Shell tool not working on a lofted body

[m_eckhard]

Hi all,

I'm currently working on a proof of concept. For this I created a body using the loft tool. Since the final model is symmetrical, I'm only creating half of it. Now, I want to hollow out the body using the shell command. However, when I try to create a 1.5 mm shell, it does not work. It only lets me create a shell of up to 0.2 mm. The body in question:

Error when creating shell:

The body was created via a loft between two profiles with two guide rails. I turned on the sketches for better clarity:

The weird thing is: I had a design in the past where there were no problems with the shell. But I changed the profiles and guide rails since then, worked on other parts, etc. before moving the timeline back to the shelling tool so I don't really know what change broke it.

I tried isolating the issue along the body. It seemingly happens in a random place:

At the section in the middle there is no shell possible.

What's so special about this area? Does anyone understand why the shell tool is not working here? I've been trying to find a solution for this for hours now and I don't seem to understand it. It seems to happen at random when I edit the underlying sketches.

Any help is much appreciated!

[TheCADWhisperer]

@m_eckhard 

Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

[m_eckhard]

Hi @TheCADWhisperer, thank you for your reply!

There is only an option to export it as .f3z file. I have exported to .f3d before. Has it moved? Is .f3z sufficient?

[jhackney1972]

An F3Z file is sufficient.  You may have to ZIP it if the Forum will not allow you to attach it.

[m_eckhard]

Alright, I have attached it to the original question. Only "Fuselage_2" is of importance though.

BTW: did I miss something or why am I not able to export as .f3d?

[jhackney1972]

First, your Shell is having a hard time due to the extremely narrow pointed end of your model.  I do not hold out much hope for the shell to succeed at a 1.5mm thickness you desire.

As for the reason that your model cannot be exported as a F3D file is because F3D models CANNOT contain External files.  You have 5 external files in this assembly denoted by the Chain Link Symbol in the Browser.  If you had Copied Base_Mockup component and Paste/New to a new file, you could have exported it as a F3D file.

[TrippyLighting]

There are problematic. The sharp edge @jhackney1972 has pointed out is one, although the sharp edge itself is not necessarily the problem. THe problem i that there are two faces the transition from a curvature continuity to sharp. That is always a problem.

Then the  lofted surfaces in the highlighted areas also have strong curvature that is too small to offset 1.5mm.

A 0.1mm rounded offset works, increasing it to 0.2 takes quite long to compute and that's about it.

[m_eckhard]

Thank you both @jhackney1972 and @TrippyLighting for your quick replies. I appreciate it!

Also thank you @jhackney1972, I didn't know this was the case with imported components.

So, the main problem still is unclear to me. Especially since it has worked before. Even with bigger shells. I also don't really understand your concerns about the sharp trailing edge. I quickly tested it by creating a lofted body from a few very basic sketches. The second lofting-sketch even features a triangle - this is as sharp as it gets.

Here, creating a shell does not raise any issues:

Even much greater shells are no problem:

I also adjusted the sharp edge in the original body by inserting a line in the underlying sketch.

The problem still remains.

So I don't think the sharp is the root cause. But I also cannot exclude that it will never create an issue.

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@TrippyLighting, I'm not sure I can follow. I know what surfaces you are pointing out, however I never had an issue with that. My crude lofted body above also reiterates this. There's also curvature that is abruptly transitioning to a straight face.

So let me rephrase the question a bit: What would you change so that the shell is possible with a bigger radius?

[TrippyLighting]

, there are at least tangent For a start, I would NOT use imported geometry with bad curvature.

Loft profiles should be spiffy clean with smooth curvature.

I don't understand what surface transition you are imagining here. It implies tangency, but there is no constraint for it in the sketch. A rail that is tangent to a loft profile is generally not such a brilliant idea.

The shall command creates surfaces that are offset from the surface normal of the originating surface. In that area, the shell command would have to create a very sharp-edged surface if used without the rounded option. 

Not only is this sketch not dimensioned, but it also requires at least tangent constraints in the highlighted area. Otherwise, near-tangent conditions also can effectively prevent shelling.

With the sketch inputs fixed, and some changes to one of the loft rails, and enabling the 'keep tangent edges' option on the loft (essential when profiling sketches contain tangent constraints), this shells to 1.5 mm. It ain't pretty, but it works.  

Your term "crude" means sloppy. Lofting creates complex NURBS surfaces that don't like sloppy 😉

[johnsonshiue · Accepted solution]

Hi! There are multiple ways to work around the issue. In this particular case, the precise shell or offset cannot be created, probably due to high curvature area at the bottom and near the tail.

Without changing the original design intent, I used Surface Modeling technique to simplify the geometry and create a hybrid precise inner surface to hollow the solid body.

Please take a look at the features I created after Loft2 and see if you can replicate the process. It is highly doable but it does require a bit of good understanding of how Surface Modeling works.

Many thanks!

[TrippyLighting]

@johnsonshiue Nice work!

[m_eckhard]

Thank you all very much for taking the time to help me!

I understand both of your solutions, however I'm not quite sure about the "why". I am still not sure how to avoid this issue in the future or what's the best practice of designers to create fuselages that blend fuselage and wing. I also think this is important for people that stumble across this post in the future.

So in the following I'm trying to explain what I find hard to understand.

