Since the OP mentioned being on the GrabCad site, I'm surprised he didn't stumble across BoltGen. It's a free plug-in for Rhino.

The plug-in author's resource page is additionally listing a standalone GUI, but it's unclear whether it feeds the resulting bolt model back to Rhino or just leaves a bolt model file -- presumably in Rhino's 3DM nurbs format: http://mcvanaero.com/resources