Interesting "Bug" In Sweep Curves (Wildfire 4)

Yesterday and today I set out to draft a cooling coil for a coworker. It's a simple, flattened spiral of copper tubing lying on its side,
with two "risers" coming up from it (to come up and over the side of the tank the coil will be sitting in). Think oblate spring lying on its side, with its two ends pulled out and running vertically upward, then bending over the lip of a tank. The first riser descends to the bottom of the first coil, and then the tube turns in 6 coils. The ends of the coils are basically 180-degree, 2-inch-radius bends of the tubing. At the end of the last coil, the riser comes up off the top of the coil, rises up the side of the tank, and over (shaped like a candy cane).
I started out with a single set of Datum Points, using Offset Coordinate System Datum Point and Cartesian coordinates, defined in the order they would occur along the tube. The end of each coil would consist of two 90-degree bends, and thus had two of these Datum Points. Each coil-end was progressively defined to be ~1 inch further in Y than the previous end (those two points' Y-coordinate was 1" larger than the previous two points) to get the spiral effect. I then used this set of Datum Points to define a single Datum Curve, using "Single Rad" and "Whole Array", starting from Point 0. Then, I created a Swept Protrusion along this Datum Curve, using "Insert", "Sweep", "Protrusion", "Select Traj" (here selected the first segment of my Datum Curve), "Curve Chain", and "Select All".
Originally I drew my sketch (for the sweep) of the cross-section of tubing so that it would start out on the inside of the curves of the riser... however, this would put it on the outside of the curves of the loops, making the tube itself have different bend radii depending on if it was in a loop or in the risers, because the driving Datum Curve was using only one radius, Single Rad.
So instead, I changed my sketch so that the sketch would sit 90 deg at the side of the Datum Curve (90 degrees to the plane of all the "bends"), and the plan was it would stay at the side of the curve all the way through all the loops, thus keeping the same tubing radius throughout the part. Here's where the bug appeared-- in practice, as ProE drove the sketch along the length of the Datum Curve, after a loop or two the profile started to "migrate around" the Datum Curve until at the end, it had moved to the top/outside of the Datum Curve instead of sticking to its side!
Of course, this resulted in the tubing's "bend radius" gradually changing throughout the part, rather than staying the same as I wanted it to.
If I make the Datum Curve use multiple radii instead of Single Rad, and put the sketch back on the inside or outside of the turns, this "sort of" fixes the problem-- but Pro/E still restricts my choice of radius to the max for the *smallest* turns... so the bigger turns look more like rounded corners than actual 180-deg bends of tubing.
Any suggestions?
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I'd start with a horizontal center line through the trajectory, then snap the circle to that and dimension/constrain to trajectory curve. I've found that swept sections always behave more predictably with a horizontal constraint to additionally guide the section's path. Then spline>single rad should work fine.
BTW, have you thought of an "X" for establishing the curve points? The "X" could actually represent a mounting frame for the tubing, distance between points=tubing dia+spacing x 1.414, a simple pattern, although the 3rd leg of the "X" will be a tubing dia+spacing less than previous two lengths (maybe not a "simple" pattern).
David Janes
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