Is there a logic to seams between faces of a loft?

wito_roz137

https://cad.onshape.com/documents/96d2f63c98091d8538521590/w/9d7f859ef9f9ee0591254fbd/e/ba12e8bf890a79a99deb2a8b

I did 2 lofts. One ended up with a seam across the face, the other one did not. Now I cant draw a bridging curve as there is a edge on one but not the other. Sketches for both have splines tangent on both ends so all transitions between part of a sketch should be smooth. I don't understand why one of the faces have seam but not the other. 

imants_smidchens

I don't have nearly enough knowledge or experience to say what is causing this, but a solution (not a good one, mind you) which eliminates that seam is to check the "match connections"  box and misalign the sides slightly (shown below)

Note that this does change the geometry of the part slightly:
the volume goes from 58495.69 mm³ to 58493.15 mm³ (difference of .0043% so idk if it's particularly important)

Just to reiterate, this is neither the best solution nor an answer to your question, but perhaps it is helpful nonetheless

John_P_Desilets

@wito_roz137 On sketch 24, there is a projected sketch entity from sketch 22 for the arc. If you redraw this arc without projecting it, that should do the trick. 





After redrawing the arc. 



There are a few ways that you can create construction geometry for a bridging curve. You can either split the face of each part with a plane, or create a sketch that uses the intersect option. 


Plane inserted in the area where the split needs to be. (Point plane option)





Split faces using the plane.  The edges formed by the split can now be used for the bridging curve.




Option 2.

I used the same plane as before to create a new sketch. With the "Intersect" sketch feature, select the faces of the part or parts to be intersected. This option will create the necessary construction geometry needed for the bridging curve, without changing the face geometry. 




If you want learn more on how to use these advanced sketch constraints, check out the Advanced Part Design Learning Pathway, Advanced Sketching.  Onshape Learning Center


Link to the copied document. 
https://cad.onshape.com/documents/f62279415c32017e3b10597c/w/c6d91cfc9758b6cd4f5ee217/e/4128c2d938a11b1eeb2fea1a

wito_roz137

Thank you for your reply.

Even if I redraw arcs without projecting them I still get the seam as soon as ends of arcs and splines on two sketches are aligned with each other.  As soon As I make ends of arcs not aligned with each other on 2 sketches seam disappears.

I would really like to understand when seams like this are created to have a more predictable design flow. At the moment I do not understand how to predict if seam will or will not be created.

Thanks for showing me how to create geometry which can be used to make bridging curves. 

Alex_Kempen

Onshape generally creates seams or edges between non G2 continuous intersections along a face. That is, edges tend to appear where the edges of the face are not G2 continuous. Notably, G2 continuous is a higher standard than, say, tangent connected; although a filleted corner is generally considered to be "smooth", the points of transition between the flat faces and the fillet are only G1 continuous, so they exist as edges in the model despite there being a smooth connection. This isn't a hard and fast rule - edges can always be created using features such as split face, and other exceptions can arise - but it does explain a fair amount of behavior, especially in the context of lofts and other features.

EvanReese

if you want to make sure it doesn't have a split, but you don't want to barely tweak the connection like you're showing, consider converting your profiles to a single spline using Fit Spline in Edges mode. I do this a lot for surfacing work because it reduces the chances that you have a bunch of little unstable sliver faces farther downstream, and you can add or remove edges to the spline chain without breaking the spline references. Here's an example doc.
https://cad.onshape.com/documents/1d0963ec3ed860096bbe6c1b/w/b4c844efbb4cb4d094530d8c/e/1d947eb0365c03b1e0c3cf79