Tangential constraints and lofted bodies

Beginning with your solution @TrippyLighting. I added dimensions and also added tangent constraints in the sketch areas you highlighted here 

and here 

.

However, I'm not able to see improvements regarding the shell. Yes - adjusting this guide rail 

helps. But I'm not sure why a vertical constraint is not helping here which in my mind would make the spline (red) perpendicular to the horizontal, projected line (purple). Instead you chose to tilt the control points of the spline to the right. The reason of which is not clear to me.

You also wrote that

"a rail that is tangent to a loft profile is generally not such a brilliant idea.

The shall command creates surfaces that are offset from the surface normal of the originating surface. In that area, the shell command would have to create a very sharp-edged surface if used without the rounded option.".

- Why is that the case? Isn't calculating a shell of a tangential guide rail which leads to a tangential surface easier to calculate?

"Your term "crude" means sloppy. Lofting creates complex NURBS surfaces that don't like sloppy 😉"

- Exactly! That's what I was trying to show. I created this crude body as an example that creating a shell is even possible with a body this sloppy :). This was to elaborate my confusion.

Surface modeling (workarounds) instead of shell tool

@johnsonshiue I like your approach, thank you very much!

However, I'm not sure why surface modeling is the (best) way to go. Isn't this more of a workaround or would you design a similar fuselage in the same way? What would you say is the best approach to create a fuselage with blended wings? Isn't this what the loft and shell tools are for? Is there even a way without surface modeling?

Ultimately I'm still unsure what I did wrong like I said in the beginning. What bodies are "shellable" and which aren't? What guide rails are best? And so on.

[TrippyLighting]

---

@m_eckhard wrote:

...I'm not sure why surface modeling is the (best) way to go. Isn't this more of a workaround or would you design a similar fuselage in the same way?

---

Nope!

Under the hood, Fusion and any other solid modeling software work with trimmed analytical surfaces  (simple shapes such as planes, cylinders, cones, etc.) and NURBS surfaces for free-form shapes. Those are stitched together into a BRep.

There are many shapes that can only be created using surface modeling.

Solid modeling was initially invented because, for pure mechanical designs, creating geometry with surfaces was very time-consuming.

The exterior and interior A-Surfaces of most, if not all, commercial aircraft, automotive designs, and industrial designs (Apple, etc.) are created with surfacing software only. Autodesk Alias is one such software. Once these surface models are made, they are handed off to engineering software.

Fusion and most other mainstream solid modeling CAD have "some" surface modeling abilities.

[m_eckhard]

Thank you very much for the explanation!

I tried replicating your approach (message 9). I'm sure I copied every step exactly like you did. But I'm not able to create a shell at the end. I'm very sorry to bother you all so long with this problem. I still cannot see why it's so difficult for me to solve. It seems so simply and straight forward... I would appreciate it very much if you could take a look at the surface modeling steps at the end of the timeline. Maybe you can spot a mistake I'm not able to see at the moment.

[johnsonshiue]

Hi! To add to what @TrippyLighting already mentioned, Solid Modeling and Surface Modeling are almost the same. The similar set of tools are available to create the desirable geometry. The major difference between the two is about the ability to manage intersection. In Solid Modeling, the intersection is largely due to geometry created by the features. The user manages the intersection by changing the output type (Join, Cut, Intersect, and New Body) to ensure the body is watertight. Surface Modeling offers much more flexibility in this regard. Each piece of surface body is an independent body. You only need to form a watertight volume when you want it to be a solid body. Also Surface Modeling can help define curves when they are extremely difficult to create.

For a curvy model like this, I would use Surface Modeling techniques. In the meantime, I would make sure the geometry stays as simple as possible. I wouldn't create additional sketches when the body geometry is readily available.

In terms of the Shell failure, the geometry as is has very little chance to get a precise offset (every point on the model has to be offset normally to a given distance). It is because there are quite a few high curvature areas on the model. The offset in those areas likely will self-intersect, leading to Shell failure. Alternative approaches as @TrippyLighting and I demonstrate here are needed.

Many thanks!

[m_eckhard]

Thank you so much for your elaborate answer! I think other users will find this helpful as well.

[m_eckhard]

@TrippyLighting, is it possible for you to take a look again at my attached file "Remedy_UAV_Mockup_ v13.zip" in message 14? I'm not sure if my message was lost in the buzz (I'm sorry if you just hadn't had the time yet 🙂 )

[TrippyLighting]

I have previously identified the problem areas. Here is a video that visually explains, using another CAD application, why the tail area remains a problem.

[m_eckhard]

Thank you very much for the visual explanation! I appreciate it very much!

Alright, I will use @johnsonshiue's approach then. I will also mark his reply as the "accepted solution".

The confusion as to which I asked you again to look at my (updated) model, arose from the fact that you were able to use the shell tool in the end after creating the body with surface modeling. I tried it step by step as well but I could not get it to work. However, if you say that @johnsonshiue's approach is more consistent with this geometry then I will use it instead and stop worrying about that. Also since I don't see a way around the sharp trailing edge because that's just a result of the shape of the airfoil which will happen with every blended wing fuselage if I'm not mistaken.

[TrippyLighting]

I want to add one more thing. You asked what you did wrong. I don't think you did anything wrong here!

For more complex geometry, often the solid modeling tools reach their limits, so you need to move to surface modeling.

Surface modeling can be very "finicky" in Fusion and often requires a bit of experimentation.