I have an aluminum tube that I need to bore about 1" at end out to at
least 5.57" -- it varies from about 5.47" to about 5.60".
Unfortunately, this is too big to put on my Sherline lathe--it won't
fit onto the outside of chuck that I can find for the Sherline. I
have this vague notion that there might be some way to do this by
putting an end mill in the headstock and rotating the workpiece so
that the end mill removes a fairly consistent layer. This doesn't
have to be spectacularly precise. I tried this approach, and except
that I couldn't figure out a way to get the workpiece to turn
consistently (since it wasn't supported), it almost seemed to work.

Does anyone have any clever ways to do what I am trying to do, without
breaking down and buying a bigger lathe? Some clever way to mount the
tube on a 3.5" OD chuck, for example?