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Rather than spoonfeed you steps, why don't we see what you've learned so far?
Here are 2 images from your attachment. Do you see anything that might be an issue?, particularly with the green and yellow surfaces?
Some things to consider and/or hints:
you have to finesse these surfaces. It's not easy, either. It's a bit of give and a bit of take.
Are you happy with the pole structure in the image above? If you are, you need to rethink things a bit
Ask yourself, is this a surface that looks like it flows well from one direction to another (in regards to the pole and patch structure). If it doesn't flow well, can breaking it into smaller surface patches (pieces) make it flow together better?
I'm not sure whether below is the right way of "breaking it into smaller surface patches" or not. The pole structure does look much better.
One practice that won't ever fail you is if you're going for G2, then use degree 5 splines. G1, you can relax a bit and use degree 3 splines.
In my understanding, for the current example, "degree 5 splines" are for the guides/rails. I'm not sure whether the sections should also be "degree 5 splines". I 'm not sure whether I understand this correctly or not.
Your brief comments would be appreciated!
For this instance, I would just use degree 5 all over if it were me. I don't believe I'd ever mix degree 3 with degree 5 in the same areas if I could help it. The scope of the desired end results should be your guide. If the desire is Class A, degree 5 is pretty much the preferred choice.
The more poles you have, the more the surface should relax as continuity locks down the poles near the ends but the con for high degree splines is the math gets more complicated and "heavier". We don't want to over do it - degree 7 or 9 at the highest but to be honest, I don't recall what the preferred maximum degree might be.
Hi @TimF Thanks for the commens on the choice of spline degree! Absolutely helpful!
I do want to ask another question. I think I'm a little greedy. If you think you spent too much time on this thread, please simply pass on it.
Very often, one or more input curves are edges left by Trim Sheet, or curves from Combined Projection. Generally they are of bad quality for surfacing. To refine them by Advanced Curve Fit without damaging the model continuity seems to be a big challenge for me. I wonder whether there is a better method to do the job.
IMO, the surfaces should be developed such that whether trimmed or not, the result is as needed. This is more than likely why I and others kept stating the root of your original issues were in the adjacent surfaces - they needed to be rebuilt such that they were usable rather than causing more issues.
I feel that if you continue to refine edges with secondary curves, you're potentially going to run into issues sewing the surfaces at some point....just depends on the tolerances and how much your refined curves deviate from the trimmed edges. The more stuff you put between the edges and the end surface, the more tolerance you might be adding into the mix.
Again, you're asking advice based on 3 surfaces - and we all know that's not the entire job, only part of it. To go further, I think we'd need to see the big picture at some point.