When a curve is tangent to another, I learned, when I was younger, that there are 2 possibilities for such tangent : a curve has 2 possible orientations, in a 3D affine space.

But SW designers haven't learn such basic concept.

So a relation of tangency give one choice, and get by with that....

http://www.solidworks.com/pages/programs/letsgodesign/index.htm?pid=3&erank=3

I suggest that you spend your money with professional developments.

This begins with correct basic concepts with the affine geometry in affine spaces of dimension 3.

Message was edited by: Richard Doyle - Removed potential offensive word.