How can I loft this tapered cross section on a tapered coil?

[dannym9999]

I'm trying to do a coil with an ellipse profile.  Both the coil and the cross section taper

But it needs guide rails, otherwise the cross section twists around before meeting the top oval.    So I can do guide rails, but first error is they don't intersect the profile.
So I came up with the idea of putting a line through 2 points on the square coil profile that extends out and splits the ovals in two, then I would loft in 2 ops.  Weird but that should make 2 rails- the diagonal corners of the square coil- that touch both profiles.

It looks logically solvable here, but I get a big yellow mess and "Rail cannot be tangent to profile".  I don't see the problem- tangents shouldn't just be legal, they should be *mandatory* for this op.

Then I remembered I've been stuck on this before- I can make up anything for a cross section, and it could follow the coil's taper.  That worked if the actual cross section didn't have to taper.  I couldn't find any way to make two different cross sections loft together with the coil guides, even though this looks perfectly correct and maybe the only way to do it.  But wouldn't work, not by any method I could find.

And I thought "hey maybe I should make a  

Any help here?

Like then I tried to switch to a triangle profile and make a new guide triangle from one side of the coil to a centerline point on the ellipse.  Planning to use the new triangle as a better guide.  But, same problem.  Says rails cannot be tangent to the profile??

[dannym9999]

The other thing I tried was to loft the new "inner triangle guide" on order to create a new coil line that intersects the top centerline point on each ellipse.  It will give a result with "centerline", but this makes the cross section rotate and is useless.

I went back and looked at Sweep.  Sweep will be able to "loft" the tilted ellipse profile on the bottom of the screen all the way up the tapered coil without rotating the profile in the wrong way, it just needs "rail + guide surface" on the triangle coil profile.  But, this can't be used to make the ellipse taper into the smaller top ellipse. 

This is so weird- Loft is basically doing the same thing conceptually as Sweep, but Sweep has the Guide Surface option whereas Loft does not, and it seems essential to be able to make this thing at all 

Here's the Sweep successfully controlling the angle of the ellipse (with a Section Analysis turned on)- that part is good, but the cross section isn't able to taper into the smaller top ellipse profile as required.

[davebYYPCU]

No problem my end, maybe it's your data.  Loft needs enough definition to understand the requirement, otherwise unexpected results are better than 90% of the time.

I estimated dimensions to look right.

Taper Coil.

Project and Project > Intersect the coil to a sketch.  Added 2:1 ellipses and 45 degrees to the cross sections.

Loft with one coil edge as Centre Line.

May need some additional profiles on the left side (quadrants?), as the Loft function tends to shrink wrap or sag and appear to be disproportionate without them.  (Yuk! Only good enough for Govt work)  

Might help....

[KristianLaholm]

My workflow uses Sweep with Path and Guide rail.
Path and Guide Rail are based on surface bodies, they are trimmed to start/end correctly with sweep path.
The sweep profile is on a plane aligned to the sweep path.

"First spiral" sketch is driving most of the geometry and dimensions.

2 last sketches are only for checking the geometry.

Zebra analysis in image for easier visualization. 

[etfrench]

Is this what you want?

This was done with a Surface workspace sweep plus split face command, followed by a loft command.

[dannym9999]

Unfortunately that's not addressing the problem, the profile is still twisting.  

[davebYYPCU]

I saw that with Ed’s.
Use @KristianLaholm method..  Simple to set up, Sweep not Loft.

Might help….

[etfrench]

You will need to add guide rails in order to keep it from twisting.

[dannym9999]

How would you get usable guide rails that follow a tapered cross section up the coil? If I had an op that created a usable guide rail like that, that op would already have created a tapered profile to do it. It would already have achieved the goal.

[etfrench]

A cone with the ID of the desired elliptical coil. A cone with the OD of the coil.  Split face command on both of those using the original surface helix to create the major diameter rails.   Copy the original surface helix, one above and one below to create the minor diameter rails by splitting the original cone.

Note: The cone shapes may need more adjustment to perfect the rails.

[davebYYPCU]

How would you get usable guide rails that follow a tapered cross section up the coil?

Did you check @KristianLaholm file?  Pic was captured from his file.

You need 2 coils, and then Sweep with Path and Guide Rail. (Very clever)

If I had an op that created a usable guide rail like that, that op would already have created a tapered profile to do it.

Correct.

Might help....