Assume a 2" steel pipe, with a same diameter pipe welded at
90° in the middle.
Is a stronger joint (tension, compression, shear, torsion)
1. Drilling the base tube and inserting/welding the vertical
2. Fish-mouth the vertical tube and weld onto the base tube.
Dweller in the cellar
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
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...
"That's for the courts to decide." - Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms
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.
JR North wrote:
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
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.
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
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
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.
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
Otherwise, and if it doesn't need to pass a flow, I'd drill just one piece -
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
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.
Dweller in the cellar
JR North wrote:
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.
JR North wrote:
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.
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.
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
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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