Oil canning is the enemy
As you MIG weld, you’re moving electrons in the substrate. The electrons don’t exactly like to do this. This electrical resistance is what causes panels to heat up. That heat causes metal to expand and contract. All welds cause a little bit of heat expansion and contraction. However, excess heat causes your panels to swell, that’s when you have a problem. The metal doesn’t contract back to flat. It cools at different rates. Hence, you have oil canning/warping.
On thicker material, you don’t need to worry about this much, as it doesn’t react as easily. Sheet metal is a different story entirely. Sheet metal is typically used for panels where being straight is important. Rocker panels, floor pans, patch panels, the straighter the better. Point being, the key to straight panels is heat mitigation.
(Side note: you’ve just turned your car off and you hear a pinging noise from the engine bay. That sound is metal contracting as it cools back down. This is totally normal, and the usual culprit is the exhaust manifold.)
Stitch, let cool, repeat
There are a couple of ways to mitigate excess heat. The most obvious is to skip around to different areas of your weld, doing a short stitch weld then waiting. This spreads the heat out over a larger surface area, and prevents one spot from soaking too much. To speed up the cooling process, you can also blow some compressed air over the weld. The fast moving air is going to take heat out of the panel through convection. If you really need to minimize warpage, wait until your weld is just cool enough to touch before laying down another stitch.
I use these methods for every important bead that I lay down. Take this custom motorcycle seat pan that I made for my CB750 for instance. It required a long weld in thin material. It required an oddly shaped butt joint, and any warpage here was going to ruin my fit up. I also employed these strategies for my custom electrical tray, and the rear cowl.
Let’s talk about stitch welding. This is where you’d weld a short series of spot welds, and then move around to a different area to minimize heat. Depends on the weld, but I keep these stitches shorter than 1 inch at a time. Sometimes I’ll even do a single spot weld and move, sometimes I’ll do three in a row. There’s a little bit of feel involved in knowing when to continue and when to wait. Again, this all comes down to practicing your heat management.
Don’t turn the power down
One thing you want to resist is the urge to turn your power down. This is going to result in poor penetration and therefore a weak joint. As the saying goes, “welders need a hot rod to get good penetration.” Ultimately, you’re going to have to move slower to get the heat you need for penetration into the panel and that’s usually when you have problems with warping or even worse, blowing a hole straight through your substrate.
(Smaller) things to watch out for
Also, you want to keep that stickout consistent. Reason being, the longer the stickout length becomes, the lower the current output will be. Likewise, if you use a short stick-out, current levels will be higher. You must have consistent stick-out length for a good-looking weld, any sway in either direction will affect your amperage.
If you really need to reach into a tough spot, you can sometimes get away with a longer stick out (increase wire speed to compensate), but for ultimate consistency, you want that stickout to be the same every time. Eastwood’s MIG welding pliers will help with this. These cutters are offset. Press them against the nozzle and cut for perfect stickout every time.
Back it up
Try supporting your metal from the backside with something. Having something like a table underneath or even a little copper backer will help support that molten weld pool and it will act like a little bit of a heat sink. You want to avoid welding sheet metal that’s floating in the air if you can.
Similarly, you want to make sure that there’s no air gaps in between your pieces. The fit up is everything. Let’s say you’ve been doing some welds and one of your panels has pulled away from the other. You want to either apply some pressure with your hands, a stick, or even better, another clamp to ensure that those materials are tightly pushed together. People say “if I can jump across it, I can weld it.” while that is funny. Try to avoid air gaps where possible.
Another thing to watch out for are those little balls that form on the end of your wire. It might seem small but that little ball of wire takes just a little bit longer to get up to the right temp. Always snip that off to keep your welds consistent. This is especially important if you’re doing a spot weld.
You want those to be in-and-out, really quick with a fast zap. That will make your weld nice and flush with the surface. Any inconsistencies in your setup, settings, or material will slowly add up to an ugly weld. If you’re going to try to replicate some factory plug welds, you really need to have everything perfect to get the right look. Replicating these welds is possible.
As always, thank you for reading,
-Joe Dick
Media Host/Eastwood Content Creator/Honda Motorcycle Wrangler