Q: My 16-year-old son, Jack, has started a project that we're having a lot of fun with. He's building a clone of a Series II Lotus 7 from scratch. I've got a real Lotus 7, and we've found some pretty decent chassis blueprints, so the intent is to have this be a true clone, not an interpretation like most of the 7s you see. I don't know if he'll finish it but I really don't care; the fabrication/welding experience is invaluable.
It's built primarily out of 16-gauge 1-inch-square tube, plus some 1-inch and 3/4-inch DOM round tube. Jack's been laying things out on one of my fabrication tables and tacking the tubing together with TIG welds. Of course, when he puts down a tack, there is some shrink and things move a bit. I've been having him put complimentary tacks on opposite sides, so that things generally move back into position, and overall we've kept the subsections very precise; they're within about 1/32 inch on the diagonals. However, there is a little twist, which is harder to measure, as my tables do have a little variance on their surface (they are not surface-ground).
We're just about to take the flat, top, and bottom sections and turn them into the 3-D chassis. So here's where I'd like your advice. My feeling is that I'd rather have him continue to tack everything together and once we've got the full chassis built, go back and do all of the finish welding. We'd continue to diligently make measurements and if anything gets too far out of line, make adjustments. It's easier to make these adjustments when the joints are only tacked. On the other hand, perhaps things would move a bit less if the joints were finished welded. And of course, if the joints were finish welded, there wouldn't be much risk of them moving again, as could happen if he finish welds a fully tacked.
My overall philosophy with welding these days is to not jig things tightly and force them into place, but rather to use strategy with the initial positioning and weld order so that the welding will tend to naturally bring the parts into position. The short way to say this is I don't want something to go "bonk" when it comes out of a fixture. So I'm inclined to have him tack the whole thing before finish welding.
Jack is learning a lot. He hasn't done a lot of TIG welding, but the hundreds of tacks in this project are good practice before he does the bulk of the finish welding. He's also learned a ton about cutting and fit-up. Probably the most important lesson he's learned is if the part fits perfectly, and it's clean, the welding is a piece of cake. If the fit or cleanliness is poor, so is the weld. I wish I had learned that when I was 16.
A: What a fabulous project for your son. He's really "upping his game", and I'm sure he'll learn invaluable lessons in the process, plus you will both take a lot of pride in the project. I wish I got more reports like this from father-son teams.
If I hadn't seen your pictures, I would have recommended tack-welding the entire chassis, then aligning everything, and carefully finish welding; placing small welds in diagonally opposed areas, moving around the chassis until finished.
However, seeing that the sub-assemblies are all in one plane, I think it makes more sense to finish weld these, and then straighten each sub-assembly, using your best table as a reference for flatness. Next, I'd lightly tack weld the lower assembly to your table, position the upper assembly, and fit all the uprights between them. If any twisting happens when welding the sub-assemblies, it will be easier to identify and correct it by referencing against the flat table surface.