Doug Miller wrote:

I needed to shorten the tailpiece for a bathroom sink drain: chrome-plated
brass tube 1.250 OD with 0.030" wall.
OK, no problem, I'll chuck it up in the metal lathe and part it off.
Should be easy, right?

No sooner does the parting tool start removing stock, than the tube begins
slowly but inexorably sliding out of the chuck.
Machining thinwall tubes is very difficult. One way to do it is to have a
mandrel just a couple thousandths smaller than the ID, the chuck jaws
squeeze the tube onto the mandrel and then it will hold. Six jaw chucks
help, but not too many home shops have these. Or, make soft jaws for the OD
of the tube, a grip around the entire OD of the tube will also be more
secure.

But, holding a 1.25" tube with .030" wall in a 3-jaw chuck is not going to
work well.

I'd just cut it with a hacksaw and then clean up the edges.

Jon