Unable to flatten filled & thickened surface made entirely of splines

c_h367

I'm trying to create a cup-/dome-like shape using splines, but it's resulting in this weird wobbly/uneven-ness to the bottom surface.  I'm using a Fill followed by Thicken to create a non-zero-width wall for 3D printing.  The Fill produces an error unless I switch it to Sampled, but that rendering looks completely fine, until I add the Thicken.  It produces the cupped bottom edge as indicated in my screenshot.

I had quite a bit more weirdness around the back and front just using Fill, due to influence from the spline control points, but ensuring the controls were at 90° angles helped resolve it (and improved the wobbliness a bit).

Does anyone know how to flatten this surface out?  Here's my model:

S1mon

This can be much more straightforward. You can make the main shape solid with one more fill, and then shell can be used to thicken to the outside, while keeping the bottom flat. The cylindrical part doesn't need so many steps to add onto the main body.

glen_dewsbury

The way you got to your shape will always give that wonky edge.
1 way to fix is move face to extend part past where you need it then use an extrude/subtract to clean up.
https://cad.onshape.com/documents/fcfbd10383d5605fd32ed846/w/c5296aeece6364080b1bb8e6/e/7e5bf2994749c72f3d48d439

tim_hess

Those weird bits are likely occurring because the sides of the surface aren't perpendicular to the bottom plane. 

I'm not a surfacing expert at all, but you could try a few things:

• make sure the ends of the splines are perpendicular to the base plane before creating your surface.
  
• You might be able to use "replace face" on the bottom surface to flatten it out.
  
• You could extend your surface further bigger than it needs to be (extend it further downwards) and then trim the body after the thicken operation.
  

Someone else may be able to provide better guidance on the right way to approach this, but those are my thoughts. 

S1mon

This can be much more straightforward. You can make the main shape solid with one more fill, and then shell can be used to thicken to the outside, while keeping the bottom flat. The cylindrical part doesn't need so many steps to add onto the main body.

c_h367

Thank you for the tips everyone!

S1mon said:
This can be much more straightforward. You can make the main shape solid with one more fill, and then shell can be used to thicken to the outside, while keeping the bottom flat. The cylindrical part doesn't need so many steps to add onto the main body.

This is very helpful as a beginner, thank you!