Three profile loft's default rail twists the feature, and I can't untwist it
@jordanJM463 - It looks like this is a bug in loft point mapping in Fusion. Rail curves are the usual way to control this, I'm surprised it does not work. Can you share the model here so we can investigate?
Jeff Strater
Engineering Director
While Jeff might be surprised and unhappy that this does not work I am not 😉
It simply isn't a good idea to create a loft like this because it usually generates terrible geometry.
Below is an image of the curvature map (Inspect-.Curvature) map.
The patchy look is likely reason that this does not thicken.
The video is ages old and shows a specific workflow, but I would not consider that a best practice.
I would not call the spline buggy before I had a full understanding of what that spline represents.
A Fusion 360 fit point spline is a multi span 5 degree spline, a very complex beast.
Interestingly enough the kernel can really only to 3 degree NURBS surfaces so an approximation is made somewhere.
there are several layers of math dnd algorithm at work here probably down into the geometric modeling kernel.
Can you be more specific on these requirement ?
In general it isn't a bad idea to break this into several individual lofts.
@jordanJM463 wrote:
Also I see you don't work for Autodesk, so snark is definitely not pointed towards you!
Well ... I hope you don't get snarky with Jeff either 😉
The tutorials is ok for what it demonstrates and it also does not create that twist.
If you turn the Inspect->Curvature map on on your straight lofted structure you'll see it looks quite OK.
However, as soo as you move the spine point I'll likely burst into rainbow colors and get all patchy.
Would you be able to post that filleted version so I can get a better understanding of what you're after ?
to answer the original loft question: The behavior you show in your screencast is definitely a Fusion bug. Editing the loft and adjusting the point mapping seems to be failing - the point mapping is not remembered. That bug is FUS-49011.
Usually, the way to force point mapping is to use rail curves. In the screencast below, I added rail curves. Does this produce the geometry that you need?
dca8db9a-5cbb-41b8-9c60-a10559acd3d1
Also, with this change, I was able to Thicken this to 1 mm (probably more) outward, and 0.4 mm inward:
The curvature map for this still shows areas of high curvature, which will cause Thicken issues:
Jeff Strater
Engineering Director
I used this line between the bottom and top triangles, project/intersected it into the middle sketch to size the middle triangle with.
And then I adjusted the fillet radii of that middle profile by eye after the fact.
Now I've realized I could have made that line - not from the construction triangles - but instead from the profiles themselves, and used a tangent arc coincident to that intersect point to let those radii automatically be correct. That would be better than eyeballing it.
I'm also wondering now if the middle profile is even necessary. Is there a design constraint that requires conical corners down to a certain point before the transition can begin? If you just loft bottom-to-top only, it's even smoother.
sigh, my screencast is not posting. Try again. Here is the direct link: https://knowledge.autodesk.com/community/screencast/dca8db9a-5cbb-41b8-9c60-a10559acd3d1
Jeff Strater
Engineering Director
