Mitre joints
There are a NUMBER of ways to accomplish this:
* Select/Vertices all the vertices at one tube end then
Modify/Vertex/Rotate/Selected (having selected local axis first). This
will rotate the circular end of the tube around its center stretching the
sides of the tube appropriately.
* use a block solid that you create just for mitering, possition it at the
appropriate point on the tube, and boolean subtract it from the tube (you
may have to tesselate the intersecting face, first).
* if you have ACAD and AME, use it's boolean operation to do the same as
above (AME's boolean is much less hassle cause you perform the boolean
before meshing the object).
I'd say the first option is probably simplest from a pure 3DS point of view.
Dave