3D sketch points 'flip' 180¦ randomly

[Simon205]

Hi,

I have a series of 3D sketch points (3DSketch4 in attached model), each of which is constrained by two 'on plane' constraints, and one 'on surface' constraint. The surface is an extruded ellipse (nearly cylindrical), which means the 3d points could mathematically be in one of two points.

Problem is, when scrolling through configurations where other geometry changes, the 3d points 'flip' 180° to the other side of the cylinder, causing the 'web' features in the model to fail.

If anyone cares to open the attached file, configurations 03 and 08 are correct, all the others show problems!

Any help appreciated

Simon

 

[SnowCrash]

"Future version", so I can't open and help, but I think you need to add another relationship since the math does provide two solutions to the existing restraints. Could you split the surface and confine the points to one side of the split?

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[JohnRBaker]

Many systems which depend on the solving of simultaneous equations suffer from something known as 'chirality'. And while there are ways to make code less susceptible to this behavior, it's very difficult to avoid it 100%.

Below is a link to some more information about what and why it is what it is, and while it's not really anything which will help the 'victims' of this (i.e the CAD user), at least you'll know that it's a real issue and not something caused by cosmic rays or the fact that you your company refused to hire a 'feng shui' consultant when they last remodeled your office, although on second thought, particularly after reading the various articles on 'chirality' (like the one below), one comes to believe that perhaps applying a bit of 'feng shui' rules to software design just might help ;-)

http://en.wikipedia.org/wiki/Chirality_(mathematics)

Good luck...

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
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UG/NX Museum:

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

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[Simon205]

Thanks for the replies guys (and the background, John).

Snowcrash, yes, "..confine the points to one side of the split" is exactly what I need to do. But there's no 'stay on this side of the right plane' constraint!

Yep, it's a 2012 version, and as you all know - I can't save backwards.

 

[Updraft]

Can't you add another constraint, such as a dimension or even Fix to keep the points in place?

- - -Updraft

 

[mjcole]

John,

Thanks for the link
I had to scroll up for a moment because I thought I might have wandered from the SolidWorks forum. Did you read the post about Negative dimensions on SolidWorks? It referred to a method I used on UGv18 when 0 value dimensions weren't allowed

SolidWorks currently has problems with their swap direction on dimensions in that if you swap direction it shows a temporary minus value but then shows a positive value in a negative direction. If changed to zero and then given a value it swaps directions.

Does NX now support Negative Dims?

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[JohnRBaker]

Without intending to hijack this thread, YES, in an NX sketch you can enter nagative values for dimensions which constrain the position of a sketch object. However, you cannot enter a negative value for dimensions which constrain the size of an object.

For example, if I have a pair of parallel lines, the dimension constraining the distance between the two lines can be positive or negative, however the dimension constraining the overall length of either of the lines can only be positive.

Is that what you were asking about?

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.

 

[kellnerp]

You might turn on the option that shows the two sides of a plane using different colors. See if the planes flip between configurations. That might provide a clue.

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[Simon205]

Thanks for the replies on this.

If it helps anyone; in the end I conceded defeat, extruded all the ribs vertically upwards, then surface cut them with a revolved conical sheet as the trim plane. Not quite what I needed, but good enough for now.