How do you end a double-helix with a 3D curve that looks good?

[LMF5000]

Firstly, what's the best practice for making a double helix in Fusion 360?

I've discovered two methods:

1. Make a coil, then use the "move" command to create a copy that's rotated 180 degrees from the original.
2. Sketch two circles and sweep them with a large twist angle (360°-1080° depending on length).

My main question is, how can you link the two ends at the top to make a neat coil that's tangential (or even G2 continuous) with the two helical parts?

I've managed a rather clumsy approximation by drawing a 3D spline between the ends and using it as a guide rail for a loft between the ends. I've also tried using the pipe tool on the 3D spline... but both relied on moving the fit points to roughly align them to the coils of the helix by eye. I was wondering if one of you seasoned professionals could suggest a way to do this in a more elegant, parametric way.

The shape itself isn't that crucial, as long as it looks aesthetically pleasing. I've tried to terminate it with a shape that tapers as the function of this device will eventually be to hold rolls onto onto it to keep them organised (so a taper at the end helps it self-align with the hole in the roll), but terminating it "flat" (like a compact-flourescent lamp) is another option.

[laughingcreek · Accepted solution]

probably something along these lines. the curvature constraint between spline ends and helix insures g2

[LMF5000]

Interesting! So if I understood the timeline right, you swept a line in surface mode with a twist to make a helical surface, then imported that as 3D geometry into a sketch, drew a spline with a curvature constraint and made a pipe?

For importing the helix, did you use sketch -> create -> project/include -> include 3D geometry?

Edit: Just followed what I wrote above, worked perfectly. You've managed to answer both my questions! 🙂

[laughingcreek]

I didn't project anything. I worked directly with the edges of the sweped
surface. I also used a plane on path, and used that to draw the circle
profile. Then sweped the circle. Didn't use the pipe command, but could
have.

[LMF5000]

Ah, got you. I see that now. Thanks for posting your solution 🙂