Gregory, This is quite easy. Just start a sketch on the "cut-off" plane you are referring to, then pick the icon with the "Create an entity from an edge" description. For the ellipses formed from your angled cylinders you can just pick directly, but the Chain and Loop options are especially useful when picking up more complicated boundaries. I'm not sure about your "plate to fit inside after", but you can create a second part within a "real" or temporary assembly and subtract the intersecting geometry of another part. Need to know a bit more about what you are trying to do. David