Strategies for Minimizing Distortion
Q I’m boxing the frame on my ’31 Chevy. I have the 10-gauge plates cut and tacked in place. What is the best way to weld them without warping the frame? LarryMerrill
A Well, the short story is that welding nearly always causes distortion (warping). When you make a weld, you are fusing red-hot metal to the base metal and there is a zone of metal next to the weld that gets heated at the same time. As this hot metal cools, it shrinks (pulls) the metal around it. If you take a bare C-channel framerail and weld a boxing plate to it all of the welding will be on one side of the rail, and the rail will cup an alarming amount when the weld shrinks. Fortunately, there are strategies for limiting this distortion.
Your best “ace in the hole” are the chassis crossmembers. These go a long way toward keeping your frame from bowing as you weld in the boxing plates. If you are using the stock crossmembers, I’d recommend keeping them in place, and fitting the boxing plates around them. The shorter the unsupported length of your frame is, the less distortion you’ll get.
If you plan to build your own crossmembers, I’d use another strategy. I’d lightly tack the boxing plates to the bare frame rails and then tack-weld the crossmembers into place, joining all the frame elements into a rigid unit before you do the finish welding. Of course, it’s important to get everything plumb and square before you start welding. If you start with any twist in your frame, the finish welding will lock this in, creating a big problem.
Not all frames are the same, and usually the newer the car, the more crossmembers it will have. Model A Fords are particularly flimsy, with only three crossmembers and very long sections between them. On a chassis this limber it would be a good idea to tack in some temporary braces halfway between the crossmembers. These could be made from 1-inch square tubing, and I’d recommend one brace tacked to the top flange of the frame, and one on the bottom.
Another important consideration is the sequence and length of your finish welds. You’ll get the least amount of distortion by making short welds, and staggering their location. For example, your first weld could be at the top corner of the boxing plate at the rear crossmember. The next weld could be at the front crossmember on the bottom of the rail. Next, you would shift to the rail on the other side, and make similar welds. I’d recommend making welds about 1½ inches long throughout the process.
If you follow this strategy, alternating your welds from top to bottom, front to back, and one side of the frame to the other, you’ll keep the distortion to a minimum.
Professional shops use a heavy chassis jig to hold everything in alignment for both the tack and finish welding, but you can get decent results without such an elaborate piece of equipment if you work slowly, and check for twist and squareness frequently.
After all the welding is completed, you can check the frame with diagonal measurements to make sure nothing has gone out of square, and if you have a flat surface to measure from, you can support the chassis at a convenient height, and measure up to identical points on the right and left side of your frame to check for twisting.
Note that concrete floors are NOT flat, and can’t be relied on for accurate measuring.
If you don’t have a good way to check your frame for twisting, you could take it to a body shop with a frame machine, and they can measure it for you. They also have the ability to correct any distortion that may have occurred during the welding.
This is an example of a nicely stepped and boxed frame.