I weld for my own stuff, not professionally, so I am speaking in terms of the "amature" weldOr and what basic stuff he should know.
Electricity will flow through the path of least resistance. So for both the best weld AND keeping the current where it belongs, the weldEr ground cable should be close to the weld being laid down. Disconnecting the battery isn't going to save the computer, since its grounded. While it sounds like a good precaution, I have yet to see any solid proof and factual details of how welding shoots the computer. That said, better safe than sorry, so unplug the computer at its harness.
As far as frame welding goes, its generally accepted as a taboo on most high tensile frames, but it can be done if ya use the proper stuff. If you stick weld, which is best (i think) for large hitches etc, keep in mind the structure of the weld and the area around it on the frame. Drilling the frame is about as bad, given that most frames that are labelled not to be welded are also labelled not to be drilled.
The comment on welding along the frame not across it is dead on, for a number of reasons. If you find that weld across the frame is necessary, weld a plate along the frame, and weld across it for the project. Use a "stitch weld", beads of 2-3" long and breaks of 1-3" between. This shortens the stress area and reduces the possibility of fracture. Do NOT weld across the ends of the plate, as this defeats the purpose of having a plate. Welds across the frame produce a lateral weak point. Welds along the frame create a longitudinal weak point, which is not as likely to fracture.
Next, if you stick weld, I recommend getting one of the free books mentioned, or invest a bit of cash, and get a good book. I have one pocket welding manual, and one textbook. Both have good info. Also, for a frame weld, find a good 70 series rod; I prefer a 7018. 70 is the tensile strngth in thousands, 18 is the position and coating of the rod. 1 is a general purpose rod, can be used in horiz, vert, and OH welds. 8 is a low hydrogen coating, which aids in a strong weld.
For welding horizontally, you need a bit of practice. Welding vertical, the pro's go from bottom to top, its a stronger weld. Guys like me run top to bottom, allowing the puddle to sit on the tip of the rod. Its not a great way to weld, but it gets the job done. Overhead welding takes lots of patience, practice and stamina. I recommend avoiding it unless you MUST do it. Of course, if you plate the frame, one of the welds will be on the bottom of the plate. Keep in mind, puddled metal is affected by gravity just like all else. Move at a decent rate, and keep the heat in one spot enough to melt the steel, but fast enough to allow it to solidify quickly. Yeah, its a practiced art, so go practice.
Speaking of puddles, I have found that if I keep a steady hand, and observe the puddle, it makes a nice whirlpool motion. If it does this, the weld will come out picture perfect. Some things that will screw ya up: Oils, dirt, rust, etc. The best tool a weldOr has is a nice hand held power grinder. Use it. Clean metal is the key. And that frame under your truck? Its dirty... has paint all over it. Thats gonna suck when ya try to weld it.
Good luck, and make sure you factor in the the possibility that its just easier to hire a pro.
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