Self intersecting loft when using offset contour

[karlheinzstoeber]

Hello Loft Experts,

have a strange problem. The outer lines in the two drawings are each intersections of the body with the planes. If I select the faces formed by these intersection lines, the loft is created without any problems.

 

The inner lines are derived from the intersection lines by the sketch offset function. If I use the faces formed by this, the warning loft intersect itself will output and the loft will not be generated.

 

Does anyone have an idea why this might be the case?

Best Karlheinz

[g-andresen]

Hi,

Please share the file.

File > export > save as f3d on local drive  > attach it to the next post.

günther

[jeff_strater]

can you share the model here?  It's difficult to tell from just the screenshots.  Thanks.

[karlheinzstoeber]

Dear Günther and Jeff,

is a little difficult to share the file to the public now, because there might be some copyrights for the used profiles.

I would suggest to share a public link and send you the password by email.

https://a360.co/3cpca0X

Is this OK for you?

Best Karlheinz

[karlheinzstoeber]

Dear Günther and Jeff,

is a little difficult to share the file to the public now, because there might be some copyrights for the used profiles.
I would suggest to share a public link and send you the password by email.

https://a360.co/3cpca0X

Is this OK for you?

Best Karlheinz

[jeff_strater]

@karlheinzstoeber - I sent you a direct message that you can reply to with the password.  Thank you!

[karlheinzstoeber]

Looks like the first link doesn't work. Here is a new one:

https://a360.co/3kPtDV0

[karlheinzstoeber]

When I try to open, I got the message empty model. But why?

[jeff_strater]

if the model contains only sketches, you will get that (misleading) message - it just means that there is no "viewable content" in the model, because the viewer only renders solids and surfaces, but not sketches

[jeff_strater · Accepted solution]

@karlheinzstoeber - thanks for the model.  The problem is what we call a "point mapping" problem.  Loft, when given two profiles with sketch points has to decide how to map points on one profile to another.  In this case, we get it absolutely wrong.  You can tell from this image:

What we need to fix this is a rail curve to help guide the solution

that will produce what I expect is the desired result:

If it is OK to share a screencast, I can show you how I did that, or I can send it via PM.

[karlheinzstoeber]

Dear Jeff,

Thank you very much for the solution. I could understand you well and everything works fine.

I had already thought about rails, but I assumed that the mapping itself would find the right solution for straight connections. Well, I was wrong.

 

Are there best practices for when and where rails must be used? Depending on the design, many rails are possible. So far, I have always used rails when the connections between the surfaces are not straight.

Protected elements in my project are only the splines used for wing construction. Everything else could be published.

Regards
Karlheinz

[jeff_strater]

"Are there best practices for when and where rails must be used? Depending on the design, many rails are possible. So far, I have always used rails when the connections between the surfaces are not straight."

Not really.  Mostly, your understanding is correct - rails are primarily used in cases where you want to diverge from the default shape produced by directly connecting the selected profiles.  This is sort of a secondary usage, when we don't get the default point mapping correct, rails can be an easy way to "guide" the solution.  You can also drag the point mapping points, to manually correct point mapping, but for reasons I don't understand, this did not work in your model.  Both that problem, and the fact that the default point mapping is wrong are bugs that I need to investigate, but I wanted to send you an immediate solution to get you going.  Sounds like you were able to do that.  Glad to hear it.

I suspect the screencast below is no longer necessary, but here it is just in case...

[karlheinzstoeber]

Dear Jeff,

thank you for the screencast. Just to help you, here is another case for your investigation... Same problem.

 

As you can see, the rail will solve the point mapping problem.

Common to both cases is that at least one of the profiles is an intersection of another loft. Maybe some jitter is the source.

Here is a link to the project: https://a360.co/3HTtbPA

Same password. If you like, please download the project. I will remove the copy in a week.

Regards

Karlheinz 

[jeff_strater]

thanks, but I'm not sure that is the same category of failure here.  It does not appear to be a point-mapping problem, but a legitimate self-intersection near the pointy end of the shape.    The rail fixes it by forcing the geometry to curve inwards at that point.

[GRSnyder]

Just as a suggestion for Fusion 360, it sure would be nice to have the option of drawing correspondence lines that are not rails. Correspondence lines would simulate the effect of manually moving the correspondence points while executing the loft but would not oblige the lofted shape to conform to their actual shape, as rails do.

Why? Two reasons. First, the manual correspondence points are inexact. They do seem to have snapping (thanks for that!), but only to "significant" points of F360's choosing. I'd like to be able to place them with precision. 

Second, lofts often break and have to be re-created from scratch. Anything that lets you substitute pre-established lines manual twiddling is a boon. 

Why can't you just use rails? You can, as in the examples above. But if the loft is smooth or complex, you don't necessarily want to have to nail down the exact cross-section of the result at any particular point. F360 often does a nice job of creating smooth, curved surfaces on its own, as long as it understand the correspondences correctly.