Creating a transition while keeping a set length of line

[on4now4]

I am looking to form a tube. For example lets just say I want to go from a round 60 mm tube to a square shape tube. But in the transition area I want to retain a length of line through the transition... Is there any way to do this besides cutting lots of sections and measuring each one making lots of sketches and modifying slightly...

I would think there should be a way to do it in the blend command but I have never found anything.

 

[jackk]

A transistion can be created with either Multi-Section Surface or Multi-Section Solid to fit between two or more sketch profiles. Use a Spine, Guide Curves, and/or closing points to control the shape.