Depending on how big your tube is, there may be an easier answer: a
What I do with mine if I need a perfectly square end on a tube is
to grab it in the chuck of my flexshaft, and then spin it against a
file, such that the file pretends to be a lathe, and cuts the end
This only works if the tube is small enough to be grabbed by the
flex shaft. You put the file straight across the end of the tube,
and get the flex shaft turning at a few thousand RPM. (middling
speed) then carefully run the end of the tube against the file. Make
sure the part of the tube that’s hitting the file is going the right
direction for the file teeth to cut. I do that to both ends of hinge
knuckles, and the hinges come out smooth.
Hope that makes sense.