Hi Mike,

The problem is you have overlapping surfaces, when you swept your profile Boolean union is getting confused because it's trying to union the spiral body to it self as well as the cylinder.

When you look down the centre of the spiral body the lines indicated there are underlying surfaces within the body.





Hiding the front face reveals the overlapping surfaces inside.




I've attached a cleaned up version where I removed the inner spiral overlapping surfaces, extruded another cylinder surface, trimmed it up and joined it to make a solid, it should union to the cylinder now.

~Danny~