I'm building a new chassis right now. Its made from 2X3" .125 wall channel steel. I have the boxing plates all tacked in and I'm ready to finish weld the center of the chassis and front clip. I'm making the rear clip seperately and will join them later. How do I go about weling the plates? Do you start in the middle, side to side, bounce around? Here's the chassis, sans the plates afixed to my new welding table so its not going to move around during the process.
I would tack every couple of inches then back step your welds. do a few inches at a time and move from end. Move from side to side. By back stepping you keep moving the heat away from the direction you are going.
Get all your crossmember in place befor starting, the rails can move without those members in place. Everybody is going to suggest that you weld a bit and skip, jump from side to side etc to spread the heat. I tig weld, and heat/molten puddle is what it is all about. After about 30 years of doing this i have yet to figure how by skipping around there is less heat. So, i give a good firm tack so the boxing plates won't shift and i start welding from one end and go straight to the other as fast as possible. Everybody will tell you that you'll wind up with a pretzil if you do that. A complication is the amount of radius in the corners of the channel, i don't know how that is going to react. The key to welding is getting the heat even across the entire length of the weld, if you linger in one area and focus the heat in a particular spot then you can have problems. When being welded the steel will expand at a fixed rate, when it cools it contracts at about twice the expansion rate and this contraction is greater with higher temps. That is why i start at one end and buzz to the other, it has the least amount of heat and spreads it evenly from one end to the other. I suspect you can weld it however you want as long as you don't linger, if you do something stupid just keep going and come back to it later. The key is not to linger.
My first hot rod, back in the 70s, ended up with warped frame rails because I arc welded the boxing plates from one end to the other.I was just learning to weld by myself with a borrowed welder and no one advised me that the heat gets so hot it warps the frame. Well, it did. I must have lingered..Good that you ask. A lot of knowledge on this site.
I've used this method on several frames without a jig and it works very well with NO warping. First, tack all the plates in identical spots every 12 inches from front to rear at the top and bottom of the rail. Starting at one end, weld about 1-1.5" on one side on the bottom of the rail. Immediately go to the other rail and weld the same ammount on that side in the same location. Moving back to the first side, now weld another 1-1.5" bead in the same spot only on the top of that rail. Again, move back to the other side and weld the same ammount in the same spot on the top of the rail. Move up to your next tack and start the process all over again. Repeat until you reach the end of the rails, then go back to where you started and weld another 1-1.5" bead starting at the end of your first bead. Reapeat the whole process until your frame is boxed. This method allows for fully penetrated welds without concentrating heat in any specific area. Also by doing the same exact operation on either side it won't allow the frame to move an appreciable ammount in any axis. Each pair of weld operations counteract the previous ones. I have heard some guys say they can just weld a frame from one end to another without issues, sounds like a recipe for disaster to me. Good luck, have fun!
This is a REALLY good point that I neglected to mention. If you're going to flush-box them make sure that you have ground a bevel on the edge of the plate and the rail for your weld to lay into. Step boxing, obviously they need to fit the rail just right, no gaps.
Thanks for all the good advice. I had the fab shop make these rails a half inch longer on the bottom viewed from the end. So the boxing plate actually rests on this little ledge and sits flush with the top of the rail. I did this with the idea of protecting any fuel/transmission/brake lines from a jack when servicing the car.
1/2" longer on bottom? so the bottom leg is longer than the top. That will complicate things. The shorter upper leg will send heat to the corner quicker in greater amounts than the lower leg - the heat has 1/2" less distance to travel to get to the corner. The bottom will roll inward and the channel won't be plumb on the outside. You might get lucky and not have that happen because of the crossmembers, there will be a lot of tension in the frame that you might want to stress relieve by appliying heat on the outside of the channel bends, esp the lower. Hopefully it is a non-issue. Tons of frames get welded on and boxed with little or no preparation.
OJ, I think he said the plates would be mounted straight up, so the distance to the corner would be the same top and bottom. We teach to weld 3" sections and IMMEDIATELY move to the same spot on the other leg of the same rail (i.e. top, then bottom). The trick is to get both edges of the boxing plate welded while HOT. NOTHING you do will prevent the rails from bowing in to some extent. No matter what, you are putting more heat on the inside of the frame... Proper fit of the boxing plate is critical. You get it welded with the least amount of heat input. That is NOT the same thing as turning the welder down...one of the biggest tricks to MIG welding is to get enough heat in the weld while still minimizing distortion. Don't even think about TIG welding over that much area unless you pre-stress the frame or plan on spending some quality time with a torch pulling it back straight...
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