On a large rectangular frame made from square tube steel, how do you make sure that it's flat? Flat meaning all the tubes in the same plane. In the past I've cleared out the garage and done layout and tack up on the concrete floor, but it's obviously not very flat. I've always had to tweak stuff back to flat by bending opposite corners.
square is easy to check with a corner to corner measurement but I can't figure out how you check for flatness over that kind of distance.
I may need to make up some 12'x5' frames out of 2" square tube for a yard project soon. Flat and square will be important.
Absent a really flat starting point, I would use spacers and shims on the garage floor to take out some of the error, tack one together and then tweak the tacked frame into perfection before welding out.
Then I'd use that perfect one as a fixture to build the rest on top of.
When #2 is made, flip it upside down (and then backwards) on #1 and you'll see any error.
I'd argue that a 12'x5' frame is going to be pretty flexible anyway depending on bracing, so flat is going to somewhat relative.
Locost guys do a build table for this kind of work.
you could use a lazer level and shimming similar to what jamescal is mentioning for larger constructions.
How flat does flat need to be?
Clearing the floor and using a level with shims to level everything out is about as good as you can do without a giant surface plate. Need to make sure your alternate welding corners/edges to prevent distortion.
Lots of measuring and shims.
Know anybody with a bodyshop that has a frame rack? They have a flat surface with multiple tie-down points and it would be easy to tack, square/level, hold down and final weld in place. Your material will twist, pull, suck at each corner when heat is applied and it will be damn near impossible to end up with 4 90* corners once they are welded unless it is done in a jig of some sort. Just my experience in the past. I have a frame table that I use to clamp everything to when I need a square weld.
