Self-intersecting loft / sweep

piotr_smektała

Is it possible to create such a geometry using sweep / loft function in Onshape?

Using such a set of sketches:

I can't figure out how to do that.

Am I missing something or that is a big flaw in functionality?

NeilCooke

Parasolid does not permit self-intersecting geometry in a single feature but you can make it in two or more features if you really need to.

NeilCooke

Parasolid does not permit self-intersecting geometry in a single feature but you can make it in two or more features if you really need to.

paul_chastell

The specific issue in this particular case is that Onshape is trying to make a single path and it is that which is crossing itself and Onshape doesn't like that but Onshape will be happy with the final result. As Neil says, by sweeping with two sweeps using two paths that are not crossing each other you can achieve that result. See https://cad.onshape.com/documents/b2724ca437fdb4b7ea41abb3/w/266674bfc17c15439f8fe4d7/e/d7b17eb691374acf4f80888b

paul_chastell

piotr_smektała

Well, Neil's answer says everything :-(

Just an inherited property of the geometry modeling kernel.

Model presented by me was just an example of a problem.

It is pretty easy to create such a geometry using two consecutive sweeps.

What I was thinking to use OS for, was something like this (this is a simplified example also):

And in this case I'll have to use 3 sweeps and 5 sketches (2 of them (circular intersections) with dimensions calculated in advance) to create one solid body, instead of just 3 sketches and start/end diameters.

Well, it's just not as simple in OS as in other tools.

P.S. Well, tried to create such 3s 5s trick in the meantime and no success so far ;-(

owen_sparks

Could that be use case for a featurescript that splits your sweep path into many "micro-paths", executes "micro-sweeps" on each in turn and then Booleans it together at the end?

Could possibly test for fail condition and say double the number of segments as see if it passes then?

Owen S. 

paul_chastell

I threw this together at lunch but I'm not happy sharing the document yet. Still, it is possible. The key is that the body can self-intersect as long as the faces don't. Hence the two lofts, one for the 'left' and 'right' forming a single solid part.

MBartlett21

@piotr_smektała
See https://cad.onshape.com/documents/990065be862441f76bbd90eb/w/29b6322a1e8e7dab00dbdf74/e/ab266ba0e372b4d738118cf3

It is just one sweep

piotr_smektała

Paul, ... I can see the light ahead.

Tricky, smart, maybe not applicable to more complex models but really encouraging to experiment with.
Thank you so much for this remark about  body/face difference.