Lofting: Problem with guide lines from a square base to an ellipsoidal shape

derek_vair

Here's the onshape document URL:
https://cad.onshape.com/documents/9ff53daf0bb504ea1eb7fa44/w/00ca9d592e3ddaeb3ca01cc4/e/cc6f08cdd81203f8d71f968e

The end result is supposed to look like a gravy boat with no handle, and be 3D printable.  I have a much larger model that does print, but it gets messed up when I try and scale it down in onshape, and using Cura to scale it down results in walls that are too thin...

Similar to, but unlike the classic "square to circle" that gets twisted, I've got more vertices on the ellipsoidal shape than on the square, as it's made up of an ellipse, with the end of one major axis cut off, and replaced by a tangent line, an arc, and another tangent line (to make a kind of spout).  The basic problem is that onshape generates guide lines toward the front, giving an odd shape to the lofted shape.  I thought I could use three manually drawn pairs (L & R) of guide lines in the loft - for the rear, the middle/center, and the front of the square, the front being the face of the square closest to the spout.

The rear guide lines work as I expect.  However, the center ones don't.  I think it's  because I can't get the point of intersection of the guide line with the top of the shape to "lock in".   A side effect (I think) of the drawing as it is, is that I can't get the shell function to work properly to make the generated loft hollow.

I've tried using intersecting planes, lines drawn on the extrusion face (and then "Used" them in the TrayTopOutline sketch, but can't get a clean intersection point (like I have with the rear guide lines).  If there's another technique than loft/ shell, I'd be happy to use it to "step around" this problem.

dirk_van_der_vaart

Have a look at this approach,
https://cad.onshape.com/documents/0cb2f0c3a0760634dacf9032/w/899a7f528138072f6d54c2e3/e/51c8fe5c5f4eb85c5ed46ea4

John_P_Desilets

@derek_vair Lofting sharp and round profiles can be tricky. Often times this will produce creases or folds in a loft. Does the base need to be square? I made a quick example that doesn't have a square base. Maybe this can help? 

https://cad.onshape.com/documents/f51be1a8bde220decf0f9476/w/78055e0033e65fdab3f872ed/e/b7ee086adee98a9e74d5714e

bruce_williams

@derek_vair

You have poked the forum team and now you will receive more than you imagined!  LOL

Good ideas coming out and you will need to give it the design intent you want.

Here is my quick rough in.  Besides the advice @dirk_van_der_vaart & @John_P_Desilets offer, John has also pointed out in past sometimes best to do lofts in smaller sections and I also rounded the rectangle base slightly.  This has some crude stuff so hopefully you can refine.

dirk_van_der_vaart

Another tip, if a shape is symmetrical, design only half and mirror at the end.

leonid_raiz

I believe you are not taking advantage of all the controls that the loft tool provides. Take a look at my version of your document https://cad.onshape.com/documents/be3fb088f663c6eb8fcebdac/w/3d56ed8c5d7b15683149c62d/e/0530e65ced9558c880e73374
 
I activated loft Match connections and defined the way how to connect sharp corners at the bottom to intermediate positions on the top edge. The resulting shape is much nicer. I also changed it from surface to solid.

The subsequent shell feature still does not work with your thickness because rear guides are too curved at the bottom; there is not enough room for shell thickness. 1 mm thickness works though. I would also suggest editing TopTrayOutline sketch. The sequence of curves there seems to be changing curvatures too abruptly. 

dirk_van_der_vaart

Have a look at this approach,
https://cad.onshape.com/documents/0cb2f0c3a0760634dacf9032/w/899a7f528138072f6d54c2e3/e/51c8fe5c5f4eb85c5ed46ea4

John_P_Desilets

@derek_vair Lofting sharp and round profiles can be tricky. Often times this will produce creases or folds in a loft. Does the base need to be square? I made a quick example that doesn't have a square base. Maybe this can help? 

https://cad.onshape.com/documents/f51be1a8bde220decf0f9476/w/78055e0033e65fdab3f872ed/e/b7ee086adee98a9e74d5714e

bruce_williams

@derek_vair

You have poked the forum team and now you will receive more than you imagined!  LOL

Good ideas coming out and you will need to give it the design intent you want.

Here is my quick rough in.  Besides the advice @dirk_van_der_vaart & @John_P_Desilets offer, John has also pointed out in past sometimes best to do lofts in smaller sections and I also rounded the rectangle base slightly.  This has some crude stuff so hopefully you can refine.

dirk_van_der_vaart

Another tip, if a shape is symmetrical, design only half and mirror at the end.

derek_vair

@dirk_van_der_vaart That technique will work for the model I have in mind - and thanks for the symmetrical design / mirror tip.  Lots to learn about this CAD stuff ....
@John_P_Desilets This approach will work, too, even with my constraint that the bottom part be square - I found a lots of answers to the "loft square to circle"  that I can use for that bottom section !
@bruce_williams I'll have to play with the "sectional lofting" technique you used here;  hopefully, I'll get it to work for me.

Thanks, all, for your responses.  

bruce_williams

@John_P_Desilets

I finally got to dig into your Gravy Boat model.  That is very instructive!  Thanks for posting tips and examples like this; I am learning a lot from your stuff.

The edge on this is especially interesting, how you used surfaces and loft settings.  Powerful! 

leonid_raiz

I believe you are not taking advantage of all the controls that the loft tool provides. Take a look at my version of your document https://cad.onshape.com/documents/be3fb088f663c6eb8fcebdac/w/3d56ed8c5d7b15683149c62d/e/0530e65ced9558c880e73374
 
I activated loft Match connections and defined the way how to connect sharp corners at the bottom to intermediate positions on the top edge. The resulting shape is much nicer. I also changed it from surface to solid.

The subsequent shell feature still does not work with your thickness because rear guides are too curved at the bottom; there is not enough room for shell thickness. 1 mm thickness works though. I would also suggest editing TopTrayOutline sketch. The sequence of curves there seems to be changing curvatures too abruptly. 

bruce_williams

Nicely done @leonid_rai    Using Match connection is indeed more elegant and versatile.  

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