I just found this cool program that creates Celtic Knots in 2D and 3D. It outputs in stl and dfx so if you have the stl importer or the dfx importer you can have some fun with this program. It also exports obj and the output is very clean quad geometry if you want to edit it further it makes it a snap.
http://www.stevenabbott.co.uk/Knots/knots.html
I also did a little write about it here http://www.cncmodelclub.com/celtic-knot-cnc-patterns/ with a couple of quick tests that can be downloaded.