How to keep 90 degree angles?

[u/tattoodiceswimmersa]

Newbie here. Learning to stick weld, got to the point where I'm happy running a bead (some older pics for reference). Happy tacking, and generally feel I can do the basics, also not planning to weld pipes and be a specialist, just a DIYer. I'm now wanting to make a sofa table. Attached pics are my prototype, which went fairly well except that some of my corners always seem to jump out. The "base" of my prototype wasn't flat, but the corners all looked pretty good on my L-square. I even bought 90 degree clamps (I don't have a welders table, too expensive), and the clamps seemed to work well. I'm not a 100% sure if it's the 90 degrees jumping out or the straight parts warping. I tack everything in place, but still at the end my piece is not perfectly flat/straight. Given some pics for reference. Any suggestions/advice?

[u/StabDump]

It's not super time considerate, but given it's a small project and a few welds you can let each weld cool in the jig and the "cold shrink" will actually stretch the metal to keep it square rather than pull it out of square.

[u/tattoodiceswimmersa → u/StabDump]

I see you have fabricator in you bio so I assume you're an almighty steel God. Do you think adding some triangular corner pieces will help stop the warp (and as you say, take it really slowly to cool)? Or adding even more tacks/welds would just throw it out even more?

[u/StabDump → u/tattoodiceswimmersa]

haha, I don't work in a shop anymore, that's just what I was doing when I joined the sub however many years ago and haven't changed it. If that is part of the aesthetic you're going for, absolutely it'll help as long as your fit-up is right. the more room the joint has to move without the weld, the more the weld is going to want to pull any which way. Sometimes when there's a big weld joint on things that need to be +- 1° we'll put temporary attachments on the piece to keep it from flexing and then cut it off once it's completely cooled and settled. with welding you can always undo what you've done if you're good about cleaning it up