch heavily forward in the direction the weld will travel, so pre-heating of the weld area is not excessive.

Of course, how all that would translate into useful MIG or TIG technique is up for interpretation! But, this is how I do it with the torch. One thing to remember is that the acetylene torch always "pre-heats" the metal ahead of it more than a shielded electric arc does, and that's part of what makes this work. I can imagine that with stainless, it might be good to lightly coat the welding rod as well as both sides of the adjoining sheet metal surfaces with a high temp flux before you try this.

L Cottrill