Make a curved cut on a curved surface of a free form object

[Anonymous]

Hi, I am having trouble trying to make a shallow cut on the surface of a form from the "create form" tool. I have followed previous advise on splitting surfaces and then trying to offset the surface onto the main body, but instead it seems to shrink or expand the body as a whole instead of the area of the face created to cut. 

What should I do? Below is a step by step of what I did and what is going wrong. Thank you! 

1. sketch; 2. extrude-new body; 3. split face; 4. the new face to offset; 5. the offset gone wrong.  

[jeff_strater]

it worked on a simple model.  Share your model for more help.  There are some oddities to Offset Faces on a model like this with all tangent faces.  You might need to use Split Body instead of Split Faces, do the offset, then Combine the separate body back to the original.