tolerant edge

To my limited knowledge, surfaces in NX might have four types of edge:

 

     ● analytical curve (line, arc)
     ● spline
     ● intersection curve
     ● tolerant curve/edge

 

"analytical curve" and "spline" are easy to understand. "intersection curve" is simply a precedural curve, just as Offset Surface is a prcedural surface. All these three types are mathematically precise.

 

The difficult one is "tolerant curve". I've read the doc again and again, but still failed to understand it. The doc states: "The system represents a tolerant edge by combining spline curves on adjoining surface edges within a tolerance distance from one another."

 

(1)

 

In my understanding, an edge is an edge. For example, Swept gives a heavy surface. But even a heavy surface is a mathematically precise surface, which has mathematically exact edges, just as a light surface given by Studio Surface.

 

Why does NX need aother type of edges? Can anybody kindly give an example?

 

(2)

 

Trim Curve has a special option -- Output Exact Geometry. The doc states: "Outputs Intersection curves whenever possible. If that is not possible, a tolerant curve is produced."

 

When is Outputs Intersection Curves "not possible"? Can anybody kindly give an example?

 

Thanks!