So I have two rings, each made from tube.  One ring is inside of the other with space between them.  I would like to insert another material in between the surfaces of the two rings. 

While I can just create the material using the same method I used for the other two rings, I would like the material defined as a function of the surfaces of the other two rings.

When I create a part from within my current assembly, and select the same plane I've used to create both other rings, I start to create the revolve by sketching two circles near where they should be. 

I'm stuck now as to how to define the edges of these circles.  I tried to create a dimention between the circle in my sketch to that of the inner surface of the outer ring, but I get the message:

"Selected silhouette cannot be uniquely identified for the current operations.  Please create a split line by using Split Line operation with the Silhouette option."

When I try to select the inner surface of the outer ring.

I'm guessing there is even a better way to mate/define this, but I don't know where to start looking.

Thanks

Miles