Symmetric Spline problem
Symmetric Spline problem
(OP)
Hello everyone,
After creating a spline I want to mirror it at both endpoints. In the spline dialog there is a continuity type called "Symmetric Modeling". I can apply this successfully to one endpoint, but when applying this continuity to the other endpoint as well, NX gives the message "Cannot compute bcurve". The attached image shows how the spline is mirrored at both endpoints using symmetric continuity at one endpoint and tangent at the other. As you can see, the spline is not continuous where the tangent continuity is applied.
What can be done to achieve perfect symmetry with splines at both endpoints?
After creating a spline I want to mirror it at both endpoints. In the spline dialog there is a continuity type called "Symmetric Modeling". I can apply this successfully to one endpoint, but when applying this continuity to the other endpoint as well, NX gives the message "Cannot compute bcurve". The attached image shows how the spline is mirrored at both endpoints using symmetric continuity at one endpoint and tangent at the other. As you can see, the spline is not continuous where the tangent continuity is applied.
What can be done to achieve perfect symmetry with splines at both endpoints?
NX8.5 Win7SP1 64bit i7-3770K@4.3Ghz 16GB Quadro2000





RE: Symmetric Spline problem
RE: Symmetric Spline problem
NX8.5 Win7SP1 64bit i7-3770K@4.3Ghz 16GB Quadro2000
RE: Symmetric Spline problem
How many control points do the Spline has? If you wish to have perfect G3 (G2 continuity is already there on both planes as i can see from the the image) then you must have at least 7 control points(or may be more).Actually once the G2 continuity is achieved then G3 can be achieved using the middle pole.
Or the best you can do is....when you achieve G3 at one end then use X-FORM to fiddle with the middle control point (while keeping continuity check (G2/G3 ) through BOUNDARY CONSTRAINT.This way you wont loose your contiuity at the opposite end.
Best Regards
Kapil
RE: Symmetric Spline problem
NX8.5 Win7SP1 64bit i7-3770K@4.3Ghz 16GB Quadro2000
RE: Symmetric Spline problem
RE: Symmetric Spline problem
NX8.5 Win7SP1 64bit i7-3770K@4.3Ghz 16GB Quadro2000
RE: Symmetric Spline problem
If so, - what curvature does a line have ?
Zero. And that is what i see in the image. Since a line has zero curvature, the spline matches that and the end of the spline has therefore zero curvature.
But, if you mirror that over YC in the image, the result is actually G2, - Curvature continuous Since the G2 definition only describes that the curvature on each side of the point is equal.
What you desire is that the rate of curvature change is zero at the point. - No acceleration/deceleration of curvature in/at the endpoint(s). - which is what the Symmetry condition defines.
To accomplish that I think you need at least 8 poles. ( it takes 2 poles at each end to match G1, 3 poles at each end to match G2, 4 poles at each end to match G3. Symmetry should be a special case of G3.)
Now note that "symmetry" isn't a guarantee for a good looking curve... It's very difficult to handle the shape of the spline if the 4 poles at each end are controlled by the system, -You have very little freedom to manipulate the shape left. Also 8 poles does allow quite a lot of undulation in total.
I have attached an example. Edit the spline and play around with the poles to see what i mean.
Btw, the way i set the symmetry is to select, in the dialog not in the graphics, the first or the last point, then specify symmetry + the direction. ( again dialog)
Regards,
Tomas
RE: Symmetric Spline problem
However, in your example, you use a degree 5 spline with multiple segments (there are 2 knots visible). After playing around I've discovered that checking the box "single segment" in the spline dialog increases the degree to 7, and that makes it easier to control the shape of the spline because the poles seem to be more evenly spread apart. Another advantage by making the spline a single segment is that when creating a surface based on that spline (eg swept using the spline as guide and an arc as section), that surface has fewer poles, seams and patches than a surface based on a multiple-segment spline, thus creating a cleaner surface.
NX8.5 Win7SP1 64bit i7-3770K@4.3Ghz 16GB Quadro2000
RE: Symmetric Spline problem
Regards,
Tomas
RE: Symmetric Spline problem
Regards,
Tomas
RE: Symmetric Spline problem
The way I would do it is first sketch a rectangle, then create a single segment 7 degree/8 pole spline with symmetry modeling continuity associated to the mid points of the rectangle, then mirroring the spline to create 4. The advantage with this method is that although the initial "setup" is more time consuming with adjusting the poles, when you've reached a satisfactory spline, later you can simply change the size of the rectangle, and the 4 spline curves will update and still retain the original curvature, including perfect symmetric curvature.
NX8.5 Win7SP1 64bit i7-3770K@4.3Ghz 16GB Quadro2000
RE: Symmetric Spline problem
G0 (position) locks in the end pole; G1 (tangency) locks the next pole inward; G2 (curvature) locks the 3rd pole from the end; G3 (acceleration) locks the 4th pole from the end of the spline.
I'm not trying to imply that a parametric approach isn't correct, as it's mainly a preference from one designer to the next. However, it's usually easier to deal with splines by leaving them just associative and not trying to completely lock it down at each and every pole. The position, slope and magnitude will affect the majority of the spline's shape with these 5 and 7 degree splines. Eyeballing will suffice in most instances unless we're worrying about aerodynamics or matching existing surfaces.
Again, not trying to imply you're right or wrong, just pointing out some small things that are typical to spline behavior.
Hope some of this is helpful to you.
Tim Flater
NX Designer
NX 7.5.4.4 MP8
WinXP Pro x64 SP2
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB
RE: Symmetric Spline problem
Tim Flater
NX Designer
NX 7.5.4.4 MP8
WinXP Pro x64 SP2
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB
RE: Symmetric Spline problem
NX8.5 Win7SP1 64bit i7-3770K@4.3Ghz 16GB Quadro2000
RE: Symmetric Spline problem
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:
To an Engineer, the glass is twice as big as it needs to be.
RE: Symmetric Spline problem
NX8.5 Win7SP1 64bit i7-3770K@4.3Ghz 16GB Quadro2000
RE: Symmetric Spline problem
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:
To an Engineer, the glass is twice as big as it needs to be.