one surface - 3 curves

 From:  Michael Gibson
1817.2 In reply to 1817.1 
Hi Olga - for Network all the curves involved in the Network need to form a grid, similar to the way the lines of latitude and longitude form a grid on a sphere.

In your case there, your combination of curves cannot be "unwrapped" to a 2D grid like latitude or longitude lines so Network isn't able to use that arrangement.

If you split your outside curve into 2 pieces so that there were 3 pieces in the long up-down direction, that would then form a grid layout and could be networked.

This previous post here shows some illustrations describing this on a similar layout, it may help you to see how the curves for Network need to be organized.

However, having said all of that, this is a good example where I would recommend a completely different method - instead of trying to build the initial surface to that final outline directly, instead it is better in cases like this to build a larger more simple sheet first, and then trim it.

You are actually all set up very well already for this other method!

These are the steps for that.

Start by selecting this curve:

Now run Construct / Sweep, and pick the other long curve that crosses it as the rail curve. That will create this surface, this is now the larger "sheet":

You can now delete the curves used for the sweep or hide them to get them out of the way:

Select the surface:

Run Edit/Trim, and select the outline as the cutting object:

While you are in the Trim command after you have finished picking cutting objects, it will ask you to select pieces to discard, select these 2 pieces to discard:

Then when finishing the Trim command you are left with this result:

This kind of "construct larger extended surface and then trim back" type approach can often give you much cleaner surface quality results than trying to construct a surface directly to a complex boundary. With "direct construction" to an outline (like trying to sweep with the outline as a rail path, etc...), it is difficult to get a very clean surface when parts of the outline boundary may pinch together.

It is much better to have a complex outline as a trim curve that is on top of a more simple surface. Such things will have a far better behavior in other operations such as shelling and filleting, the overall quality of the underlying surface is much more uniform this way, the more parts of a surface that are squeezed or pinched together tends to make for less quality in the surface.

One other note - to get just a single surface from the sweep instead of 2 surfaces joined together you would need to fuse the 2 segments of the rail curve into one segment - the way you can fuse segments currently is by turning on control points and deleting the control point where the 2 segments touch each other. That combines 2 segments into one segment.

- Michael