The thing about gaps between welding surfaces: The bigger they are, the more they will draw closed potentially throwing the corner out of square. Where you start on a gap will also help or hurt the squareness. To reduce the draw, tack weld a brace across the right angle in triangular form. Make sure the brace is of very ridged material. It can be ground/cut off after all welding is done. That will help reduce the pull as the welds cool. With what you have there, I'd start with the inside joint. And run it. After the joint is done, check for squareness. Then go to the outside joint. If it's still close to what you want, (for the side joints now) start at the corner of the inside joints and run to the corner of the outside joints on the sides. I've also tack welded corners a little over square to compensate for the pulling of the welds as the material cools. That takes a bit of practice to get it close to right. But most of the time, I can get it within +- .030". And have even held tighter tolerances of +- .015 on some aircraft parts after cooling.
There are tools to help keep corners square also. Some are two axis vises, some are three axis vises. I've also used very long table clamps, come-a-longs, chins, turn buckles, pneumatic jacks, bottle jacks, portable hydraulic jacks, and all sorts of stuff to get projects and joints square, and keep them square. It all depends on the project. And what resources you have at hand.
If the weld is a repeatable project, a welding fixture can be built from the original to act as a welding jig for following products. It can be built on a flat surface out of various piece of scrap and angle welded to it. Or using the original piece with flat outer and inner surfaces tack in place to hold loose pieces in place by laying pieces on top of the original. Kinda like stacking the non welded parts in their respective positions on the finished product before welding. Once all parts are completed, grind or cut the flat surfaces off the original, with a hand grinder or cutting wheel.
There are tools to help keep corners square also. Some are two axis vises, some are three axis vises. I've also used very long table clamps, come-a-longs, chins, turn buckles, pneumatic jacks, bottle jacks, portable hydraulic jacks, and all sorts of stuff to get projects and joints square, and keep them square. It all depends on the project. And what resources you have at hand.
If the weld is a repeatable project, a welding fixture can be built from the original to act as a welding jig for following products. It can be built on a flat surface out of various piece of scrap and angle welded to it. Or using the original piece with flat outer and inner surfaces tack in place to hold loose pieces in place by laying pieces on top of the original. Kinda like stacking the non welded parts in their respective positions on the finished product before welding. Once all parts are completed, grind or cut the flat surfaces off the original, with a hand grinder or cutting wheel.
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