Well, I've had a go at doing the inner skin for the chassis rail and overall I'm really pleased with how it's gone. It's been a real challenge for my skills and I've learnt a lot as always. However, I simply cannot convince myself that the final result is good enough from a structural perspective and that would always be on my mind. My doubts are twofold: (1) the quality of my plug welds as usual particularly when joining thicker to thinner material, (2) a few bits of poor execution on my part to do with attaching to the firewall which I'll explain below.
As a result I'm going to take the whole of the chassis rail out and put a complete new one in. I've finally bit the bullet and ordered a spot welder and I'm hoping I can master that well enough to achieve a better outcome - time will tell I guess! Anyway, it's all good fun albeit a bit costly!
Anyway, a few pics and a little bit of explanation for posterity.
I bought a rail off Andy at Auto Panel Solution and it contains internal strengthening which I had to remove as it was right where the inner skin would go:
I'm totally clueless when it comes to CAD solutions but I figured that I've got no chance at all at making this fit unless I produce a model of the part to see how I can form it accurately so I gave Autodesk Fusion 360 a go as I'd read it was easier than something like Solidworks for total n00bs. They do a hobbyist license which is free and it was surprisingly easy to get something together although having a bunch of YouTube videos on sheet metal modelling was essential. Here's a rendering of the part:
I used 2mm sheet for it as I figured that would give me good strength and also pretty much match the chassis rail which was 2.4mm (I wanted to match them for ease of plug welding...). As some of you will know, forming flanges on 2mm sheet is not easy! Owing to the shape of it I could only form one edge with my metal brake but the other two had to be done by hand using 5mm angle iron clamped to the bench for a support and some severe "encouragement" from a lump hammer with some protection on the face. Anyway, it turned out quite well:
The above image was just a test fit but shows that it fits pretty well - it was nice and tight. After that image was taken I drilled spot weld holes into the small left hand section and encouraged a better fit for that to the original chassis rail. I then welded the whole of the left section into the new chassis rail piece while it was on the bench as that gave me the best chance of making nice plug welds. Note that I was going to weld a small section across in the inner skin across the gap that you can see but that hasn't been done yet (and probably won't be now). It actually went pretty well and here's what it looked like before welding into the original chassis rail section:
At this point I was reasonably happy with things but then I made a mistake which I'll put down as a learning experience. As you can see I Cleco'd the flange to the firewall and I actually put a jack under the bottom of it to ensure that was solid into the underside of the floor/toeboard. However, that was all I did and I didn't tack the flanges to the firewall which meant that with the large amount of heat used to weld the chassis rails together it pulled the rail away from the firewall and very slightly to the right. I was so peed off I cannot describe. I can guarantee you that it was dead-on balls accurate straight before I started welding as I'd used a laser to throw a straight line and also had the crossmember bolted in as several advised me to. This image isn't the final fit-up as it's not straight but shows what I was mean :
The front flange had pulled away from the firewall by about 2mm and the from section was pulled off to the right slightly. I'd also say that it was raised up slightly so overall I was not happy at all. Regardless of this though I carried on and plug welded the flanges to the firewall! Looking back I should have just stopped there as I had to fold the flanges over in-situ to meet the firewall owing to the newly created Mariana Trench of a gap that I had just made. The long flange going down the toe board actually wasn't too bad a could be plug welded OK but the top flange was a disaster zone as not enough of it met the firewall to plug weld together (incredible as it sounds you just can't weld across thin air!). Anyway, I seam welded the top flange.
The "final" version of this looks like:
You don't really get the misalignment from that angle but I can assure you that it's there. It actually feels really solid and possibly I'm being too worried and bothered but I'm just not happy with the outcome at all so I'm going to start again with a fresh chassis rail and a spot welder. Obviously the spot welder will hopefully come in very handy in other areas as well.
The one thing that I have learnt from this is that copper backers really do let you pile the heat into metal without blowing through. I kind of knew that and I use copper backing sometimes but I didn't really realise just how much heat you can put into a thin panel (< 1mm) if it's got a copper backer on it. This came into great effect when doing the spot welds from the front onto the firewall metal where the flange is significantly thicker than the firewall that is being welded into. My current favourite copper backer is this bad boy as it does allow you to really get a very solid connection with the metal (I do use the Frost 4x2 inch plates as well):
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