Help with a Model and the Shell command

john_gentilin

With this model, I want to apply the shell command but I had the hardest time making it work. According to this post there was a mention about the smallest outside radius had to be bigger than the shell thickness. So I kept reducing the thickness till the shell command worked. 
https://forum.onshape.com/discussion/7191/trying-to-shell-out-a-part

My question is, can someone tell me where my radius are too small, most of my revolves have huge radius's, maybe it's an intersection of two revolves.  I would like to have a shell thickness of 2-3mm, and not .3. Here is a link to my model.
https://cad.onshape.com/documents/46fa6561c3c0d839fb353ade/w/c9b9b354f1c329909efc014b/e/c676b3104fc0136db7a1726a

Thank you
-John

andy_morris

Hi John,
The problem is as described in the other post, but your case has some additional challenges. When you create the revolve for the top of the head you create a degenerate point at the outer edges - this where the curvature tends to zero and the tangency to the adjacent faces is hard to resolve.

The shell works far better if you extrude the additional material afterwards, not before the shell, as there are fewer adjacent surfaces. With this change you can create a much thicker shell - I tried up to 1/2 in (12.7mm).

I'd also recommend that you try and shell the simplest shape and then remove the excess material with an extruded or revolve. So at the base I would use a simpler loft with less curvature at the back.

I created an example that I hope will help https://cad.onshape.com/documents/b5ef13232f8d227f5bf0dd03/w/fc4482426570dea0f897266b/e/92c18425ec06b419eaaaff95

Best regards,
Andy

Marc_Miller

@john_gentilin The zebra stripes (curvature visualization) can be turned on as one of the "shading" options.

john_gentilin

I made a revision named "Shell Thickness" if I happen to move on to other parts. 

andy_morris

Hi John,
The problem is as described in the other post, but your case has some additional challenges. When you create the revolve for the top of the head you create a degenerate point at the outer edges - this where the curvature tends to zero and the tangency to the adjacent faces is hard to resolve.

The shell works far better if you extrude the additional material afterwards, not before the shell, as there are fewer adjacent surfaces. With this change you can create a much thicker shell - I tried up to 1/2 in (12.7mm).

I'd also recommend that you try and shell the simplest shape and then remove the excess material with an extruded or revolve. So at the base I would use a simpler loft with less curvature at the back.

I created an example that I hope will help https://cad.onshape.com/documents/b5ef13232f8d227f5bf0dd03/w/fc4482426570dea0f897266b/e/92c18425ec06b419eaaaff95

Best regards,
Andy

john_gentilin

Andy, thank you for the quick response.  How did you produce that B&W curve plot ?   The back of the neck is not complete yet, I was just getting started with the shell to see how it would look then ran into problems. I still need to model the back of the head and make a real neck.

Do you have any pointers to learn more about degenerate points/geometry ?  I think I understand your explanation, but need to do some more research.  I did notice that the "thicken" command was complaining about that point specifically but the error message just said it could not rebuild the geometry. 

Thanks again
-John

Marc_Miller

@john_gentilin The zebra stripes (curvature visualization) can be turned on as one of the "shading" options.