Tangent Rails Error Code!!

[Anonymous]

I keep getting the error code that 'rails cannot be tangent to profiles', when trying to loft a surface.

But why not? Surely this is necessary in order to produce tangent surfaces (like what is required here).

Is this a bug? or is there a solution??

Please help as this error code has now appeared in multiple models and I need to find a workaround/solution.

[jeff_strater]

If you share the model here, we can take a look at this case.  Just from the picture, it looks like it should succeed, but it's hard to say for sure without the model itself.

Jeff

[Anonymous]

Here is the link:

http://a360.co/2kNvgV0

[jeff_strater]

Hi @Anonymous,

Thanks for the model.  To be honest, I'm not entirely sure.  I'll need to check with the geometry kernel guys to be sure.  But, I suspect that the root of it is a conflict between the tangent constraint on the first profile, and the rail.  In a way, they are competing to say the same thing.  The loft succeeds fine with the rail, or with the tangent condition, but not both.

Oddly enough, it does succeed with a "smooth" (G2) constraint to that profile.  That surprised me because G2 is a higher-order constraint than is tangency.  Another thing to investigate.

I thought for a while that it was because there as no tangent constraint on that curve in the sketch for the rail.  But, I added that, and it did not help.

[edit]  Forgot to add the screencast:

More later, as I learn it.

Jeff

[Anonymous]

Thank you for getting back to me.

The problem is, even with the smooth constraint the surface isn't smooth. It appears to being 'pinched' in the corners. (I've added an image to show what I mean).

This seems a relatively simply surface to create so I am struggling to understand why Fusion is struggling.

[michallach81 · Accepted solution]

Hi Russel,

Your main problem is how you've articulated your design. You have far too many sketch features. That amount of sketches (especially abuse of projection) leads to small in size but significant inaccuracies, that causing troubles. Also how you choose profiles to create dome-like surface is not most fortune one. While making Lofts or Sweeps try to not using rails or guides. Forced algorithms will give "forced" results, more elaborated artifacts. Below is my screencast where I try to recreate your design, following your design intent. I've also attached my Fusion file.

[jeff_strater]

Hi @Anonymous,

What @michallach81 says is consistent with the information I got back from our core geometry team.  The bottom line is that Loft is a picky tool.  You have to get it really accurate in order to get good results.  I will try to summarize the concerns that the team found with this design that are contributing to the failure of the loft with tangent conditions.  Michal's workflow avoids many of these problems.

First, sketch46, which is one sketch used for Loft15, does not have any constraints:

This causes many downstream problems, because, although the curves look like they are OK, they are not quite tangent in all places.  These near-tangencies can cause lots of downstream problems for Loft, Fillet, and other Fusion features.

Loft16 has a few smaller problems.  Sketch2 is just a bit off-center, and the rails used in this loft are a tiny bit off from the profiles, as well.  These might be very small problems, but a lot of small problems can have big effects.

It's hard to see, but if you rotate the model a bit, you can see that Loft16 is not symmetric:

This is probably a result of the above errors.

Loft 18 also does not produce a good-quality surface, again because of the accumulation of errors from above:

So, the reason why Loft19 fails with a tangent condition is also a factor of the problems up to that point.  You can see that, even without the tangent condition, the loft is not of good quality:

So, my recommendation is the same as Michal's:  You will get much better results if you carefully reconstruct this model.  Use projections, and project->intersect to make sure that you have precise sketch points, use edges directly in profiles when you can (don't create a sketch where you don't have to), etc.

Jeff

[Anonymous]

Thank you so much guys these were really helpful and solved the problem perfectly.

It's great to learn the best practice for fusion 360 as I have been teaching myself so far.