Aircraft weldors have always used fishmouth cuts to join tubing of two different sizes. Also, a double fishmouth sleeve of larger tubing can be used to join two sections of same-sized tube. Here is a technique that allows you to butt two sections of tubing together with a wonderfully strong splice made of the same tubing size!
What you do is cut your two fishmouths so the notches of the cuts are a little over one OD apart. Then, you simply split the tube between the notches with your hacksaw so you now have two equal halves. Finally, butt your two equal-size tubes together and clamp the two fishmouth pieces onto the joint with a C-clamp or Vise Grips(R). Because the half sections must be forced to spread out around the tubes, the joint is essentially self-aligning! Once you have a few tack welds in place, the clamp can be dispensed with and you can finish weld all the edges, INCLUDING the entire length of the longitudinal 'split' cuts on each side.
If you carefully file the butted ends before joining, this method will give an extremely smooth interior surface to the joint, since the only full-depth weld will be the two tiny butt joints that appear between the edges of the longitudinal split on each side. The resulting joint is far stronger than a simple butt weld because the total weld length is far greater.
L Cottrill
Strong Welded Butt Splice for Tubing
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larry cottrill
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Strong Welded Butt Splice for Tubing
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- The final appearance of the joint, fully welded. Photo Copyright 2005 Larry Cottrill
- Fishmouth_splice_welded_crop1_small.jpg (179.23 KiB) Viewed 3970 times
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- After applying a few small tack welds, the clamp is removed. The joint is now ready for finish welding. Photo Copyright 2005 Larry Cottrill
- Fishmouth_splice_tacked_crop1_small.jpg (182.01 KiB) Viewed 3968 times
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- As you apply your clamp, try to get the two split halves well aligned, and the gaps approximately the same on both sides of the tube joint, then finally tighten the clamp in place. Photo Copyright 2005 Larry Cottrill
- Fishmouth_splice_jigged_crop1_small.jpg (165.81 KiB) Viewed 3969 times
re: Strong Welded Butt Splice for Tubing
Very nice Larry.
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larry cottrill
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re: Strong Welded Butt Splice for Tubing
Thanks, Dave!
I just recently thought of another variation that might be even better: Cut out your double fishmouth section and just split ONE side! Then, use a screwdriver to spread the ring just enough to slide it onto one tube end, and when inserting the other tube. That should be self-aligning without clamping, and there would be a little less welding. The finished joint would look just like the third photo on one side, and like two normal (un-split) fishmouth joints on the other. Don't know why I didn't think of it sooner ...
Glad you like it!
L Cottrill
I just recently thought of another variation that might be even better: Cut out your double fishmouth section and just split ONE side! Then, use a screwdriver to spread the ring just enough to slide it onto one tube end, and when inserting the other tube. That should be self-aligning without clamping, and there would be a little less welding. The finished joint would look just like the third photo on one side, and like two normal (un-split) fishmouth joints on the other. Don't know why I didn't think of it sooner ...
Glad you like it!
L Cottrill