I'm new to this forum and though I have been using Solidworks since 2005 I would place my skill level at "enthusiatic amateur". I am moldeling a gear box housing from which I will be creating patterns for casting the part out of aluminum. I need to model internal oil passages on the inner surface of the gear box housing which would be easy enough if the surface was a flat plane. But of course it isn't. The inner surface of the housing is a large oval which morphs into a smaller circular surface. There is a pretty generous radius that joins the two surface features. I want to create an oil passage that lies on the inner surface of the housing. This oil passage will be .375" in diameter and have a .250" wall thickness. Basically, I want to create a tunnel and attach it to the inside of the housings compound curve surface. I am thinking that the way to do it is to extrude a profile along a centerline that is placed equidistant above the housing's inner surface. How do I project a centerline off of a surface? Or is there a better or easier way to do this?