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Why is not possible to shell this part ?
hugues_vanneaud
Member Posts: 7 ✭
Hello,
I would like to shell this part : https://cad.onshape.com/documents/fae4112140c20f5304977118/w/26d5455114e8c54e14cef249/e/3b6e1a584d69c36b4e6e17ad
I succesfully shelled a very similar part before : https://cad.onshape.com/documents/ffdaf56f802233253a8ae9e6
I am a hobbyist and Onshape is my first experience with a CAD sofware. I was wondering if somebody from the forum could be kind enough to help me with this?
Thank you very much.
Hugues
I would like to shell this part : https://cad.onshape.com/documents/fae4112140c20f5304977118/w/26d5455114e8c54e14cef249/e/3b6e1a584d69c36b4e6e17ad
I succesfully shelled a very similar part before : https://cad.onshape.com/documents/ffdaf56f802233253a8ae9e6
I am a hobbyist and Onshape is my first experience with a CAD sofware. I was wondering if somebody from the forum could be kind enough to help me with this?
Thank you very much.
Hugues
0
Best Answers
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EvanReese
Member, Mentor Posts: 2,188 ✭✭✭✭✭
Seems like you have some accidental sliver faces that are probably causing issues.
Evan Reese5 -
EvanReese
Member, Mentor Posts: 2,188 ✭✭✭✭✭
got it working
you were right it was something with the way you made that one surface. For surfaces that don't clearly have 4 distinct edges, loft is usually not a great choice. I used Fill surface instead. Here's what your surface's UV curves look like with loft. See how wonky it gets toward that left curved edge. My guess is that the surface became hard to offset because the normal direction of the surface becomes a bit unpredictable in that area.
Fill on the other hand, interpolates a surface through the selected edges, and trims it for you, so those UVs are cleaner, and you can shell, thicken, or offset that face with no trouble.
Evan Reese7 -
EvanReese
Member, Mentor Posts: 2,188 ✭✭✭✭✭
Sure, the short version is:
- Every surface in Onshape (or Solidworks, or Rhino, or whatever) actually has 4 sides... All of them. I think this is because it's a requirement of the math that's used to produce them, but I'm not 100% sure.
- To get non-4 sided surfaces they are trimmed, but the 4-sided surface is still in there
- Every surface has a UVW grid. U,V, and W are comparable to X, Y, and Z, but in the domain of the surface. So W is always "up" from the surface, aka normal.
- sometimes one of your sides can get really tiny, or even be a point (like when lofting to a point). That's called a degenerate point and can cause problems when offsetting and the like. Imagine trying to calculate the W direction at an edge that's infinitely small (a point). All CAD has a tolerance limit, but it's so small that you pretty much never notice, but in cases like this it can show up as an issue. There are times degenerate points are ok or needed, but should be avoided unless intentional.
- The cleaner your UV grid, the fewer unexpected issues you'll have.
- The Face Curves tool I used is one way I evaluate that UV grid if I need to.
Evan Reese8
Answers
Looks like a cool project. Marble run or something?
I managed to fix the sliver face, but it did not solve my original problem. Thank you anyway, it would have caused some big problems later on.
I think I have a problem with the surface which split the main mold in two halves, but I have not found the solution yet.
you were right it was something with the way you made that one surface. For surfaces that don't clearly have 4 distinct edges, loft is usually not a great choice. I used Fill surface instead. Here's what your surface's UV curves look like with loft. See how wonky it gets toward that left curved edge. My guess is that the surface became hard to offset because the normal direction of the surface becomes a bit unpredictable in that area.
Fill on the other hand, interpolates a surface through the selected edges, and trims it for you, so those UVs are cleaner, and you can shell, thicken, or offset that face with no trouble.
I know this is a little bit off on a tangent for this thread, but I'm super curious. Maybe it'll be educational for some other surfacing newbies as well.
I also found your video on curvature. Other people having problems with curves should watch it : https://www.youtube.com/watch?v=X714INhNYL0&t=277s
I saw your Face Curves tool and I thought it was just drawing new curves on the face of surface. Your explanation above makes it seems like its actually pulling the UV information about relating to how the surface was originally defined and using that to draw the curves? Am I understanding that correctly? If so, that sounds awesome.
Yep, UV coordinates are an inseparable part of what defines and makes a surface. I suspect most modern CAD packages try to hide it to make the software easier to learn, and usually it works out, but there are edge-cases like the one above where some understanding about your surfaces can help.
And yes, Face Curves is creating curves from the pre-existing UVs. Also, I could only make that feature because the dev team at Onshape made some new stuff available in Featurescript with the last update. From there it was pretty straightforward, so thanks, dev team!