#Mitre joints
In addition to the answers you've already received, if _all_ the angles of your
tubes are to meet at 45 degrees, you can loft the effect. Simple example,:
create a quad and a circle in the Shaper, input the quad as the path and the
circle as the shape. Turn off Tween, turn on Contour and loft. Result is a
tubular shaped quadrilateral with 45-degree bends.
– Jack