How the heck do you weld around a circumference?

Hello,
I am working on an exercise contraption that has a number of pieces of horizontal pipe, welded on the main structure, which provide a convenient place to hang the weight plates when they are not in use.
I am at the part where I have to weld the pipe sections on to the main structure. The pipe sections are five inches long, and are 1.25" schedule 80 pipe, and they will be fillet welded around the circumference where they join the main structure (the main structure is made from 2.5" square tubing).
So I'm all ready to weld these on, I have the main structure laying on supports on the shop floor, and I have the first one clamped up, ready to be welded around
Here's my question: how the heck to I maintain the correct rod angles as I go around the circumference?
I played with a couple pieces of scrap yesterday, and while I can get my weld started just fine, when I go to move *myself* around the work, I find it nearly impossible to maintain the proper angle/distance/speed of my electrode.
So how do you do it in a situation like this? Is the trick to give up on the idea of doing one continuous bead, and instead do the weld in two or three sections, so that you don't have to move your entire body while trying to weld at the same time?
What's your technique?
Jon

On 4/18/2013 9:33 AM, Jon Danniken wrote:
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Welcome to the joys and tribulations of aircraft construction. At least you are welding to a square tube and don't have to fish-mouth the joints. (Count your blessings!).
I've always done this kind of welding with a torch instead of stick. It's easier (for me anyway)
But you figured it out already. Do it in sections, and when you start a new section melt into the existing weld.
At the end of the day you will find muscles complaining that you haven't heard from before...

Jon Danniken wrote:
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Weld positioners exist for a reason, move the part, not yourself.

On 04/18/2013 07:44 AM, Pete C. wrote:
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I would have considered this, and while it would have worked on the test pieces, the actual part I am doing the work on is too big (7' x 3') to rotate in my shop.
Jon

I have one for sale, it is a welding lathe
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Are there pictures?
Not that I "need" a third positioner, but "want" is another matter...
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On Fri, 19 Apr 2013 19:12:47 -0700, Ernie Leimkuhler
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Ernie! Since you're here, what do you think about the issue of "cold starts" with MIG when you weld around the circumference of tubes in multiple steps?
It's not something I'd do, but I note that the people who make tube frames for kit planes go to great lengths to avoid it, to the extent of making elaborate rotisserie-type fixtures to rotate the frame while they're welding a joint. Some say that cold starts, which supposedly result in high local stresses, make MIG a questionable method for those life-critical applications.
What's your experience?

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I would never recommend MIG for air frames. MIG is a very fast process and results in welds that are lightly "quenched" by the surrounding cold metal, which can make the welds harder than the surrounding metal, which can lead to cracking under stress. Cold Starts can be caused by old or dirty contactor relays in the MIG machine that cause the wire to hit before the full welding current and shielding gas are present. One way to prevent a cold start from occurring in the weld is to start the weld bead off the joint and walk it over. Similar to how older Aluminum boat hulls were MIG welded before the pulsers took over.
Industrial machines are more tune-able and can have a pre-flow of gas and usually have more substantial relays. Another problem is Whiskering, where the wire continues to feed after the welding current has stopped, which causes the wire to fuse into the cooled weld pool. The problem with some aircraft kits is that below a certain size of plane there is very little regulation or inspection.
TIG is a slower process that uses less overall heat and results in welds that are more consistent with the surrounding metal.
I imagine if you had a high-end, digital, pulsed, MIG machine you could likely dial it in and achieve reliable welds that would rival TIG welds, but you are talking a starting price of $6000 for that machine.
I would still like to see some destructive test data, and proof that the welds were properly engineered and specified in a Weld Procedure Specification (WPS) and that the welders were certified to that Procedure.

On Sat, 20 Apr 2013 23:27:01 -0700, Ernie Leimkuhler
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Thanks for the insights, Ernie.

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Are the pipe pcs vertical? That will be much easier than if horizontal.
Also, idn't sched 40 normal? Sched 80 very heavy? I"da thought sched 40, in 1.25, would be more than strong enough.
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Proly difficult without a rotary sumpn or other. I would certainly break it up.

On Thursday, April 18, 2013 10:33:02 AM UTC-4, Jon Danniken wrote:
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Just last week, I had an exhaust leak in my car. The source of the leak was a flange connecting two pieces of pipe between the catalytic converter and the muffler. Since I don't currently own the proper equipment to do this m yself, I took it to a local muffler shop.
The guy cut out the offending flange with a sawzall and spliced in a new pi ece (about 5 inches long) of pipe. He supported the existing pipes with som e pretty neat adjustable jackstands and then tacked the new pipe in place. Then he migged the circumference of each joint in four or five sections. Th e part of the joint at the top of the pipe had barely enough clearance to f it the torch, and I was a little worried about the quality of the weld ther e. The guy gave me a mirror, and I took a look. I have to say, I'm impresse d. I could not see where one section of weld ended and the next began.
Though my upcoming welder purchase will almost certainly be a TIG, I now ha ve a new respect for MIG.

wrote:
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Don't know if it's the way the pros do it, but I tack opposite sides 6-8 times, then weld between tacks. I pretty much had to do that with a muffler that split circumferentially or it would have distorted too badly to make up. Going to have to have a healthy bevel to get penetration on that schedule 80.
Stan

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If you're not an exaggerated case of athletic eye-hand coordination, and you can't put the work on a turntable, you're pretty much limited to "segment" welding. If done well, it can look good, though not as good as a continuous bead.
LLoyd

On 2013-04-18, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:
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I can weld around a pipe in two halves.

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That's still 'segments', Ig. You must be more coordinated than the average Joe, but you still can't WALK around the circle and keep your angles, can you? <G>
Lloyd

On 2013-04-18, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:
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Lloyd, I will try practicing without the arc, it helps a lot, maybe I can learn to do it in one circle.
i

On 04/18/2013 09:39 AM, Lloyd E. Sponenburgh wrote:
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Hey Lloyd, the tricky part (at least for me) is joining the two segments. I've learned how to reverse the travel angle to fill in the crater at the end of a run, which at least helps to finish one segment.
Jon

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If you think about it (and have HF arc starting), you can position the rod exactly as it finished, and at the same point in the air as it was when you finished (usually lower, but then raise it back to the right arc length quickly), and except for some thermal distortion at the spot, you'll continue the weld almost invisibly without 'filling a crater'.
LLoyd

On 04/19/2013 05:26 PM, Lloyd E. Sponenburgh wrote:
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Thanks Lloyd, I'll give that a go. One of the things that the Navy manual doesn't cover is how to tie in a bead to an existing welding segment. It's a pretty important concept IMHO, and one which I wish was covered by a manual or reference work somewhere, preferably online (like the Navy manual).
Jon

On 20/04/13 01:26, Lloyd E. Sponenburgh wrote:
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I thought that using HF start for MMA wasn't recommended, not quite sure why but with my Hitachi inverter unit and others I've seen the instructions for it wasn't to use HF for MMA. I tend to leave the stick welding to my old oil cooled welder and leave it on 80V OCV and strike an arc with a new rod on a piece of scrap and I then find I can just place it on the weld site and it'll light up nicely, at least that's the case with 6013 and the anvil hard facing rods I was using a few times recently.