Loft issue

elizabeth_giovanardi

My student is trying to make a totem-like shape. He created multiple sketches and guides and then lofted them, but somehow there is a twist for some reason that I cannot figure out. Any suggestions? Thanks!

https://cad.onshape.com/documents/39bfcad22ed5305a3d33af91/w/665f0a30a6bec085bf7196dc/e/cbc7e5b88e43e9f524d69fbb?renderMode=0&uiState=66508dc38b969a737dad21bc

john_lopez363

First off, that link is view only so making assumptions here...  I'd say the simple fix is to define "Connections" in the Loft.

John_P_Desilets

@elizabeth_giovanardi Thank you for sharing! At first glance the profiles and guides of this model are not all properly constrained. The guides need to be pierced to the profile sketches. 

Also, it might be better to use a fill surface with guides for this particular shape. Take a look at this example. Hope this helps!



https://cad.onshape.com/documents/6812bf2c92e011be99d8fed5/w/e0bad6b6b7a8f78b8e4895d0/e/3225d320d03c6f6ce3a17046




Final part

elizabeth_giovanardi

Thanks for the response - I had made it public so I thought it would work. Maybe this link will work? 
https://cad.onshape.com/documents/39bfcad22ed5305a3d33af91/w/665f0a30a6bec085bf7196dc/e/cbc7e5b88e43e9f524d69fbb

elizabeth_giovanardi

I played with the connections and improved it slightly, but there is still a second 'line' that it seems to be following.

The second question that relates to this same part is that this student created a sketch filled with splines and was hoping to extrude it to the surface with a slight offset, but this also fails. My suspicion is that I cannot say extrude 'to face' if the face is a lofted surface? 

That just made me wonder... would it help if I just created a surface and then thickened it?

glen_dewsbury

Note that the original has a cut off that causes the twist you asked about since the loft is trying to make point connections.
I took a few minutes to review the total of the document when I noted how may sketches and locations. Here is a sample that will get to the basic shape definition based on the splines and arcs used in the original. Note the relocation of the original image to simplify down stream features. The perimeter wants to be a mirror of the traced spline for symmetry. This can be done as a loft. I did it as a couple of fills that closes to become a solid.
https://cad.onshape.com/documents/df90895bbbe02ce48f5f3f24/w/550e73517c73d59426ebb537/e/dadea2567218866bedcea956

elizabeth_giovanardi

Wow - thanks for the suggestion of a fill surface with guides - I have never used those tools before but it worked perfectly! I will pass this on to my student right away!!

elizabeth_giovanardi

@John_P_Desilets
I just tried to recreate the same fill in my original doc and it appears that you edited sketch1 and the centerguide in a way to allow the fill to happen. I changed all the horizontal lines to construction lines and made the edge splines symmetric, but that still doesn't allow me to create the loft. Is this why I cannot recreate your fill? Thanks!

john_lopez363

@elizabeth_giovanardi... turns out if you fix the issues with ALL of the underlying sketches then NO "Connections" will be needed.

Every single Sketch had issues....mostly related to improper constraints:

• Sketch 1 is missing many constrains, most importantly horizontal and vertical constraint.  Also the splines that define the outside boundaries are not the same.  Use Mirror when and where possible. Both of these, if not corrected, drives errant behavior later on in the Loft operation.
• One end of the centerGuide sketch was not coincident to sketch 1 (needs to be to use Guides correctly)
• Every one of the arch sketches (sketch 2 thru 7) are not properly constrained to the centerGuide sketch.  It was mentioned above, but the correct constraint to use here is the Pierce constraint.

Won't get into here, but there are many similar issues with the Mask sketch and the approach that I've inferred was trying to be used.  One rule of thumb... the fewer Spline points you can use the better!  I took a stab a finishing the Mask...but it may not be the final result that was going for.

Here a link for you to follow along:
https://cad.onshape.com/documents/506c204584d417fff9b0ec3b/w/31c92bc6dcc65d69f66a6826/e/3d5164a81ad5e62a988224ce

john_lopez363

  

glen_dewsbury

@elizabeth_giovanardi
Hadn't used the new thin loft to date so thought I'd go back and give it a go.
Very nice! No shell or thicken needed.
https://cad.onshape.com/documents/df90895bbbe02ce48f5f3f24/w/550e73517c73d59426ebb537/e/a8c6287ea18c8a614aca9934

john_lopez363

@glen_dewsbury...very nice job cleaning up that "Mask" profile sketch.