Basically you drive a pin into the hole of the tube and that causes the tube
to expand in diameter a bit. The pin will have a tapered end on it so that
it starts into the hole without forcing. Lubrication is a necessity when
doing this!
If the tube is going to be permanent and is a tube just for price
conserations or weight considerations, I'd just find a ball bearing (or turn
a similar pin) that is large enough to force a good fit and drive that into
the hole of the tubing with a press and you will have a nice tight fit.

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Why do penguins walk so far to get to their nesting grounds?