topic Re: Problem with sketches that wont close, and rails that misbehave in Fusion Support Forum

Problem with sketches that wont close, and rails that misbehave

simon.dyer — Mon, 29 Jul 2019 05:42:42 GMT

I thought Id post my file, which represent a problem that I get all the time.

Problems:
1. Cant close the sketch - I go around it again and again without success. - Ieventually solved it by creating the shape from the inner shape with a larger offset - it closed up the part I wanted closed even though I deleted the outyer offset. This might be a bug.

2. When I loft between the two shapes (Only works from one direction) , then the rails refuse to behave.
I get messages "modelling error, rails cannot be tangent to profiles" (They are not ) - havent found a workaround.
3. Also get other rail messages depending on the order that I click on them.

Re: Problem with sketches that wont close, and rails that misbehave

jeff_strater — Mon, 29 Jul 2019 18:54:19 GMT

thanks for sharing the file, @simon.dyer - we will have to dig into this a bit. Definitely something bad going on here with the fact that Loft only works one direction, and the errors with the rails. Not sure I followed what happened with the profiles not being closed, so if you have more info on that (maybe a model with the bad profiles), it would help looking into that part of the problem. Thanks.

Re: Problem with sketches that wont close, and rails that misbehave

simon.dyer — Mon, 29 Jul 2019 22:35:37 GMT

The file uploaded should include the bad profile sketch - can’t close the sketch. I solved it with a workaround - drawing an outer area and then deleting it. Then you can try the other two issues.

Re: Problem with sketches that wont close, and rails that misbehave

davebYYPCU — Mon, 29 Jul 2019 22:46:14 GMT

I agree with Jeff the file arrives with closed profiles.

I also can Loft them in one but not both directions.

Jeff has more tools to decipher the rails problem.

I can select two good rails, and get a weird preview.

I found Jeff had replied, so did not add to the thread at the time.

How can you be happy with the purple curve inside the larger profile.

I also wonder how that projected curve happened.

I was going to suggest, the profile revert to sharp corners, and add model fillets.

Might help...

Re: Problem with sketches that wont close, and rails that misbehave

jeff_strater — Mon, 29 Jul 2019 23:57:40 GMT

Sorry, I'm having a "slow uptake Monday". Can you explain what you mean about the profiles not being closed? In the design you shared there is sketch 1:

The profile here is closed. In sketch4, there is this:

Which is also closed, but has one additional region, which sketch1 does not. Is that the issue? I found that if I edit sketch1, I can add a line to the sketch to get that region for sketch1 as well:

So, I'm still not quite certain what the issue is here. I am able to reproduce both the order of selection problem, and the problems with the rails causing the loft to fail. We are looking into those.

Re: Problem with sketches that wont close, and rails that misbehave

simon.dyer — Tue, 30 Jul 2019 05:22:05 GMT

Hi Jeff,

The profile wont close problem disappeared after reloading the model - you can see in my original post that the profile 4 sketch is not closed, which is when I exported the file to 555.f3d v4
It can be quite tedious to try and fix the profile not close, then to find reopening the file fixed the problem. Strange.

As for the rails problem, Ive made a quick test video to further demonstrate how I made the rails etc. You will see that selecting alternative rails, the problem actually moves.

https://autode.sk/2ymLQAH

https://knowledge.autodesk.com/community/screencast/d1f543c7-97ab-42ec-b894-80001bb48b40

Re: Problem with sketches that wont close, and rails that misbehave

jeff_strater — Tue, 30 Jul 2019 17:10:03 GMT

I suspect that all these problems have a common root cause. My guess is that it has something to do with the profiles, and the problem that shows itself as the loft failing one direction, but not the other, is somehow involved. We can reproduce that and are looking into it. A couple of suggestions that I suspect won't have any effect, but might be worth trying:

put the rail curves in their own sketch (you can put all 4 in one sketch). I don't think that will have an effect, but I noticed that when you first create the 3D splines that most of the points fall on the sketch plane, and you then need to move more of them to get the curve you wanted.
when selecting rail curves, try un-checking "chain selection". Sometimes, especially if the curves are in the same sketch, Fusion will try to chain the rail selection beyond where you want it to be. Again, I don't think that is the problem here, but is just a good practice. Especially when you are getting that error that says the rail intersects the profile more than once, it led me to wonder whether the rail is going beyond the single spline you are selecting.

