Swept sections

[bb1982]

Good morning everybody,

I'm struggling trying to avoid that one section curve of a swept feature get reversed after an update of the model (e.g. new set of parameters). Do you know any trick or workaround that would solve this problem?
I'm using UG NX 7.5

Best regards
Bruno

 

[JohnRBaker]

Without seeing your model there is very little that we can say about a possible solution let alone commenting on what the problem might be.

One question, is the section being swept a Sketch?

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

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To an Engineer, the glass is twice as big as it needs to be.

 

[bb1982]

Hello John,

Thanks for your reply.
I uploaded an example of what happens: sections to be swept are not in a sketch but come from a "trim curve" feature.

 

[JohnRBaker]

Can you upload the part file itself?

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

UG/NX Museum:

http://www.plmworld.org/p/cm/ld/fid=209

To an Engineer, the glass is twice as big as it needs to be.

 

[bb1982]

Hello John,

unfortunately I cannot upload the entire part. I can try to extract only those features involved in the problem.
In any case, thank you for your interest!

 

[bb1982]

Ok, I have just uploaded the .prt file which has the issue together with two expression files.
Part file comes with the "39.exp" parameters' list already built-in, and if you feed the model with the "40.exp" you will see where and how the problem occurs.

Many thanks for your collaboration!

 

[MickyV007]

If you reverse the vertical direction on swept 122 it goes well, but if you feed the exp39 file back again you need to reverse this direction again. My first guess is that the expression files have one direction opposite to the other so try change (reverse to positive or negative) the x or y or z value (you should search which one is the correct one)

Best regards,

Michaël.

NX7.5.4.4 + TC Unified 8.3
Win 7 64 bit (Intel(R) Xeon(R) CPU X5650 @2.67GHz)
24.0 GB
NVIDIA Quadro 4000 + NVIDIA Tesla C2050

 

[bb1982]

Hello Michaël,

first, thanks for having given a try to my model. Then, It has to be said that the curve that flips is always the section no2 (the no1 is ok, it is important for the face normal) and that I don't specify any direction in the expression file.
The string that is used for the feature is a Trimmed Curve out from a Joined Curve, and the direction of the joined curve is right.

Regards
Bruno

 

[JohnRBaker]

This sounds like a chirality problem which happens in parametric systems when you try to make changes to a bunch of simultaneous equations all at once, or when making a big change to only one or two. For more information about this, go to:

http://en.wikipedia.org/wiki/Chirality

I just want to warn you that you won't learn anything here that will help much except to know that this sort of issue is real and while it talks alot about mirrored objects the issue is really tied to trying to change one shape into another all it one big hit. In the case of NX (and other CAD systems) which use sketches that are controlled by the solving of simultaneous equations, the issue of something being 'chiral' is fairly common and while we've attempted to limit it's effect, there is only so much that can be done to prevent it altogether (in fact, it generally accepted as impossible to create a system like this which is 100% chirality proof).

That being said, you may wish to contact GTAC and have them look at this problem since the developers may be able to detect exactly where the issue is and how you might go about getting the desired shape while avoiding this situation. If you were only changing a few expression values it might easier to recommend what to do, but with this many changes it's beyond the simple suggestions which have worked for me and others when we've encountered this sort of behavior.



John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

UG/NX Museum:

http://www.plmworld.org/p/cm/ld/fid=209

To an Engineer, the glass is twice as big as it needs to be.

 

[bb1982]

Hello John

Thanks for the very accurate technical explaination of what could be the root cause of my problem.
For the time being, I will live with it

Goodbye and have a nice day!