In Fusion the extend feature does not allow the user to move the surface boundary inwards.
This is pretty common in my surface modeling work flow.
IE: Create surface, move boundary inwards, then loft to anther surface - just as an example.
As far as I can tell there is no work around either.
This is the goal - to move the bottom edges of the surface to the red line (~20mm):
Creating a ruled surface normal to the original surface, offsetting the ruled surface, then using the offset to trim the original surface looks like it works but the surface quality is very poor. Edges are not continuous and the result is not usable:
It's not like the extend feature can't figure out how to move these boundaries.
Here I can extend the internal rim of the original surface if the top is trimmed off.
However it seems there is no way for users to use "extend" or any other features to move the boundaries of a surface inwards in a way that subtracts from the surface. In most CAD software you should just be able to add a negative distance within the extend feature.