Re: Problem with sketches that wont close, and rails that misbehave

jeff_strater — Tue, 30 Jul 2019 18:51:05 GMT

also, can I ask where the geometry for the profiles came from? Did you create those profiles in Fusion, or was the data imported from somewhere else? The reason I ask is that some of the curves do not look like native Fusion curves. Our geometry kernel team took a look at this design and found some small gaps and small curves in the profiles. The investigation revealed a few issues for us to look into, but none of them will be simple to fix, to be honest. Would it be possible to re-create the profile geometry using Fusion curves? I gave that a try and was able to create a model where the loft works both ways, by replacing some curves in the profile with Fit Point Splines, and making sure that both profiles have the same number of curves:

I still have some problems with the rails that we need to debug, however. 3 out of 4 can be used, but that one causes the "rails cannot be tangent to profiles" error.

Re: Problem with sketches that wont close, and rails that misbehave

jeff_strater — Thu, 01 Aug 2019 21:54:02 GMT

after lots of discussion with the kernel team, we've come to a few conclusions.

The inability to loft from one direction (minus rails), but the other succeeds is because of a couple of reasons. One is what we call "curve mapping". Loft needs to map points in each profile to points in the other. In this case, the original profiles had different number of curves, which complicates this a lot:
Second, we noticed that the profile geometry itself was a little messy. One of the arcs seems to overshoot the adjacent curves:

and two of the vertical curves have funny tangency conditions with the arcs:
Finally, the insight that I was not aware of: These profiles, in comparison to the distance between them, are fairly "thin". This works fine with no rails, but if you throw the rails in there, the Loft tries to react to follow those rails, and self-intersecting geometry can result. This is mostly a bug in the kernel, but it is good to be aware of this for the future.

So, this model revealed a few bugs that we are tracking, and also some slightly bad data in the profiles. Hope this at least lends some clarity to the reasons why Loft is struggling with this model.

Re: Problem with sketches that wont close, and rails that misbehave

TrippyLighting — Thu, 01 Aug 2019 23:00:30 GMT

One shall never say never, but I would NEVER try to recreate that geometry that way.

This cries for a few overbuilt surfaces, properly trimmed, stitched filleted and then thickened.

Re: Problem with sketches that wont close, and rails that misbehave

simon.dyer — Fri, 02 Aug 2019 08:39:50 GMT

Thanks @TrippyLighting - being somewhat 'newb/lazy/lean', I do everything in Model env. Its a tool in my busy day - modelling appears quicker than surface/patch builds. Am I wrong?
@jeff_strater has some great info above. I'm going to Accept his earlier comment as the solution for others to follow - rails in their own sketch works much better ! But how do you put the vertices into free space except after closing line - seems unnatural??

Re: Problem with sketches that wont close, and rails that misbehave

simon.dyer — Fri, 02 Aug 2019 09:06:39 GMT

Thanks for taking such a detailed look into this

I@jeff_strater -
Re: Profiles
I concur my arc in V4 (Now on V21) had little bits I hadn't seen were overlapping so didnt trim off. (Perhaps you could write us a little python routine , to remove little overlapping or loose bits? [like that great unfixall you wrote for me].😀 )
BTW I made the sketch tracing canvas with fitpoint Spline, then attached arcs to sides and trimmed around.
Re: Rails
You solution above is best practice - draw rails in new sketch. Much better results.
But please how do I draw dots in free space during tracing?🤔

[ When I accept the earlier rails solution, will it close this discussion?]

Re: Problem with sketches that wont close, and rails that misbehave

TrippyLighting — Fri, 02 Aug 2019 12:09:06 GMT

Attached is how I would approach this.

Super basic sketches. Lines and splines. No closed profiles.

Lofts with center rail for the sides, lofts with no rails fo the front and back, extrudes for the top and bottom.

Boundary fill, fillet, shell, done!

There idea here is to identify primary and secondary surfaces/features. Fillets are often secondary.