There’s a misconception among enthusiasts and even some professional body guys about welding sheet metal. The fact is that no matter how good or careful you are, metal WILL warp when you’re welding on sheet metal. We’ve heard all of the tricks in the book, but none of them completely stop the warping; only reduce or control it. The secret is that the heat and cooling from welding will shrink the metal around the weld seam to some degree. The only way to reverse the warpage is to stretch it back out. The best way to reverse it is to hammer on-dolly on the weld to stretch the metal back out. Gene Winfield has mastered the technique of planishing the welds or “hammer welding” (modern slang term, we know the blacksmith term technically means something different!) over the years when Oxy-Acetylene welding sheet metal. I decided to ask Gene for a quick how-to on this process and he was nice enough to let me shoot a short video of the process as he described the way he does it. Below is my abbreviated version of the process.
I’ll preface this by saying I know the technical term “hammer welding” is an old blacksmith term in which metal was heated so hot you could literally hammer two pieces of metal together. BUT, over the years that term has morphed and is now also used to describe the process of welding two pieces of metal together and planishing the weld joint flat while it’s still red hot.
The two types of welds that easily lend themselves to “hammer welding” are Oxy-Acetylene and TIG welds. That’s because they stay soft and can be planished and shaped like the base metal. MIG welds tend to be brittle and harder than the parent metal. They also cool VERY quickly giving you an extremely small window where they’re soft enough to hammer on. Oxy-Acetylene welds are the softest of the bunch, but controlling the heat in the panel can be VERY difficult when welding. The more you heat the panel, the more you will warp the metal and the more you will have to hammer and dolly the metal.
Gene is demonstrating this process on a 1950 Shoebox Ford being chopped at the 2015 World of Wheels in Chicago. The metal around the back window was all scratch built from four separate pieces and needed to be welded together. Gene mentions he starts by fitting the panels as tight as possible before welding. A tip he shared is to make the panels a little larger than needed and allow them to overlap. You can then tack weld the perimeter of the panels to hold them in place while you work. Then take a thin cut-off wheel and cut through excess that lays underneath. This allows the lower piece to sit flush with the other panel and can be clamped and welded with very minimal panel gap, all while keeping the overall shape consistent.
With the panels butted together and the edges tack welded Gene starts welding the seam up with a fine flame and a 1/16 filler rod. As he melts the parent metal he pushes filler rod into the seam for a few inches.
He then keeps the flame on the weld seam to keep it hot and swaps his filler rod for a post dolly. The heat from welding the panel seam has already started shrinking at the weld seam and has begun to sink down. He takes the post dolly and bangs it from inside upwards to bump the major sag out of the seam, he then quickly pushes upwards on the dolly on the seam and swaps the torch for a cross chisel body hammer.
He then proceeds to hammer on-dolly quite hard on the weld seam. You can hear a distinct ringing from the hammer striking the dolly through the metal. If you do not hear the ringing you need to push up harder on the dolly or make sure that it is centered under the area you’re striking. This process is done while the weld seam is nearly red hot to allow the metal to stretch back out easier. What’s happening is that the weld seam and the metal adjacent have a lot of tension from the molecules in the metal being condensed in a focused area due to the extreme heat and cooling of welding. By putting the metal between a dolly and a swinging hammer you are slowly stretching the metal out by striking on it each time (think of kneading pizza dough). This relaxes the metal and flattens the weld which in the end allows you to control the warpage in the panel. This is very important to keep the shape of the panel you’re welding. This can be done once the metal has cooled, but it does take more work and time to move the metal.
Some people like to take breaks between welding to allow the metal to cool or jump around on the panel to keep it from heating up too much. I’ve also seen some high end welders and coach builders weld a seam in one continuous pass. A weld seam becomes wavy and unevenly warped because of the variance in the heat affected zone of the weld. What does this mean? If you put more heat into one section of weld than another or if you weld longer in one section than another the heat affected zone changes and so does the amount of panel warpage. So if you have a TIG welder and can move quickly you can weld a seam continuously and keep the heat affected zone small and consistent throughout the panel. This means the warpage is the same throughout the seam and it takes equal amount of planishing to get the weld seam relaxed and back into the shape you want. I wouldn’t suggest trying this method until you’re VERY confident in your metal bumping and welding skills.
Gene is a master at controlling the heat and bumping the weld seam up as he goes. He likes to keep moving to finish the job quickly and commented that after the weld seam is completely hammer welded he will come back with the hammer, dolly, slapping and vixen files, and bullseye picks to fine tune any small low spots. He will the work the panel until the metal is ready for lead, filler, or primer and matches the shape of piece he’s welding.
As with anything everyone has their own way to attack a project, but I think everyone can take away something from watching an experienced metal fabricator or welder work. As Gene says “Everyday is a School Day”!