Hello all.  Perhaps someone here can give me some insight on what I'm not doing that's preventing me from not being able to  use "cut with surface" within an assembly.   I'm following along in a book on an exercise where a simple sphere is its own part.  Then an assembly is created and a 3d cube is drawn (using a 3d sketch) and 14 spheres are mated at specified intersection points and the midpoints (see .jpg).  I've gotten this all completed, but the last step in the tutorial just says to "cut away parts of the model to create a unit cell" without any explanation of how to achieve this.  I've attached the part as well as the assembly.  Basically the intent is just to cut away the outer parts of the spheres where the 6 faces of the cube are.  I'm at a loss for what I'm missing.  Thanks in advance.