# What is the Difference Between Bonded and Merged Surface Edges

When you have two surfaces meeting, edge to edge, Pilot3D allows you two ways to attach or connect the surfaces. One method is to “bond” the two surfaces and another method is to “merge” the two edges. Both methods allow you to move any point on the common edge and watch both surfaces update at the same time. The difference between the two surface relationships is in how each one affects the operation of certain commands. First, you need to understand the difference between the two methods.

Two surfaces have a common “merged” border edge if each point along the edge of one surface has a “Merge Point” relationship with each point along the edge of the other surface. This means that if you move any point along the common edge, then both surfaces will update appropriately. There will be no gaps along this edge as long as all edge points are matched up one-to-one. There will be one “Merge Point” relationship for each point along the edge.

Two surfaces have a common “bonded” border edge if the two surfaces edges have been matched or merged AND then bonded. Bonding is usually done by defining a bonded relationship at the common surface edge using the Surf-Bond Surf command. There are some cases, however, where bonded edge relationships are created automatically. One case is when you split a surface into two surfaces along an internal row or column. A bonded edge relationship is created for the resulting common edge. Another case is when you extrude, sweep, or loft a curve that consists of both polyline and curve sections. The resulting surfaces will be automatically bonded.

Actually, there is another method for joining two surfaces along an edge. You can use the Edit-Match Point-Match 3D command to match (move) each edge point of one surface to each edge point of another surface. This technique, however, does not create any relationships or constraints related to the geometry. This means that if you move one of the common edge points, the two surfaces will separate. If you want the two surfaces to remain attached, then you must either merge the edge points or bond the edges together.

If you want two surfaces to match exactly mathematically along a common edge (no gaps or leaks), then the two surfaces MUST match EXACTLY along the common edge. This means that the two surfaces must match everything point for point, the common points must have the same weight factors, the two surfaces must have the same polynomial degree along the edge, and the two surfaces must have the same knot vector along the edge. This can be important if you expect to transfer the NURB surface model from Pilot3D to a solid modeler with no problems with gaps or leaks.

The bonded edge relationship is “stronger” than the merged edge relationship. This means that the program will always maintain this exact point-for-point, no gap relationship between the two surfaces. This problem arises when you want to add a row or a column into a surface where that row or column runs into a bonded surface edge. In order to maintain the proper bonded, point-for-point relationship between the two surfaces, the program must add a row or column into the adjoining surface. Thus, adding a row or column into a surface will propagate through all connected, bonded surfaces. If two surfaces are merged along an edge, then the process of adding a row or column into the surface will NOT propagate to the adjoining surface. This means that there is no guarantee that the common edge will stay exactly the same for merged edges.