Perfectly square - square tube frames?

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Looking for some input on fabricating frames with 1" square mild steel tubing.I've tried the "welders" magnets - I find them helpfull. The stronger the better - until you get too close and the arc wanders.  One time I wasted a full hour trying to square up a simple rectangular frame of 1" square tube. After getting the first corner square with a metal frame square I checked the frame diagonally - no good -longer one way then the other. So I squared the frame with a tape diagonally and rechecked with square- ng. In typical fashion I assumed that I must be at fault. After wasting an hour I figured out the problem. My square was no longer square. After trying another square everything was great except I felt like an idiot.Now I'm able to get frames perfectly square but I have trouble keeping the frames from "tweeking into the other plane. ( keeping it flat) without clampingit to the bench with heavy parallel bars. Even then I still get some twisting.After some carefull observation  I also noticed that placement of the tube in relation to where the seam weld is also has a measurable effect on distortion.Anyone have some ideas, tips, tricks? I'm working with a flat steel table. Acorn platen or 3D table not in the budget yet.
Reply:What's perfect?I build lots of hand rails and cages.  On my table I have a "square" border made of channel.  The border runs across the top and down the left side.When I build a square frame I slide one clamp one piece of material against the top border and the other against the left border. I add the opposing pieces and tack.  I keep a chip brush near to clean spatter and scale from causing error between the sqare and the material. If I am in the field and I know the material is cut correctly, then I tack measure diagonals and shift accordingly(shift is always 1/2 whatever the error reading).  No need for squares or magnets.  They are usually too short for accuracy.
Reply:tapwelderThat set up sounds like a good idea. I'm in the process of planning a new welding table. I currently have a table that is 30" x 60" x 1/4" thick. Its cramped for space- It also is home to a vise, bench shear , scroll benderand a ring roller ( one on each corner. I'm trying to build it 4 x8 feet so I haveroom to lay out railings and larger scroll pieces.  I've been getting the frames square enough for the simple projects but twisting is now my biggest problem. Sometimes the frame will twist up to 1/4 inch. This is even after tig tacking every single joint.
Reply:Magnets work OK on Ac but they create arc blow if you are using DC.  I find that making frames on tables creates problems because you can get a bit of spatter underneath or the table is warped.   I set two strips of flat bar or angle turned vee down across the table and then let my two longest members on top of that.  It is important to make sure that the angles or flatbars are LEVEL... otherwise you will put a twist in the tubes.  If you work flat on the table make sure the table is not twisted by putting a level across it at both ends.  The table does not have to be level but it has to be the same at both locations.  If I am cutting 45 degrees cuts then I make them slightly less so that I can tack on the inside corner and hinge the joint open or closed.   Turnbuckles are great for adjustment.  Put your first tack on the inside corner then tack the turnbuckle across the inside of the corner.  Adjust to your hearts content.     I you don't have a twist in the frame when it is all tacked up you should be able to prevent twisting when welding by moving around with your welding.  Beginners often want to weld up one corner at a time.  Leave you turnbuckles  and temporary bracing in until everything has cooled down.   On turnbuckles:  You can get really big ones if you have a friend who is a lineman or look around in surplus stores.