The intersection curve is calculated using the surface surface intersection technique. If you receive the message, No surface-surface intersection, then FeatureMILL cannot determine the intersection curve for your surfaces. For additional details see these restrictions.
If you receive the following message, Can’t trim, trim curve does not end on a boundary, then an intersection curve has been calculated, but it is not a valid trimming curve for your surfaces. This figure shows an example.
See Trimming restrictions for all the restrictions for trimmed surfaces.
If you have multiple trimmed surfaces, the intersection curve must form a valid trimmed surface for each of the trimmed surfaces.
If you have checked the Trim this surface also checkbox, then the intersection curve must form a valid trimmed surface with respect to the trimming surface.