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Can't split boundry using a sketch

Hello, I'm very new to Onshape and I'm trying to make a half-split through this surface cone. The problem is that it doesn't let me split like seen in the image below. When i use the part split it splits the whole part. As an end result I'm going to use the flatten tool and have a rip edge as this split line to then save it as a 2d image so i can then print it onto a piece of paper and fold it back to 2d. Please let me know how I can fix this or if there's any other way to do it. Thanks.
Comments
Have a look at flatten surfaces with a rip edge. The flattened surface has to be exported then brought back in via a sketch then thickened. Not as convenient as sheet metal tool but works.
https://cad.onshape.com/documents/7cd9097db9a39677f054b262/w/956877d55e01351d69173503/e/5ecacdc8316393a4b85885dd
The problem is that I can't have a rip edge on my cone. I'm trying to get an edge with the split but instead of 1 cut to the middle of the cone it cuts it the whole way through.
I see you didn't do the revolve the whole 360 degrees. That's my problem right, that when you revolve the cone fully you get no rip edge. Thanks for the work-around but it would be nice to have the split feature only split in a boundry
Have you tried split "face" instead of split "part"?
I might not be understanding what your trying to do, but why not just limit your cone revolve to 180°?