How create a pipe which DOES intersect itself

[graham_S_allen]

I have the following sketch so far. The blue spline is for the models hair, and I need to use pipe of sort to create the object, but I need it to intersect itself to get the desired result. Of course, when I try this it errors. How can I achieve the desire result. See images for reference.

https://poppy-playtime.fandom.com/wiki/Mommy_Long_Legs?file=Evil+Mommy+Long+Legs+Alt+Angry.pnghttps://poppy-playtime.fandom.com/wiki/Mommy_Long_Legs?file=Evil+Mommy+Long+Legs+Alt+Angry.png 

thanks,

[Drewpan]

Hi,

Probably turn off visibility of the "model" and create the pipe separately. Then use Combine to merge them together.

Cheers

Andrew

[graham_S_allen]

When I say intersect itself, I mean the pipe it will create. There is no issue with it intersecting any other bodies. A pipe (or sweep) command errors if the profile happens to intersect itself

[TrippyLighting]

Instead of using the pipe tool, the sweep tool is probably the better option. It allows self intersecting geometry and even allows to extend the sweep past the path. The latter isn't needed here, but can be very convenient for other things!

[graham_S_allen]

Hey Trippy. thanks for detailed video. I did EXACTLY as you did with sweep except that I started it at one end, and created the construction plane at one end of the spline. Is there some strange constraint that stops it from intersecting itself if it starts at the beginning of the path? Why did you start the sketch at th midpoint?

EDIT: I just saw you intersected the plane I think? I did not do that, what does that do?

[graham_S_allen]

I've created a test project to show my point. There are two splines. One is like my original and has the same issue - with this one, the spline does not intersect but the resulting body does. This is different your example above so I wondered if this is why yours worked. However, the second spline (on the timeline) is just like yours and also fails.

For the first one you see its worked, but if you edit the sketch and set the circle diamater large on purpose, then the sweep fails saying it will intersect itself. Seoncd one just fails outright.

[laughingcreek · Accepted solution]

I think you missed some important points in @TrippyLighting 's explanation.  

but changing gears. When bodies intersect, either in obvious ways like 2 ends of a pretzel crossing,

or in less obvious ways like when you do a sharp bend in a sweep, 

it can be really hard for fusion to figure out the geometry.  you have to help it along by starting with simpler geometry.  seems like I've mentioned using analytical geometry to help fusion along before?

[graham_S_allen]

Thanks for helping. I'm not sure what points I missed from Trippy though, care to explain? I watched the video clip 3 times and noticed that the way I built my sweep was the same as his except for the points I mentioned. My point being that my spline (before being sweeped) did NOT intersect itself and his did. I was asking if this is why his worked – which does not seem to be the case.

To answer your other point about you advising on intersections (from the other topic) – yes you have advised, and I understood what you said. I think you think I am asking the same question here, but I am not. This thread was not about how to handle intersections (which you helped with in the other topic), it was asking how I might achieve the desired result without intersecting (because I can’t get the thing to work). Naturally, Trippy wasn’t aware I had already discussed intersection issues in another thread, so he started to explain using similar points as you did. So, I did take on board what you said 😊, I was just asking a different question.

Bear in mind I am an absolute Fusion noob and new to CAD also, so there is huge learning curve for me, and I’ll likely ask some dumb questions!

On to your point, you did say I can use Isocurves and analysis tools, but nothing more beyond that. I turned on the curvature comb, and Isocurves but don’t understand how to read and analysis it. In your second example, what is the significance of the distance you have between the two points at either end of the spline? I feel you have highlighted this for a reason.

Also, thanks for the file, I just looked at it and have some q’s:

What is it in your design that allowed it to intersect but fails on mine? Is it the restraints you added to sketch, or is that you split the circle into two halves? Even looking at what you did and stepping through the timeline does not intuitively make me see how it worked; it would help a lot if you could explain why you did it the way you did to make it work. Your spline doesn’t look that different to mine, so I don’t understand why yours worked and mine did not.

Sorry for the long post, just trying to understand everything, please answer all my q’s.

Thanks again!

@graham_S_allen -this post has been edited due to Community Rules & Etiquette violation

[laughingcreek]

it's late, only time for a quick answer-

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@graham_S_allen wrote:...Your spline doesn’t look that different to mine, so I don’t understand why yours worked and mine did not.

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I didn't use a spline.  the path is all arcs set tangent to each other. analytical geometry.  

[graham_S_allen]

Ok I think I see what you are saying now in that it's analytical geometry. You did say that before, but I assumed a spline was too.