Stuck in Unity3D intersection. Intersect in 3d (Plane-Edge Intersect)

I have a problem with how to get the outer intersected edges of a 3D object that was cut randomly. For example, sometimes i want to cut it till i reach the middle and sometimes i want to cut it just a little(just a small cut in a random face of the object that i know it has an edge)(the cut is not actually happening to the actual object and copied the objects mesh details to other varriables so i can check the cutting details). My problem is that when i’m trying to get the outer edges i’m getting nothing as a result or something that is on a complete different position that i desire.
On this link: [c# - Stuck on Unity3D Intersect, intersection in 3D - Stack Overflow] i realized that i need to do a plane edge intersection, but whatever code i use, i never get the result i expect. Also the 3D objects i use have only outer edges, so no inner edges exist. If you go to the link above i have more info about my issue. Any help would be appreciated!!