Loft error "The attempted loft would self-intersect or have illegal singularities"

[richard.kFK4QA]

I have modelled a loft starting from a point through 11 subsequent sections. I have also modelled four guide rails on two perpendicular planes. These rails are all 2D splines.

I have added the option of replacing the two rails on the one plane with 3D sketch splines.
I also have the option of the cross-sections being pure ellipses, or approximations of ellipses constructed from tangential arcs to create a fatter cross section.

There is weird behaviour as follows:

If I use the pure elliptical cross-sections I can use all four planar guide rails, or, when the shape requires, two planar guide rails plus the two 3D rails.

However, if I use the cross-sections formed from tangential arcs, I can only use any two guide rails, ie any two of the planar rails, or one planar rail and one 3D rail. If I add another rail the loft preview disappears and when I exit the dialogue box an error box appears warning that the loft feature edit has failed and the cause is "The attempted loft would self-intersect or have illegal singularities". This is a repeatable error.

See pic attached of the successful version using elliptical cross-sections and two planar plus two 3D guide rails.

[JDMather]

@richard.kFK4QA 

Attach *.ipt file here.

[johnsonshiue]

Hi Richard,

Without seeing the actual part, I can only speculate based on the error message. I think some portion of the Loft faces self-intersect (toward either ends), kind of like tying a knot. Such geometry isn't allowed in Inventor.

Based on the image, I personally think Guide Rail Sweep might be a better choice. Please feel free to share the file here. The form experts and I can take a further look.

Many thanks!

[richard.kFK4QA]

Hi JD and Johnson
Thanks for the responses. There are five .ipt’s involved due to the parametric chain, plus two spreadsheets, so I will have to send these attached to separate posts due to the 3 attachments per post limit.
The guide rails are required to create the radius at the nose. Each rail starts perpendicular to the centreline. Without them the loft forms a sharp point. In file “Pod-elliptical sections” you will see that four rails are used; two 3D splines and two 2D splines. In file “Pod-arc sections” the two 3D spline rails are used. If you try to add the two 2D rails the error is generated. You can deselect the two 3D rails and successfully use the 2D rails, but you then cannot add the 3D rails. Each rail definitely intersects all sections.
Separate issue: I selected to be notified by email of any reply to this post, but received none. What address would the email have come from? I will check if that address has somehow been blocked.
Regards
Richard

[richard.kFK4QA]

Further files:

[richard.kFK4QA]

Further files:

[JDMather]

@richard.kFK4QA 

The usual practice is to right click on the project folder and select Send to Compressed (zipped) Folder and then Attach the *.zip file here.

As far as your Loft issue I did not attempt to resolve as I would do this geometry differently and it would take considerable time to set up.

Edit: Also, you should state what version of Inventor you are using - especially when not the latest version.

[richard.kFK4QA]

Hi JD

I'm on Release 21. (We're going to jump straight to R24 hopefully soon. Delay is because we have 70 licences in four countries on opposite sides of the globe and need to get all Inventor & Vault databases updated and synchronised simultaneously)

The two stage deriving allows the root data to be used across multiple projects, and bringing the points in via sketch blocks is useful because the sketch blocks can be scaled in the files they have been derived into.

That aside, I don't see how the modelling in the top-level model would be done much differently? You would just see all the modelling in the derived sources higher up in the tree if it was all done in one .ipt? Do you have a recommendation of a different technique? Please email me if that would be easier.

Regards

Richard

[johnsonshiue · Accepted solution]

Hi Richard,

Many thanks for sharing the files! I think I know where the problem is. This has something to do with the starting point tangency condition. For some reason, the top and bottom rails can cause unwanted bad geometry. It is quite easy to fix it. Edit the Loft -> Conditions -> Point Sketch -> select "Tangent" or "Tangent to Plane" (select YZ plane). You may want to do it for the elliptical Loft also. It is always nice to have a nice tangent transition at the start.

BTW, the Guide Rails Sweep does not help this case. I think the Loft is the right tool to create the shape in this case.

Thanks again!

[richard.kFK4QA]

Bingo! Thanks Johnson, the Tangent to Plane amendment did the trick. I had not explored that feature before. Good to learn. (had to be tangent to a work plane though, the centreline is rotated slightly)

Much appreciated.

Regards

Richard