Locating a round surface in assy

[jlbmech]

I am having problems locating a buttweld pipe cap in an assembly. I need the radius head of the cap to be a specific distance for a plane. When I use the tangent option to located it, it puts the cap off of the face and not on it. What am I doing wrong here.

 

[beachcomber]

Hi jlbmech,
Best way here is to put a reference plane in the cap model file, tangent to the domed surface. You can then use the plane for positioning rather than the dome.
What could be happening at the moment is that the 'wrong side' of the sphere is being made tangential. Try moving the part nearer to it's correct position and then do the tangent mate.
I've seen that problem myself when placing o-rings tangential to the surface of a groove.

bc.