Allan Adler wrote in message ...
(1) How would one make a long metal pipe (e.g. a few feet) which is as thin
and as hollow as a syringe needle?

http://www.smallparts.com/products/descriptions/htx.cfm
Hypodermic Tubing, 304SS, welded and drawn - which means they fold
a sheet into a "U", then close it the rest of the way and weld the
seam, then draw it thru a die (and possibly over a mandrel) to smooth
the seam and reduce the diameter. They have 3' lengths as small as
0.012 OD and 0.006 ID. (I expect this is done with multiple stages
of drawing.)

http://www.smallparts.com/products/d...tx,%20ctni.cfm
Capillary tubing - they don't say how it is made, but I imagine it is
also drawn down from a larger size. The wall is thicker, and it doesn't
come as small a hypodermic tube - they have several sizes, all 0.062 OD,
with IDs ranging from 0.005 to 0.030. It is available in _much_ longer
lengths, up to 100'. They coil it - the wall is thick enough that you
can handle it much like wire of the same OD.

(2) And what if one wanted the pipe to follow some path other than straight?
Probably it is too thin to bend without closing it off, no matter how
carefully one tries to do it. Maybe one has to make miniature versions
of all of the standard plumbing fixtures and, to make sure the connections
are strong enough, one allows the outer diameters of the pipes to increase
where the threads are. So that feature of the pipe design would also have
to be included.

The capillary tubing has walls thick enough that you _can_ bend it into
a moderately tight raduis without pinching off. The hypodermic tubing
would be more difficult. You could fill it with a low melting alloy like
Cerrosafe, bend, then melt out the alloy.

John Kasunich