To Create a Multi-point Extension
You can use multiple measurement points to create a variable distance extension.
1. Perform one of the following actions:
◦ Select the boundary edge chain of the surface that you want to extend, and click > 
Extend.
Extend.◦ Click > Extend, and then select the boundary edge chain of the surface that you want to extend.
Extend, and then select the boundary edge chain of the surface that you want to extend.The Extend tab opens with the 
Along Original Surface type selected. The selected edge appears in the Boundary edge collector on the References tab.
Along Original Surface type selected. The selected edge appears in the Boundary edge collector on the References tab.2. To define the distance to extend the surface, use the drag handle to manually extend the selected boundary chain to the desired distance.
3. To define the extension method, click the Options tab, and next to Method, select an option:
◦ Same—Extends the surface by continuing it past its original boundaries. The surface type remains the same.
Use this method if you want to preserve the surface type of an analytic surface.
◦ Tangent—Extends the surface by making it a ruled surface that is tangent to the original surface.
◦ Approximate—Extends the surface as a boundary blend.
◦ Extrapolate—Extends the surface by extrapolating it past its original boundaries.
If an analytic surface is extrapolated, it will become a b-spline (non-analytic) surface. To preserve the surface type as analytic, select the Same method instead.
4. To add measurement points, click the Measurements tab. Initially, the system adds only one measurement point, which extends the entire chain by the same distance to create a constant extension.
5. Right-click inside the Measurements tab and choose Add.
6. Specify the points's location by typing a distance along the edge (from the first point to the point being added) under Location.
7. Type the extension distance under Distance or drag the point.
8. From the Distance Type list, select one of the extension options:
◦ Normal to Edge—Extends the surface normal to the selected edge.
◦ Along Edge—Extends the surface along the side edge.
◦ To Vertex Parallel—Extends the surface at the vertex and parallel to the boundary edge.
◦ To Vertex Tangent—Extends the surface at the vertex and tangent to the next one-sided edge.
9. To continue adding points, right-click again and choose Add. To delete one of the measurement points, right-click that row, and then choose Delete.
10. Click 
OK.
OK.