On Wed, 7 May 2008 20:08:15 -0700 (PDT), wrote:

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?

Simple - if the part will fit over the ways.

Make a bit you CAN hold in the chuck. Larger is better. Bore a recess in
the face of the bit, big enough that you can reach in with a spanner and
turn the heads of four bolts that go sideways to the axis of the lathe, for
which bolts you have drilled and threaded holes. In essence you are making
a simple four-jaw chuck with outward-moving jaws. The work holding will be
poor; if you can live with marks on the inside of the non-machined section
make the ends of the bolts pointy.

Mount part and centre, bore (carefully) to suit.

If your part is long you'll have to use/make a steady as well.