I don't like welding magnets! They pull the weld. A lot if you're spray transfer welding, but considerably when globular transfer and short circuit welding too.
Rest the tube on the plate with 3 thin nails between, about the thickness of the tube. (OK, thinner when tube is 1/4") Tack it close to, but not including one nail, then remove that one. Check straightness and tack next to one of the remaining nails. Repeat for the last nail. Any remaining error should be small, and can usually be corrected by the pattern used to complete the weld. It should be completed as a series of tacks to minimize warping of the plate.
Go symmetrically if the straightness was fine after the 3 first tacks.
With a bit of training an angle grinder can be used to correct any remaining warp. Use it as you would with a scraper: take out small scallops by just touching the part slightly. |