Question on welding Fish-Mouth pipe connections

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Assume a 2" steel pipe, with a same diameter pipe welded at 90 in the middle. Is a stronger joint (tension, compression, shear, torsion) achieved by: 1. Drilling the base tube and inserting/welding the vertical tube 2. Fish-mouth the vertical tube and weld onto the base tube.

JR Dweller in the cellar

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JR North wrote:

If both pieces have the same diameter, you can't drill a hole in the base that's the same size as a vertical. OTOH, It CAN be done by heating, punching, and drifting.

I use fishmouth joints to butt weld similar and different sized pipe pretty regular. To my knowledge, none of the joints have ever failed - although some of the gates are showing evidence of some serious whiskey bumps.

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Seems the same to me, although if the welds break, the drilled one will hold in shear. Compression should be worse in the drilled one because the lack of material will allow bending in the long pipe.

Personally I think I'd go with fishmouth since it's easier than drilling that bigass hole... but then if I had a Bridgeport, everything would look like a job for an endmill...

Tim

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When I was building fishing boats we fitted 4" to 6" stack pipes all the time. We *always* used welded fishmouth joints.

By the way, you can't drill a 2.25" hole in a 2.25" pipe, you'd wind up just making two fishmouth joints!

Pipe welding is harder than it looks. I used to see these guys who could weld up pipe joints way out of position, often using a mirror while lying in a bilge under some machinery, and the welds came out looking machine made. I always thought these guys should get Olympic gold medals or something, to me this bordered on magical.

Grant

JR North wrote:

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For whatever it is worth; steel tube airplane fuselages are built with the truss members fitted to the joint contours. The longerons are nevcer drilled.

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Well, drilled "rosette" welds are acceptable practice in longeron repair, but only in addition to angle splices or fish-mouth splices. I think they're still part of the FAA repair manual, which is available as a PDF file somewhere online.

A book that tube welders would find useful, which describes this technique as well as a lot about making structural joints, fish-mouthing, etc., is the EAA's "Aircraft Welding" book. It's actually a collection of old articles from various sources, including the USAF repair manual, some of them quite old. But it's quite a bag of tips and tricks. Anyone who's welding structural joints in tubing would benefit from it.

Ed Huntress

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Several people have pointed out that, due to the wall thickness, you can't drill a hole into a pipe large enough to admit the end of a same-size pipe. It has also been pointed out that drilling such a hole is more difficult than forming a fish-mouth. So, for practical purposes the question has been answered.

However, just for the sake of discussion, lets suppost that the branch pipe is slightly smaller than the main pipe, so a hole can be drilled. It would still be weaker than a fish-mouth joint, because the metal across the side opening is gone, so it can't contribute any strength.

The only justification for drilling the hole would be if the T-connection has to carry flow, in which case you would drill a hole AND form a fish mouth.

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If you try to drill a clearance hole for a 2 inch pipe in another piece of 2 inch pipe, you'll sever the pipe. You'll wind up with 2 fishmouthed pieces made from the 1 piece of pipe you tried to drill. So now you have 2 fishmouth joints to weld instead of one. It won't be stronger than a single fishmouth weld to an intact pipe, it may be weaker.

Gary

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You COULD drill halfway through the main run pipe with a flat bottom drilling device, but I still think the fishmouth is easier and stronger overall.

RJ

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Various replies:

Why can't you? Of course you can - you're only drilling 1/2 way through to make a T joint.

Bob

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A only-slightly-better-than-ASCII sketch (577k BMP - sorry, Paint was the only thing that I could find to sketch with):

http://home.comcast.net/~bobengelhardt/t_joint.bmp

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Depends on a few things.

First, I presume you're talking about using the pipe strictly for mechanical purposes and not for carrying a liquid or gas.

Drilling and fishmouthing will give you exactly the same shape on the surface of the pipes. On the crosspiece of the T, you would of course drill only halfway through the pipe. On the leg of the T, you'd drill completely through. Drill both pieces, and it's just like fishmouthing both and would give you a joint that would allow flow.

As I said, the shapes would be identical on the surface. But the pipe walls have thickness. Drill just one piece - either one - and the full wall thickness of one pipe will be in full contact with the outside of the other. Drill both pieces, and they will be in contact only at the surface - the wall thicknesses will look FROM THE INSIDE as though you had vee'd them at 45 degrees each for weld penetration. Sort of like vee'ing out two butting plates full thickness, but then welding them from the back side. If you can weld them thay way and get full penetration and bridging of the vee, the joint would be fine.

Otherwise, and if it doesn't need to pass a flow, I'd drill just one piece - either one.

John Martin

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I don't think there's a question. You start with a sound through-member. Don't screw it up. Fishmouth the T-leg. If the wall is considerable grind the weld prep on the fishmouth. Instantly learn how to weld in all positions.

Hth, Fred Klingener

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I should have been more specific. In experimenting, I drilled a 7/8" SS tube section with a 7/8" hole saw to the centerline (female fish-mouth), and inserted a square-ended same dia. piece. It fit perfectly, mating flush with the bottom of the drilled hole edges. So the question concerns the relative strength under various forces of a female VS male fish mouth in the joint. The pipes don't carry fluid. JR Dweller in the cellar

JR North wrote:

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I think that the strength comes from the weld. And the two ways of fitting yield the identical joint, so they have the same amount of weld. I don't think there is *any* difference in strength, nor in amount of material. I'm wondering - after you drilled the 7/8" hole with the hole saw, did the waste lift out? I guess I was thinking pipe with a wall thickness of at least 3/16", not tube - I don't have any experience working with tube.

Grant

JR North wrote:

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Another question arises. Which method of fitting of the two pipes creates the most curvature of the main run during welding? In case others are un-aware, there is a tool to prevent this sort of curving during welding. It looks like a large wheel puller with hooked 'feet' to hook the main run, and a jackbolt to push on the main run at the outside of the intersection to prestress the pipe before welding.

RJ

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The waste was a curved piece of the base tube, minus the cutter width. JR Dweller in the cellar

Grant Erwin wrote:

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fitting

If you look at the forces resolved through the pipe with a hole in it, you'll see that any tensile or compressive forces transmitted along the side of the pipe with the hole have a break, where the forces now have to resolve up through the sides of the perpendicular pipe and around the edges of the hole. Where the hole is, there is no steel to resist tension or compression transmitted along that pipe.

It's much weaker and it's less stiff. If someone has the time to run it through an FEA program, it could be calculated exactly.

Ed Huntress

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On Sun, 01 Feb 2004 17:30:26 GMT, "Ed Huntress"

......and in reply I say!:

I need to put this another way....sorry.

You start with the wall of a round pipe, which looks like solid steel end on to a bending force. It is replaced by a hollow pipe being flattened side on, as far as bending that round pipe into the join. It's weaker, unless the wall of the leg of the T was really really thick.

Basically, flatten a piece of pipe end on in a press. Now flatten it side on. That's over the top, but gives an idea.

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vaguely proposed a theory

Uhhh..Ok. I think. <g>

Ed Huntress

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