Is there a way to make a midpoint constraint to an 3d arc?  I'm working on a rollcage design and can't get the constraint to keep.  When I go to draw a straight line up to the midpoint of an arc the end of the line will snap to the midpoint, but not actually go black.  It will keep a coincident constraint, just not the midpoint constraint.  Any thoughts on how I can fully define my sketch?  Thanks.