welding jig

[georgebarr]

I am trying to weld a mild steel 3" x 3" x 0.25" square tube with a flat plate measuring 5" x 5" x 0.25" (the flat plate must weld to the end of the square tube and be perpendicular to each other--this is for the gantry that will stand vertically). Think of the square tube as the gantry and the flat plate as the base that will be bolted to the floor. What are the various ways to hold the flat plate to the end of the square tube so that I may weld it and have it line up squarely? What is the best way? Since the square tube was not cut squarely, I cannot simply hold the flat plate flushed to the end of the square tube. I have the welders magnetic squares, but the holding power is too small and the larger squares cannot fit with the small plate.

[HuFlungDung]

George,

You could try to hand grind the end of the tubing. You likely already have a small try square to check it with. I consider it important to have a square cut to begin with, because you want to apply lots of clamp pressure while you tack the plate on.

If you don't have a large bar clamp that will span the length of the part so that you can clamp across one end to the other, now might be a good time to make one.

Your 5 x 5 is going to warp somewhat, too, when you weld all around. One possible method of clamping, is to use a piece of allthread rod down the middle of your tube. You will have to drill a hole in your 5 x 5 for the rod to go through. This way, you can apply significant clamp pressure to help offset the warp caused by the welding, which will always tend to pull the plate convex outwards.

[Hobbiest]

The warping does not extend from the center of the plate though, but within the heat zone...mostly the discolored area. The plate will tend to curl upwards on the edges, but shouldn't be too noticable with 1/4".