Cannot create a network surface: Root Cause?

[Anonymous]

I have searched the forum and the Internet, and to my best knowledge, I cannot find an explanation to this problem (only some workarounds). Apologize if I made a dumb mistake.

To my understanding, the "Surface Network" tool is to create a mesh grid given a sufficient number of edges that enclose it. Here, my goal is to create a surface network enclosed by 4 Splines. Here is one of my working examples (left: the 4 splines. right: surface network):

However, using the same method that created above spline (using command: create one edges, then copy/rotate the remaining edges), the following shapes (and many others) fail to create its network surface:

I tried to convert the splines into 3D polyline, doesn't work. Since these are generated by command, there isn't any offset as well (that fail to enclose the surface). Some workarounds I found include using some special scripts to generate the surface, or just move away from autoCAD.

Yet, none of these explain would this happens. Am I misunderstanding the uses of "Surface Network"? What are the minimum changes to this to work?Any help is appreciated. Attached is the .dwg file of both examples.

Edit: re-uploaded with intended examples.

[R_Tweed · Accepted solution]

It looks like the network wants to smooth out the sections when interpolating across the mesh. If you have intersections at your arcs then it will give you that error.  If your looking for sharp seams then you could loft in each direction and trim the surfaces.

I made a similar mesh using plines with fillets. I suppose you could keep reducing the fillet radius until it breaks.

See attached file.

[leeminardi · Accepted solution]

When I examined the splines in your "broken" drawing I was struck by the configuration of CVs at the cusps (circled in red below).  The spline should not show such a discontinuity in slope with the CVs you have unless the weight at the knot is very high or there is a duplicate CV. Neither was the case. It looks like you created your spline by creating one segment then arraying it and joining the result together to form a single spline.

I recreated  your spline but created 2 CVs at the cusps.  I was able to create a network spline from these splines. I used the ends of the green lines to help me create a single spline using the CV method.

[Anonymous]

Thanks a lot for your answer! However, I would like to clarify a bit, as I am very new to AutoCAD. What do you mean by adding 2 CVs at the cusps (how exactly do I do it)? And how does this affect the "broken spline" and fix it?

From what I understand of the another answer, it is that the two spline segments need to be smoothed as an arc (rather than an abrupt discontinuity in slope as you pointed out), and one method to do it is to use the fillet tool. Does your method work on such similar mechanism? Thank you.

[leeminardi · Accepted solution]

I am not sure how much you understand the math of splines and I'd like to keep my answer short and simple.  AutoCAD splines are based on the mathematics of NURBS (non-uniform rational B-splines).  NURBS define a curve via a series of Control Vertices (also called control points).   In AutoCAD you have the option of defining a splines using these CVs or via "fit" points.  The default method is Fit points. If you define an AutoCAD spline with fit points AutoCAD automatically determines appropriate CV points for the spline.

The number of CVs used to define a spline is a function of how curvy you want the spline and some other factors.

Let's assume I would like to define a curve using  5 fit points as shown below.

  

If you go to the properties panel for the spline and click  Show the CV frames you will see something like the following (I added the red point objects to highlight the location of the Control Vertices).  Each of these CVS can be moved resulting in the changing of the shape of the spline.

By default AutoCAD creates what is known as a 3rd degree B-spline.  This means that the curve is represented by a series of cubic (hence the 3rd degree) curves laid end to end.   In general the spline will not pass through any of the CVs except the first and last.  It is interesting to note that the second and the next to the last CV defines the slope and radius of curvature of the spline at its ends.  Try moving them and see how the spline changes its shape. 

It is possible to change the shape of a spline by moving the CVs, adding CVs, or by changing a weight factor (via the splinedit command).  I will ignore the weight issue in this discussion.

Creating a duplicate CV coincident with an existing CV will have the effect of "pulling" the curve more towards that CV.  For  example, the green spline below has 2 CVs at the highlighted point.  Notice how the spline is pulled towards it.

Since the spline is degree 3 we can place 3 CVs at the same point with the result that the spline will pass through the point and create a discontinuity of the slope as seen below.

The cyan colored line has 3 CVs at the noted point. 

Here's a closeup view at the CV.  The cyan curve is the spline with 3 CVs at the point and the green curve has 2 CVs there.

I hope this helps you understand splines and CVs.  Let me know if you have any additional questions.

Lee