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Hi @TimF Thanks for your correction and sharing your practice!
COS can be used as long as the resulting curve falls within the modeling tolerances (intersection, distance and angular). The same principle applies for almost all curves, depending upon the surface feature being used.
For me it's an everyday practice to use a curve on a surface to help assign continuity to the surface.
If it's also possible for Intersection Curve and Project Curve to lead to the issue as Curve On Surface described in the previous thread, I would be concerned about the robustness of all my models.
Fortunately, until now I have never observed the issue with Intersection Curve and Project Curve.
By all means I would be very careful to deal with the situation in the future.
I'm trying very hard to be patient - but you MUST get this into your head about a few things in NX regarding a curve laying on a surface, particularly how the COS command should be used.
Just because a surface is G0, G1 or G2 to another (other) surface(s) does NOT mean you can create a COS and it will AUTOMATICALLY be the same continuity to the adjacent surface(s) without EXPLICITLY assigning that continuity constraint to the COS you're creating. In that thread you're referencing, there is NO CONSTRAINT applied to either end of the COS. If there were, you'd see either 1 line (G1) or 2 lines (G2) above the endpoint handles. That MUST be applied, just like any other feature curve with continuity control. I just downloaded it, applied the constraints that were missing and the last surface appeared. The issue in the previous thread was a complete misunderstanding of how a command should be properly used if the desire is to match continuity to other surfaces, in my opinion.
The reason this must happen is because the continuity constraint can affect the end shape of the COS. It's just like any other spline - you assign continuity and the shape of the curve you've laid out might change somewhat or quite a bit, depending upon the situation and shapes involved.
I don't know any other way to explain it - that's just the way it is.
What you're in regards to Project Curve and Intersection Curve is dependent upon the geometry being input into those commands as well as the choices you make within each command's dialog - Project Curve will NOT work 100% of the time in the way you're asking. It will depend upon the inputs, particularly the way the curve is projected. It's like aiming a gun - if the sights are junk and the gun has been not cared for, do you think you're going to hit the bulls eye all the time? But if the sights are fine tuned and the gun is in tip top condition (like curves in NX sometimes need to be) then the gun should fire true, as long as the human element knows what it should be doing as well.
NX will only do what you tell it to do in some cases - and you have to learn when to specifically tell it to do something or know what you have done already will be enough.
I hope that clarifies things a bit more for you.
Hi @TimF In coming days I would try to understand again what you wrote in this thread and the previous one. I'm really appreciative for all your guidance in the past months, without which I'm not sure whether I would be confident enough to continue this career of NX design. Thank you!