CAD - rendering bent pipes

Does anyone know of a free/gpl/etc library that could be easily
used to render calculated 3d surfaces under windows or linux/X?

I'm designing some jigs for bending coils of thinwall musical
instrument tubing where the adjacent coils must touch precisely (and
be stuck together with a nearly invisible bead of solder) but not
always at the same point as the tube size and bend radius change
slightly.

I can figure out how to write programs to calculate the centerline
position of the tube in space at any point along its length. What I
would like is an easy way to render the tube given the center position
and tube radius for a set of points along the length.

So far I've just rotated a line around the tube radius to generate a
'coil' of points on the wall, and projected the resulting 3d space
into a 2d perspective and plotted it - with the result that I get
something that looks like a slinky defying gravity. It would be
really cool to render it with solid shaded surfaces though.

One I get the design all worked out, it is trivial to change my program
to spit out 3-axis g code for milling jigs to wrap the tube around.

Chris Stratton

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Christopher C. Stratton
Engineer, Instrument Maker, and Horn Player
ccs@... 617 628 1062
http://web.mit.edu/~stratton/www/brassbuild.html