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# continuity with a curve on a surface

Genius
When I need a new spline to be G1/G2 with a surface, often I would make it G1/G2 with an existing curve on the surface.

In a previous thread, @TimF and others taught me an important lesson that a spline from "Curve on Surface" can't be used for setting continuity to a surface.

I wonder whether other two types of curves can be relied on for the purpose:

• Intersection Curve
• Project Curve

Thanks!
11 REPLIES

# Re: continuity with a curve on a surface

Phenom
@surfactant,

No, that's not necessarily correct about Curve On Surface, as you can assign continuity to the curves. 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. If the result meets what you desire from a surface, then in most cases, you will have success creating a surface with continuity - there are some instances where a curve network gets to be too much and NX will give you an odd error message or warning here or there.

The creation of parent curves and their resulting children curves can lead to the reason why I prefer to not have most curves as features - one typically has to create, refine (move poles, intersect, project, trim, join, extend, etc.) then create some sort of bridge type of curve to other curves that might have to go through some sort of refinement. At some point, the history gets bogged down with older curve features and/or Sketches that just aren't necessary any longer and only make the modeling history more cumbersome than necessary.
-Tim

# Re: continuity with a curve on a surface

Genius

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.

Thanks again!

# Re: continuity with a curve on a surface

Phenom
@surfactant,

Just to be clear - the surface on which the COS resides will NOT be affected by the COS continuity. A completely new, separate surface would need to be created. The surface on which the COS resides is the parent, a child of that would be the COS and then the resulting surface created from the COS would be a child of the COS and the original surface on which the COS resides.

I hope you understand that, as it's fairly important since it has a bearing on the end results.
-Tim

# Re: continuity with a curve on a surface

Genius
Hi @TimF   Thanks for your explanation!

Let me decribe my concern better:

I have in my model:

• Surface_A     >>>   parent of COS_B
• COS_B          >>>   child of Surface_A, residing on Surface_A

Now I need to create a Spline_C and desire it to be G2 with Surface_A.

In my workflow, I would constrain Spline_C to be G2 with COS_B and believe that the resulting Spline_C would be also G2 with COS_B' parent --- Surface_A.

In pure math, this should be no problem.

But in NX, the issue in the previous thread showed that the workflow above sometimes doesn't work.

In my understanding, this is due to NX way of calculating Curve On Surface by certain approximation and without applying any kinds of constraint.

Intersection Curve and Project Curve also produce a curve residing on a surface. Therefore I'm concerned about whether these two commands might also have similar issue as Curve On Surface.

# Re: continuity with a curve on a surface

Phenom

@surfactant,

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.

-Tim

# Re: continuity with a curve on a surface

Phenom
@surfactant,

COS is much easier to use because you can FORCE the continuity - Project Curve doesn't give you this option explicitly. In regards to Intersection Curve, that should be obvious - if the surfaces have continuity, they should produce what is expected but there might be a situation where it doesn't because you might be making an assumption regarding a specific angle and it isn't correct. In those instances, you'll just have to learn by trial and error and try to remember - just like all of us long time users have done over the years. That's just how modeling can sometimes go. Many times, the end game is figuring out how to force the software to do something it really doesn't want to do - sometimes it's due to improper understanding or use of a command by the user. It's not always cut and dried like you sometimes feel it might or should be.
-Tim

# Re: continuity with a curve on a surface

Genius

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!

# Re: continuity with a curve on a surface

Phenom
@surfactant,

You're quickly approaching the realms where training would benefit you more rather than users opinions on these boards. You can be given many guidelines via this medium - but I'm of the opinion that you will learn more effectively in a classroom type of environment at this point in your learning experience.
-Tim

# Re: continuity with a curve on a surface

Genius

Hi @TimF   I would try to take some courses. Thank you!