[Goofour]

I am narrowing a Chevy 12 bolt for a HotRod I am building. I cut the tube 2" from the flange and offset half of the cut 1/4" for extra safety. Both ends of the seam have beeen beveled and the end were tacked welded back on using a straight rod going through the complete rear end. My question is using a Lincoln SP-135 set at G/3 and using .035 flux cored wire. How do I finish welding without warping the tubes and changing the alignment?

[DSL PWR]

Just tack it well, and square. Then You run light beads all the way around untill the bevel is filled. This is how pipe line's are done.

[Goofour]

Thanks DSL when you say light beads are you talking 1/4 long or maybe 1" at a time?

[Goofour]

One more question if I could. Is it better to work one end of the axle and keep the tube hot until I am finished or work both sides and let it cool to the touch before welding again?

[DSL PWR]

Thanks DSL when you say light beads are you talking 1/4 long or maybe 1" at a time?

What I mean is don't try to fill the bevel in one pass. Once the tube is tacked, you can rig the axle up on a pivot (so it can spin) and do a pass all the way around the tube at once. Just set the welder so you get penatration, but don't burn thru. After the first pass you can probably up the power on the welder so you get good penatration of the base metal.

It is also prudent to clean the weld up after each pass, even if you are using a gas mig machine. If there is ANY question of the quality of the weld (pitting, bubbles etc.) grind it down until it looks good. Remember just because a finish pass on the weld looks good, poor quality under it will make life interesting.

[DSL PWR]

One more question if I could. Is it better to work one end of the axle and keep the tube hot until I am finished or work both sides and let it cool to the touch before welding again?

You should be able to work on one side at a time.

[3t3d]

After it is done, you can set it up with the two tires on milk crates or similar.
Holding the tires still, rotate the center (pumpkin) section.
Measure the toe in/out of the two tires. Rotate center section.
If the toe changes, one of the tubes is not straight. Finding out where/if
the axle is bent allows you to use heat to straiten the axle tube after
the welding is done.
Best to get it as straight as possible at first. It can be fixed if not perfect.
Some race cars may have camber, and toe in/out bent into the axle tubes
on purpose.

Pete

[Hobbiest]

Pipelines don't have to worry about being straight and true, rear axle housings do. I would suggest you take the cast iron route in regards to welding. When you weld cast iron (on anything that can have no leaks), if it is too hot to hold your hand to it, then you should stop welding until it cools. I would suggest no more than an inch bead at a time. Unfortunately, everywhere you stop, you will most likely have a small leak. the only way to remedy this, is to grind the beginning and end of the beads, and start back from the edges, continuing into the new beads. I have spent a lot of time welding cast iron (engine blocks, water pumps, etc), gas tanks, and have even done a few rear ends myself, and can tell you that unfortunately warpage is going to occur. The aformentioned method of being able to tell where the housing is out, works very well. Just remember that heat pulls the metal together (shrinks it). Good luck. If it is too warped for you to figure out how to straighten it, take it to a decent welding shop. I have found that it is the old guys who are best with this kind of thing, and the ones who work on a lot of farm equipment are especially good for some reason.

[HuFlungDung]

A bit of theory on what is called "upsetting" is helpful to understand how to control welding warp. If a concentrated heat is applied to one spot in the center of a plate, the surrounding metal acts just like a vise: the hot spot needs to expand, but it can't expand outwards because of the vise-like effect of the cold sheet around it. So, it expands upwards. If the heat is applied long enough for the part to become a dull red (and if the material had zero stresses to begin with), the metal is said to be "upset", and a quick quench of the area will intensify this effect.

The cold upset area is now greater in height and less in diameter than it was initially. So, now it acts like a winch and pulls inwards against the cold metal surrounding it. This creates the warp factor. A little bit of practice, and you can take advantage of this property to straighten a large tube or shaft.

Beware that lots of ready made materials have built in stress from cold working. C1018 shafting is bad for this, because heating a spot on it will have completely the opposite effect to what I just described. This is because of the skin stress created in the material by being stretched in processing. So a little bit of heat immediately relaxes one side of the shaft, and the still stressed skin on the other side pulls it away. If you mill C1018 cold drawn shafting or keystock on one side, you will also see this same effect of the stressed skin warping the piece as soon as you take it out of the vise.

So, back to the welding and upsetting. If you preheat the entire area, you reduce the upsetting effect. I recommend 500°F preheat in practically all circumstances (we use lots of propane up for preheating ) Now, the red hot weld zone is only 1000° hotter than the surrounding area, rather than 1500° (approximate figures). If you are fast with the ball peen hammer, you can "undo the upset" by peening the hot weld zone (while it is still red hot). And I mean hit it, not just tapping on it to make a noise

I'd start with a few good, but short, root welds on opposite sides, but I'd use a stick welder for this. It gives you a little more time to work. But, you use what you've got. It is important to work quickly to get opposing welds laid as fast as possible. Test whether your alignment shaft is binding up as you complete each pair of opposite welds. Its the first welds that have the most effect on the outcome of the alignment. You'll have to allow the temperature around the tube to equalize completely before you'll know how straight it is. This means that it is slow going at first. I use a digital thermometer to check the temp in several positions around the tube.

I'd take the precaution of wrapping the alignment shaft with some paper, so that if a bit of warp occurs, I won't get the thing seizing in the hole when I go to drive it out.

If you test the housing back plate flanges for wobble with the alignment shaft still in place, you can deduce where the final bend state is. Than, deliberate "upsetting" can be performed with an oxyacetylene torch in several places along the tube. I recommend oxyacetylene for this because it is very hot, and successful flame straightening requires fast heat to upset the spot quickly. Quench with a wet rag.

If you bend it the wrong way, I am not responsible

[Hobbiest]

In the shop where I learned to arc weld, I was taught to heat the metal until it just barely changed color, not to an orange or red or anything, just until it looked a bit different, and to test the preheat by spitting on it! If the spit bubbled, and ran off immediately, the metal was about 400 to 500 deg. Testing this theaory with a temp crayon prooved it to work. One added point to upsetting...in sheetmetal work, you can restore dents in metal by heating a spot cherry red, then quenching it. As Hu said, when you heat one spot, the molecules want to spread apart, and in so doing, they cause the colder adjacent molecules to pull in closer to eachother. When you then quench the red spot, it cools it fast enough to keep most of your shrinking work there (if you let it air cool, some of the shrinkage will go away). Hammer and dolly-on (lightly) will then take out the high dimple of upset. BTW...when you are hammering dolly-on (sandwiching the metal between the hammer and dolly), you should not hear ringing unless you are trying to stretch the metal, instead you should hear a dull thud.