Through mesh curve NX9.0

[Karlis]

Hey everyone,
great help from you guys as always, which is what made me come here in another quest.
What I have here is a reverse engineering project.
It consists of multiple splines, generated by performing section analysis on a scanned facet body. I have also added some studio splines manually to give it some guide curves.
Here is the model:

To me this looks a lot like a need to create "through curve mesh" surface.
But whenever I do, I either get an error "strings cross or bad selection" or the surface that is generated is messed up real bad. It twists multiple times along the center axis and comes out with huge spikes. Nothing that I thought of changing really helped this problem which is weird, because I have successfully used this feature many times before on similar models.
I also uploaded the file to ENGINEERING.com so anyone willing to help can take a closer look.
Thank you for your time!

 

[Toost]

... i don't know how to say this in a way that you don't get offended...

The scanned sections are crap. Useless as is.
A faceted model is composed of small triangular areas, facets. Each facet is flat. It might look smooth when shaded but it isn't.
If you section this you will get "millions" of linear "pieces", - One facet = "one linear piece"

When sectioning and the result is splines, if the Spline degree is 1, then the spline will follow the facets very closely because degree 1 is "linear segments". The spline will have 1 segment per facet. One of the sections i picked had 402 segments. Between the segments are "knot points", the knot on a degree 1 spline are by definition C0 which means that it can be a sharp corner. In this case they are sharp.
If you would use degree 3, the spline will not follow the facets as closely but it would produce a "usable" spline.
A degree 3 spline has C2 knot points by default, which is curvature continuous. - Ok to build surfaces from.
( "Standard" degree in CAD free form is 3. Unless you know what you are doing , stay with degree 3.)
The next problem is that if you section the facets using degree 3, you might get too much data in your splines. "Overloaded".
The splines will not be smooth but have undulations because the input data is not perfect, and also contain too much data.
One needs to try reduce the amount of data in each spline to get a usable spline.
Also in the "between sections" direction there is too much data. There are 12 sections before the channels splits into 2. You might need 2 or 3 or four to get the desired shape. The remaining sections will only produce undulations in this direction if used in the surface.
- If you get undulations , do not add more data.

This model, i would build as two channels which blends together. - If i look at the last section before the channel splits into 2, it consists of 2 narrow blends and two large channels, having this sharp curvature in a sheet which is otherwise is relatively "flat" is normally not a good idea. make these 2-4 separate faces such that each has a consistent shape of its own.

Well, it's a start.

Regards,
Tomas

 

[Karlis]

Thank you for the valuable input Tomas.
I would like to note that I am a beginner in NX, coming from SolidWorks, which has little to do with surfacing.
You mentioned that I require C3 section splines in order to create a valid surface. Could you please point me where is the setting that regards this? In the section analysis feature, I could not find it.
And about the section count, I really need all of them, because my goal is to replicate this shape as close as possible to the real thing, with all its' imperfections.
I thought of making them as seperate faces, but that would require splitting my sections in multiple segments. Any idea how to do this?
Thank you Tomas

 

[Xwheelguy]

1. No, he said you needed degree 3 splines, which is different than C3 (C3 would be a higher level of continuity or smoothness between spline knots).
2. You can use all of the sections - I believe Tomas is trying to get you to examine your splines and clean them up a bit as well as make your total area from smaller surface "pieces" - don't know because my company blocks whatever image hosting site you're using.

In my opinion, you're going to need some additional knowledge of splines and how they affect surfaces as well as learning some good surfacing practices - probably more than you're going to get in a few paragraphs here. But who knows, maybe someone has the time to walk you through it.

A good alternative would be a Freeform modeling self-paced course (CAST - if it's still available) or online course from Siemens. In addition to that, you may wish to look into rapid surfacing and the reverse engineering tools offered in NX's Shape Studio (or whatever it's called these days - maybe John Baker can assist with that). I would highly recommend that, especially if you're going to be doing this sort of thing over and over and over.

Tim Flater
NX Designer
NX 9.0.2.5 Win7 Pro x64 SP1
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB

 

[MSPBenson]

A trick I use when I have a section of a facet like this is use the join curve command and then change the output type to advanced.
Try increasing the max degree. Change the tolerance to how far out you are prepared your smoothed spline to be from the original facet and see what you can achieve.
It looks like your part is symmetrical so try and model up 1 half ansuring you have tangency across your split plane.

Mark Benson
Aerodynamic Model Designer

To a Designer, the glass was right on CAD.

 

[Karlis]

Unfortunatelly, my model is not symmetrical, it is an intake port of a cylinder head, which has the entrance at an angle, thus it has been hand ported further decreasing the symmetrical properties.
Alright, I know I have a lot to learn, I completely agree. But what I need right now is to get this done within a couple of days.
As far as I understand:
1) I can make sections from the facet body as it is, but try not to create too much of them.
2) I can take the generated splines and use "join curve" command on them (one by one?), adjust the dergee and tolerance to obtain the best output.
3) I should surface the splines not as a whole, but rather by vertically split segments. (how can I split them in this case?)

Can anyone tell me if my idea is correct?

Or.. I could make the whole part by creating two seperate tunnels that intersect with each other, and trim them.
If I choose this route, what feature would you recommend to use? What I do not like with this, is that I can only use max 3 guides, I would require a bit more, since further down the path, the shape will start to change dynamically, I want the surface to follow as close as possible.
Thank you

 

[FrankSwinks]

To change a spline from degree 1 to a degree 3 spline I suggest you try the following.

For a given spline
1) Create a point set of type = Spline Points and subtype = Poles
2) Create a fit curve using
type = Fit Spline
Select the point set created
Parameterization Method = Degree and segments Degree = 3
set to closed
segments start at a low number and increase it till you get the level of fit that is acceptable.

At some point you would to remove parameters for the fit spline to be able to further edit the spline.

Frank Swinkels

 

[JeniaL]

will be better if based on existing sections you build your own sections and guides. IMHO this can take less time than fixing and playing
with original scan