Plasma tube notcher

What I call the R-theta tubing notcher is more accurately described
as a rotary x-y plasma cutter. The machine control thinks it is
controlling a regular x-y coordinate system except that x is really the
circumference of a cylinder.
This machine was designed to make tubular space frames like race car
chassis and such. Using a solid modeling package (Solid Edge in this
case) You can make intersecting cuts of almost any shape (as long as
one of them is a cylinder). Then you can "unfold" the curve and tell
the machine to cut in x-y coordinates.
A company in texas makes a machine that does the same thing as mine
except it costs about 10 grand and doesn't have bar feed. Check out

http://www.cypressweld.com/saddle.htm


One of the serious challenges I have faced is the ability to chuck
different size tube. Right now I have bushings of different I.D.'s to
accommodate different diameter tubes, and a different config. file in
the controller (note that steps per inch changes with tube diameter)
You will see from the above web site that a chuck with a 4" thru
hole is a monster. I tried collets but ran into the same thru hole
problem. 5C is limited to about 1", and larger collets are very costly,
and hard to come by.
I would like to build a chuck as is found in new pipe threading
machines. Run forward and it tightens, reverse, loosens. Can anyone
give me insight as to how these chucks are designed?