I’m sure that I’m missing something obvious and simple, but I cannot use Networksrf to create a tangent blend between these two surfaces and accompanying curve positions (file attached). I know I’ve done this in the past, but something about the polysrf is preventing me from leveraging Networksrf.
Any assistance would greatly be appreciated.
Thank you!
-Ricknetworksrf not working.3dm (937.5 KB)
Hello Reberts1,
Try hiding the surface and open extension. Then make the network surface from the remaining curves. Then show.
Thank you,
Andynetwork not working.3dm (888.0 KB)
If you use the bottom enclosed curve as an input instead of the bottom polysurface underneath, “Network surface” works well with “Edge curves = 0.01”; “Interior curves = 0.001” and “Angle = 0.01”. However, I strongly recommend you to use “Sweep 2 rails” in this particular case, because it has an option to chain the edges of the bottom polysurface. Also, it will produce a more accurate surface while keeping the amount of control points at minimum.
One problem with your profile (vertical) curves is that they have kinks (hard points or where the curves suddenly loose smooth transition), because the middle is a straight section joined at either side to two other curves that were Degree 2. This means that the resulting surfaces made by those curves will also suffer from the aforementioned issue. These curves will produce better results if you replace them with “Adjustable curve blend” (Degree 3 and consisting 6 control points).
I wonder why you don’t use “Blend surface” with the “Auto chain = Yes”? It will take you just a few seconds to build perfectly smooth blend between the existing geometry. You don’t even need any profile curves to do that. 
In the “Adjust surface blend” dialog box use “0.85” for the upper edge and “1.0” for the lower one". That will create a blend surface that’s close enough to your profile curves. Then use “RemoveMultiKnot” and “Join”.
For a much, much better result, I also recommend you to replace the vertical radius surfaces of the bottom polysurface with G2 ones made with “Blend surface.”.
Thank you!
Oddly, I used Networksrf to create/replace the polysurface and it works now. So something with the normal extrude command is not compatible with Networksrf (as input for).
I like to use Networksrf in certain circumstances where I want very specific control of the planar profiles for tooling, but definitely like Blendsrf for the quick, beautiful result!
Thanks again.
Sincerely,
Rick
Blend surface working.3dm (14.3 MB)
This is how I’d do it. Snap curves perp to the circle edges. The split the circle edge at the curve points. Then surface loft the sections with tangency.
If you want adjustable transitional surface (“History” must be enabled), I recommend to use “Loft”. There are two ways to do it with “Loft”, depending of your preference.
First, it will be nice to use “Merge” with “Roundness = 0” in order to combine all bottom surfaces into a single one (unfortunately, at the moment “Loft” will not let you use “Chain edges”, unlike “Sweep 2 rails”). “Roundless = 0” will preserve the structure of the surfaces.
Important! If you truly aim G2 blends anywhere, before you even start with the “Merge” command, I recommend to use “Change surface degree” and change the degree of the bottom surfaces from 2 to 3. Then you can use “Merge”, as I described above.
Loft 1: Extract the bottom edge of the upper circle surface and the upper edge of the bottom surface (which you merged prior that). Move these curves away vertically. This will be the G1 (tangency) profiles. Then, make copies of the curves and move them further away. They will be the G2 (curvature) profiles.
Now use “Loft”, adjust the knots with OSnap “Quad” turned on (extremely important!!!) to be in the same vertical location along the X axis, then select “Loose” type and then choose the remaining settings that will best suit your needs. Now you can move vertically one or all of the 4 profile curves to adjust the shape of the loft surface. 
Alternative loft 2: Create a closed curve profile that’s perfectly symmetrical (“Snap to grid” is a convenient way to do it), then place it somewhere in-between both surfaces. That horizontal closed curve will be used as an adjustable middle profile, which you can conveniently modify by moving or scaling its control points. Again, use “Loft”, adjust the knots, but this time select “Normal” type and also tick the boxes of “Match start tangent” and “Match end tangent”. Since “History” is on, now comes the fun time with adjusting the middle profile according to your taste.
Finally, no matter which loft type you used, apply “Match surface” to the bottom edge of the loft surface. I found that the following options work good for this particular surface:
Tangent
Match edges by closest points
Distance = 0.001 units
Tangency = 0.05 degrees
Curvature = 0.05 percent
Isocurve direction adjustment = Automatic
Both loft types are included in the 3dm file below:
Loft works nicely.3dm (9.8 MB)
This is a video showing the 2nd loft type in action:
