As linked to in Mechabits' reply, I'm (slowly, LOL!!) building a CoreXY with a welded frame. What you say is true, a welded frame will be very rigid, and will be less susceptable to losing it's "square-ness".
What many don't realise is that the process of welding introduces a lot of movement in the metal that you're welding, so it can be increasingly difficult to keep the frame square and true while you are welding it up.
Even if you have all the material cut exactly to size (+-0.01mm) and all cut perfectly square, when you make any welds, even just little "tack welds", as the weld cools it will shrink and thus "pull" whatever joint you're welding out of square. An experienced welder (which I am not) may be able to compensate for this by deliberately miss-aligning the joint to begin with and using the welding-shrink effect to "pull" the joint square, or as I did, you can create a weld-sequence that will hopefully counteract this joint-shrinkage by welding opposite corners of a frame in a sequence (so perhaps the 1st tack-weld pulls a joint out of square, so a tack-weld on the opposite side of that joint will help to pull the joint back into square).
To give you an idea of how much movement a simple Tig-tackweld can give to a 90degree joint, a single small tack (about 3-4mm) placed on the outside edge of the 90degree corner of 2 lengths of 500mm long 25mm box-steel will likely "pull" the joint out of square so that the ends are 5-6mm out-of-square (depending on how big the tack is). Of course you can clamp the frame down rigidly and tack all the joints which will keep everything square, but when you release the clamps, unless you've managed to make every one of those tacks exactly the same size, there will still be some movement as the tensions created by each tack will differ. Plus it is only possible to clamp down part of a cube-frame, so there will still be difficulties when it comes to welding the 2 "halves" of the frame.
What I did was to just clamp up each "half" of the whole frame, tack-weld up that half while checking the square-ness and compensating with tack-size, then tack welding both halves of the whole frame, getting it as square as possible. Then it was time to fully weld the joints, this has even more potential for "pulling" joints out-of-square, so again a weld sequence was used, welding only the outside seams, and welding diagonal-opposite seams in turn. All the while checking for square on all the joints after each weld, and again after the welds had fully cooled. It was by doing all this checking that it became clear that the direction that you weld in also has an effect on the movement of the joint as it cools, ie if you are welding a joint in the X/Y plane, the direction that you weld it in will also cause movement in the Z plane.
Then I welded out the side-seams of each joint, again checking for where the frame was being "pulled" out-of-square when each weld cooled/shrunk, and then finally the inner seams were welded out (again working 1st on the seams that would "pull" in a desired direction to keep/re-make the frame square). Even with all of that, when I'd finished the frame was out of square by about 1mm, which required some percussive attention (a few judicious taps with a dead-blow hammer, LOL!!)
So that's the bad news, it's not as simple as it first appeared to me. But (maybe?) the good news is that the movement gets less as the frame gets progressively stiffer as the tack/welds are made. But conversely, the stiffer the frame gets, the harder it is to get it to move into square if it has got out of alignment during the welding.
But all-in-all, I think the effort is worth it. You can end up with a very stiff frame for a 3d printer. But that in itself can add challenges, the frame is then fixed, so any additions you want to make, or any mounting holes you need to have, you either have to have drilled/tapped before the frame is welded up, be able to re-mount the whole frame in a mill/pillar-drill to drill afterwards, or you have to drill/tap by hand. Where-as if you have a non-welded frame, it's a lot easier to make adjustments or drill/tap holes.
I guess there are a lot of factors, both positive and negative, to making a welded frame. I decided to try it out as I had the equipment needed, and the time to work on it, but I've not finished yet, so I don't actually know (although I have faith, LOL!!) that it will work great. I have some model-engineering experience, but this is the 1st 3d printer I have built, so I am learning a lot, and no doubt if I was to do it again there's many things I would do different (making a complete 3d model of it would probably be at the top of that list, then I could've drilled/tapped many of the mounting holes much more accurately)
Hope this rambling helps someone who's contemplating a welded frame
Tim
(edited for spelling and to add detail of welding the side&inner welds)
Edited 1 time(s). Last edit at 07/20/2020 12:55PM by TE78.
