Steve B wrote in
rec.crafts.metalworking on Wed, 15 Dec 2010 22:27:52 -0800:

What I find fascinating is inspecting small square or
rectangular tubing. A welding crown is clearly visible on the inside, but
how in the world do they get it in such a small space? I'd like to see it
on How It's Made, or go to a factory for a tour.

Pipe and tube are made from sheet, rolled and formed into shape.
The weld is a resistance weld made similar to the way chain links are
welded, but on a continuous basis.

See: http://gobobpipe.com/goBobPipeMill.html
for a look at a machine that makes pipe.
or see a series of pictures.

http://gobobpipe.com/goBobMillTour.htm
The welding and scarfing is shown in step 7 and 8.
--

Dan H.
northshore MA.