why does this loft behave strangely?

[C.FOTI]

I'm wondering why I can't get the loft that I'm expecting from this sketch.  The sketch was made by copying/pasting an original slot, then drawing spline for rails.  In a nutshell, I don't want it to bulge the way it is, and wondering why it's doing that? Also at the tip area of the loft there is some twisting that's happening, can't figure out why Fusion is doing that.  Can anyone please illuminate me on this issue?

[TrippyLighting]

I'd love to see the final shape you are trying to create. Could you possibly create a pencil/paper sketch and post it?

I would never use a 3D sketch to create this sort of shape. Neither would I use a single spline as he rail.

Nor would it need that many profiles to create the shape I think you are trying to create.

[g-andresen]

Hi,

here is a principle of what @TrippyLighting  has recommended

günther

[C.FOTI]

The shape is as you see it, without the bulges and flares.  Imagine the loft traveling through all the profiles cleanly and following the rail path.

I understand that this is way too many profiles for this shape, which can be done with 2 profiles and 2 rails as @g-andresen  points to in his post. 

I am trying to understand why Fusion is behaving this way.

Am I running into a bug or limitation in the Loft command?  Is Loft not meant for more than X number of profiles?  Just trying to understand the innards of the software better.

[C.FOTI]

The shape in the video tapers on the Y plane, along the top of the shape.  Say you wanted a taper only in the last 2mm.  I presume you would use another profile?  What's the limit on profiles, if known?

[jeff_strater · Accepted solution]

@fotiadis1 - this is an interesting case, thanks for sharing it.  Loft is a tricky beast at times...

There are basically only a handful of knobs and switches you can use to control a loft shape, but with those few controls, you can get a  huge range of outputs.  To get the output you want, it requires understanding those controls, and a lot of patience to tweak them.  The controls are:

1. profiles - both the number of them and the geometry in them.  IMO, this loft has more profiles that it probably needs.  The geometry of the profiles, though, is nice and simple
2. rails - again, the number and geometry is a factor here.  There are two types of rails - centerline rails (IMO, these are often overlooked.  I love using centerline rails, as it is a way to "influence" the result without all the accuracy needed by "side rails".  Side rails are very precise instructions to Loft - you must follow this curve (with a few exceptions) exactly
3. end conditions - specifying the "takeoff direction or condition" is a way to influence the shape at the start or end profile.  There are some techniques that you can use which include constructing "helper surfaces" to control the loft.  There is also a "takeoff weight" knob which can be applied to these conditions, which influence how far along the first span of the Loft the shape is affected
4. point mapping.  This is actually a problem in your loft.  I'm not sure why, but it can be tweaked to help.  See the screencast below

The "bulge" you refer to is just kind of an artifact of the loft algorithm.  Think of Loft having some momentum (like a go-cart going around some sharp corners).  If the corner is too sharp, you are going to swing wide coming out of it.  Something like this:

Partly, the bulge is from too many profiles, and partly it is from those corners being too tight, and loft drifting wide to try to get to the next corner.  I'll do another reply (only one screencast per post annoys me...) on some ways to deal with this.  

The mess at the end is a "point mapping" problem.  I have no idea why the default point mapping is so bad here, but you can clean it up.  Those white points are draggable:

here is a screencast showing how to edit the point mapping on this loft.  It helps, but is not perfect:

[g-andresen]

Hi,

You have to deal with splines to get a feeling for their behaviour. In the first place you have to pay attention to use as much as necessary and as few control points as possible. The same applies to the profiles in lofts.
For this purpose, the respective tangent length must be in a reasonable relation to the distance between the control points.

günther

[C.FOTI]

I can see what you mean about distance, angles of rails points, etc.  Eventually I got the shape I wanted, with fewer profiles. 

It would be great to have a control that adjusts 'velocity' between points...that could make for some interesting possibilities.

Thanks for the detailed answer! 

[jeff_strater]

and, here is another screencast that shows how using fewer profiles + a centerline rail can help start to eliminate that bulge.  To strain my racing analogy past the breaking point, this lightens up the cart (fewer profiles), and puts on better tires (centerline loft).  The profiles at the end are still problematic.  There are too many of them too close together.  It somewhat depends on the shape you want there, but some tweaks at that end can help, too.  The cool thing about the centerline rail, if you use a spline, is you can drag the centerline, and very quickly see how it influences the shape, as seen in the screencast.

One thing I like to do with Loft is to make my profile sections on offset workplanes (or even plane along path, using the centerline rail as the path) wherever possible.  That makes it quite easy to adjust the positioning of the profiles to see how it affects shape.

[TrippyLighting]

ONe "thing" that is prevalent in almost all failed loft attempts, at least those that actually do create a solid, or surface body - is that there are curvature problems with the profiles or rails:

THis loft begs to be lofted in individual sections.

 

[C.FOTI]

I did notice that I could get it to loft a lot better when doing in several different sections, however in the end chose to reduce profiles and fix my curves.  Now if it would only Shell without giving me errors!

[TrippyLighting]

Curvature, curvature, curvature ,

 

[C.FOTI]

While I am still technically a newb when it comes to Fusion, and I understand there's a great degree of curvature but don't see why Fusion can't do it if high curvature exists.

If I exported as a faceted body, I could easily do it in Meshmixer... but I want to do it in a parametric design, not facets. 

So what would be the easy resolution if this shape won't shell?  I was thinking of using Sweep to try an assemble the shape that way, but that doesn't seem to fit the easy criteria 🙂 

[TrippyLighting]

Because mesh mixer works with meshes (finite resolution) vs. Fusion 360 works with BRep and NURBS surfaces (mathematically precise).

[jeff_strater · Accepted solution]

I created this loft as two lofts, with tangent continuity between the first and second, and tweaked the takeoff weight, and I was able to get it to shell to 2mm.  Other times I tried that, I had to tweak the rail curve on the bottom, to smooth out that area, but this time it went through just fine.  Model attached