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78finn1-Visitor
1-Visitor
July 13, 2021
Question

G3 surface issues - nose cone part (Creo 7.0)

  • July 13, 2021
  • 4 replies
  • 4797 views

I am trying to model a nose cone/end surface with G3 surface conditions (as shown in the image below/file of model attached). I am using PTC Creo 7.0/Style.

 

MR_1491202_2-1626173399874.png

I'm trying to achieve a G3 surface condition on all 3 domed sides and a normal constraint around the bottom perimeter. 

 

So far I am only able to achieve a G3 condition on the central curve/surface and a normal condition around the bottom perimeter. When I try to increase the two surface conditions either side of the central plane from a G2 to G3, I get the following error message:

 

"The cross curves are not connected with the same continuity as the connection"

 

I'm not sure what I am missing here? It would be great if you guys could take a look and tell me what I am missing? 

4 replies

tbraxton
22-Sapphire II
tbraxton22-Sapphire II
22-Sapphire II
July 13, 2021

If you are designing a nosecone for engineering purposes you will be better off using curve from equation and revolving it. If this is an academic exercise in CAD then ignore this comment.

 

Look at the curvature plots of your curve connections.

 

The curves used to define the surface must be connected with a degree of connection equal or greater than the surface connection you need. The curves are not connected with G3 continuity so you will not be able to create a surface with G3 on that boundary.

tbraxton_2-1626182911738.png

 

 

 

 

7
78finn1-VisitorAuthor
1-Visitor
July 13, 2021

OK, I'm not sure that I am understanding this then. I have made several versions of this model. The latest has been constructed all within style, every spline used for the construction is a style spline (with 7 degrees). All of the connections between the surface and the style splines are attached with G3 surface curvature.

 

The resulting surface will not achieve a G3 condition anywhere, only G2. I can't help but think it has something to do with the surface trim geom, but i'm guessing.

 

It would be really useful if someone could construct and share a part file of a successful version of this geom with G3 surface conditions.  

 

p.s. sorry if my language isn't on point...I'm new to this. 

 

 

tbraxton
22-Sapphire II
tbraxton22-Sapphire II
22-Sapphire II
July 13, 2021

This link may help explain the concepts.

https://www.linkedin.com/pulse/new-c3-curvature-options-creo-50-rodrigo-tafarelo- 

7
78finn1-VisitorAuthor
1-Visitor
July 13, 2021

Thanks, yes, I understand the concept of the difference between a G2 and G3. I  used Alias for a while and still use NX. Alias uses this graphic, very simple to understand:

 

MR_1491202_0-1626186010732.png

I'm just not sure what the "cross curves" error is actually referring to, no where else is the term "cross curve" used in Creo? I searched the help section for "Cross Curves" and nothing comes up? 

 

If is helps, I can upload the latest file I have worked on. The files is constructed in the following way:

 

- All splines with 7 degrees

- All spline to surface connections G3 surface continuity connections.

 

Result: Only G2 is possible for the resulting surface.

 

Just built the same geom in the same way in NX and it works fine. 

Patriot_1776
Patriot_177622-Sapphire II
22-Sapphire II
July 13, 2021

Well, Creo isn't quite the surfacing tool Alias or NX is.  In Creo 4 at least, good luck getting G2 continuity.  I WISH there were better surfacing tools in Creo 4.

7
78finn1-VisitorAuthor
1-Visitor
July 13, 2021

I really like Creo's surfacing tools, style is a really really nice interface. Achieving G2 is a fairly seamless and easy. G3 (for this particular piece of geometry) I am really struggling with lol. 

7
78finn1-VisitorAuthor
1-Visitor
July 19, 2021

Getting closer, but still not quite there ðŸ¤”...

 

Now trying to build the whole thing in ISDX (style), which really seems like the only way to do it because trying to manage either a G2 or G3 curve as a conventional sketch in Creo is made so difficult to control and manipulate. 

 

All splines 7 degree. The trim doesn't actually appear to be the issue...

 

As you can see, all surface conditions now G3, accept the final surface condition which is G2. Doesn't seem to matter which order I attempt (using this method - trim/top hat) the final surface condition maxes out at G2. This is probably absolutely fine for 95% of what I do, but would still nice to achieve a perfect G3...if that is even possible 🙂

 

Any thoughts?

 

1.jpg2.jpg

tbraxton
22-Sapphire II
tbraxton22-Sapphire II
22-Sapphire II
July 19, 2021

G3 connections for curves and surfaces are only supported in ISDX super-features (in Creo 4+). ISDX is the tool to use if you need G3 continuity.

 

Unless the number of points used to build the curve warrants it, you do not need to use degree 7 curves in style. Use the lowest order you can to achieve the desired shape.

7
78finn1-VisitorAuthor
1-Visitor
July 19, 2021

From what I can tell to get G3 surface conditions across all/most surfaces (as above), if you have one spline that requires 7 degrees to achieve a G3 surface condition, then all subsequent supporting splines require the same or more degrees. For the three main driving splines of the nose cone shown above, Creo automatically updates the spline to 7 degree. So for the cross splines I have used the same number of degrees. If I don't use the same number of degrees across all splines I cant achieve G3 in the way I have above. The best result I can achieve drops to G2 or below i.e. as I ramp down the number of degrees from 7 on the supporting/cross splines.

 

Maybe I am missing something here? But that seems to be the case. I'm running Creo 7.0 

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