Why are my lofts failing???

neobobkrause

Yes, another loft question. The document my question refers to is this one.

Opening this document, you'll find 2 failing Loft features. The first one is named FailingSurfaceLoft. I can get this loft to not fail by removing the two guides that refer to edges in Sketch3. Of course, in so doing I'm left with a loft that doesn't match my requirements. The question I have is what about the edges in Sketch3 are causing the loft to fail.

In truth, my design intent is to create solid lofts, not surface lofts. So the second loft feature in my document is a solid loft called FailingSolidLoft. Removing the CenterShape from the loft's profile list will result in a solid loft -- though one having the wrong contour. So the question I pose here is what about this loft with three profiles is failing.

Thanks for any help,

- Bob

traveler_hauptman

@neobobkrause I'm trying to say that your design intent is not clear. Different people would create different shapes based on your inputs. The edges covered by sketch 3 are the abiguous part of your shape.

I notice that your sections appear to have a constant thickness. Loft's are difficult to control in this way and probably not a good approach if that's important to you. Instead of a loft, I'd probably try to thicken a surface and then trim the edges with an extrude-remove.

You have chosen to work with a very complex shape. You might consider working with simpler ones as you build up your understand of how lofting works. Searching online for loft tutorials will give you a good idea what is possible with a loft even if the tutorial is for another solid modeler.

traveler_hauptman

The share is view only so we can't really look at how you set up the loft. Make your document public instead so people can make a copy and work with that.

 The surface loft error says the guides don't intersect the sketches, so that would be the first thing to check there. The solid loft has very weird geometry at the edges/ends of the solid. If I were creating that solid for you I would be asking a lot of  questions about what you really wanted for the edge conditions out of the possible solutions; the feature does not have that option... I think you will have to either give it some more intermediate profiles (I think 2 more between each profile; 1 would not be enough to define a solution I don't think) or give it an easier edge and create the edge you want in post operations.

traveler_hauptman

Actually, one intermediate sketch between each profile would probably do it. However there may be a further issue because the error I get (loft would be self-intersecting) is different than the one you get.

https://cad.onshape.com/documents/5840158f81cc43d6933ad28e/w/e424760d44012f7867cdcf63/e/c33a18724289c400d4a9652b

EDIT: I was able to recreate the error by aligning the geometry exactly as you have rather than just approximately.

chris_8

Those lofts are definitely a challenge.   I tried going for surfaces, then extruding new solids to those surfaces, then using Boolean to make an intersecting part.  Boolean failed on that, so I did some workarounds for that.  In the end it looks like 1 piece, but Boolean won't join them:
https://cad.onshape.com/documents/3c4d31dc15926a9e95c80fb0/w/c48a5dd80541bdd2c8522c78/e/3c9107b323056eba5fd0474e

neobobkrause

Thank you both for your input.

@traveler_hauptman: You have much more OnShape experience than I do. While I'm interested in actually getting the job done, and adding more intermediate curves may bring me closer to a workable solution, there's no obvious/natural place to insert these additional sketches in my design. I also don't feel that hacking my design intent in the way you suggest brings me any closer to mastering OnShape lofts for my next project or the project after that. And maybe that's reside the point. Maybe what we're struggling against is simply a bug in the loft feature.

@chris_8: The workarounds that you've engaged in are the same ones that I worked through as I tried to resolve this problem before posting this question. But as you've discovered, it doesn't actually result in a workable solution. The shape we both arrived at and that you've posted doesn't meet my requirements. Neither the solid loft nor surface loft operations accept the paths in my Sketch3 as acceptable guides. There's a reason why the loft feature isn't accepting them, even though the math related to the positioning of these paths is correct. The reason these operations are failing may be legitimate, but my sense is that there's either a bug in the code or the code is unnecessarily intolerant of some tiny, non-obvious infraction that we've both committed. Compounding the challenge of resolving the problem is the fact that the loft operations are unable to convey sufficient information to the user to explain what's causing the condition.

- Bob

traveler_hauptman

@neobobkrause I'm trying to say that your design intent is not clear. Different people would create different shapes based on your inputs. The edges covered by sketch 3 are the abiguous part of your shape.

I notice that your sections appear to have a constant thickness. Loft's are difficult to control in this way and probably not a good approach if that's important to you. Instead of a loft, I'd probably try to thicken a surface and then trim the edges with an extrude-remove.

You have chosen to work with a very complex shape. You might consider working with simpler ones as you build up your understand of how lofting works. Searching online for loft tutorials will give you a good idea what is possible with a loft even if the tutorial is for another solid modeler.