best procedure for MIG welding a butt joint in square tube so it's STRAIGHT

Problems like this are not new and are even more frustrating when you get old, experienced and too cocky :')) One trick is to cut a window in the back of the large angle enough for a tack on the hidden corner. The window has to be cut without a torch. Usually a grinder works with a cutting disc. The tacks will hold things on location. Often if you weld the joint without taking the tack out the tack will hold the joint locked. If you take out the tack then the weld will shrink. I often will weld from the open area to the tack fusing over the tack last. It is nice when you have lots of time. You can do a weld, let cool and eyeball it before doing the next weld. Doing a butt joint with a backing bar allows you to lock the backing bar and both edges when you weld. It is less likely to shrink with the backing tacked inside. The larger the gap you are filling the greater the shrinkage. Often when you have completed the joint it is bent. Instead of flame shrinking you can grind out half the bead on the outside of the bend and weld it again causing it to shrink back. Today we are welding some assemblies out of one inch and one and a half inch plate. We put a 3/8 by 2 inch strap across to hold things. The strap started to bend. We put in a half by four inch bar locked in solid with extra welding. At end of day the half bar looks OK and the bent 3/8 has straightened out. Magic!!! Randy

"Grant Erwin" wrote in message news: snipped-for-privacy@corp.supernews.com... I'm having a heck of a time. I made up a railing to the design and woops the design was too long. To fix it, I have to cut 3" out of it in each of 4 places and weld it back together, the end to be approximately 12" shorter. The first one I did was real straight, and I was happy until I did the second one. Shoot.

The piece is over 16 feet long, very awkward to handle. Would one of you experienced guys please take a crack at telling me the real way to do this?

Here's what I did: (work piece is 1-1/4"x.120 wall square steel tube)

started with 2 sawhorses separated by about 12 feet cut out the first section, deburred the ends took a piece of 3x3x3/8" angle (heavy) and clamped it to splice, 1/16" gap (NOTE: the inner radius of the angle iron is smaller than the outer radius of the tube so it clamped solidly) tacked it on the two available sides & removed the angle

-- at this point it was very slightly bent towards the tacks ground a vee on the other two faces (the ones that had touched the angle) welded one solidly then, turned up the opposite side from the one just welded ground away the tack & ground a vee & welded solidly checked - very straight then welded the 3rd side previously with ground vee then ground away the tack and ground vee & welded the 4th side

The first time I did this, it came out very straight indeed. The second time the segment was farther towards the middle, and it didn't come out straight.

Thanks,

Grant Erwin

I would use a inner sleeve tube to splice the joint. Welding into the sleeve tube should reduce the distortion.

Randy, I tried grinding a vee on the "outside of the bend" and rewelding. That works! Problem, sometimes it works too well and then you have to flip it over and do it again. You can play with the depth of the vee and how fast you weld.

This is a real good trick. Both you and Ernie pointed out the usefulness of a backing bar but for welded steel tube, that's tough. Have to file away the inside weld on both sides, and then you have to find a square tube that just slides in. Might be easier on pipe, don't know - I'm not a pipe welder. So without resorting to a backing bar, my options were leaving it as is, flame shrinking, trying to press it straight, or your trick. Your trick was very fast, I can easily grind a vee, reweld, grind the new weld flat & blend with flap disc, takes just a few minutes. I'll remember this one!

Grant

R. Zimmerman wrote:

What I do is clamp two pieces of smaller tube (in your case 1") to two ajacent sides of the pieces you want to join. This allows you to make your root the width you want. It also puts the two pieces in a true alignment. This helps a lot to get straight alignment if your sawcut is not exact.

Then I spot the corners. (MIG) Then I weld the exposed sides using a series of hot tack welds. I let it cool. Then I unclamp it and weld it out. Lastly, I grind it down to look acceptable. But by doing a series of hot tacks, it looks like it has been Tigged, and sometimes I just buff it up unless it is a place that will have hand contact.

Wurks fur me.

Steve

You can start with one tack (in a plane) _misalign_ it and do the other tack (and hope it pulls straight by your experience). If it is out of line, you weld over the tack to pull it straight (by your experience). Sequence and direction of welding plays a role. Where you stop welding it will pull more inward. In the end, you still have the flame. The problem with "by your experience" is, that the learning curve never seems to end. :-)

Nick

For lack of the right sized piece of tubing to fit inside, I've cut down pieces of 2X4 to fit inside real snug and tag weld around that. As long as you don't need to run something through the tubing I just leave the wood inside. Smokes a bit toward the final welding is the only drawback I can see. Is there anything really wrong with doing this to keep the tube aligned?

Grant: The trick for a insert is to take a short piece of your tube and slice it lengthways on two opposite corners making two "L" shaped pieces that can be inserted in the end of the tube or if you want welded together before inserting. Don't make your inserts long.... one inch maximum. They are there for backing not alignment. Another way is to cut a half in thick piece of plate to the ID dimensions. Grind a radius on the corners and set it into one end of your tube about an eighth of an inch leaving 3/8ths to fit inside the mating tube. Randy

This is a real good trick. Both you and Ernie pointed out the usefulness of a backing bar but for welded steel tube, that's tough. Have to file away the inside weld on both sides, and then you have to find a square tube that just slides in. Might be easier on pipe, don't know - I'm not a pipe welder. So without resorting to a backing bar, my options were leaving it as is, flame shrinking, trying to press it straight, or your trick. Your trick was very fast, I can easily grind a vee, reweld, grind the new weld flat & blend with flap disc, takes just a few minutes. I'll remember this one!

Grant

R. Zimmerman wrote:

Has anyone tried my suggestion of clamping two pieces of tubing on adjacent sides giving it a straight alignment, then welding the opening with a series of hot spot welds?

Steve

Clamping using two 90 degree opposed strongbacks, yes - I just did it with one piece of heavy angle instead of 2 pieces of tube. No real difference.

Welding entirely with tacks? Actually, I did do some of that. But afterwards, I'd flush off the tops, and grind a vee and run one continuous bead. Mostly because the weld has to be really continuous in order to not have little marks all around after being ground flush.

GWE

Most of what I do is ornamental metal, and doesn't really need to be flat and flush. Unless it is in a place like a handrail that will have hand contact, I kind of like the look of (fake) Tigged tubing. I sure have had some comments from people who see it who know (or think they know) about welding. (The latter category includes almost every male in the human race.) On some pieces, I think it looks better than even a good flowing smooth concave pass with the MIG. Sure probably isn't as good strengthwise, but if I an going for strength, I don't use light tubing or MIG in the first place. All the rest is decorative and stronger than it needs to be anyway.

Steve