Constraining a Spline

[Wittenborn]

I have imported a spline (an airfoil), and want to constrain the entire spline to a point. The problem arises when I try to constrain the starting point of the spline to a datum or line. The single defining point moves, but the rest of the spline stays put, and consequently my airfoil is "modified" because the rest of the spline does not follow.

Is there a way to constrain a spline relative to its own defining points? I really don't want to constrain the splines individual points, as there are about 150 of them.

Anyone have suggestions?




Regards,
Grant Wittenborn
Aerospace Engineer

http://www.darcorp.com

 

[cowski]

What does it gain you to constrain a spline to a point? I've never tried it before so I'm not sure what advantage there would be in it.

 

[Wittenborn]

Well, here is what I am doing:

I import a spline (airfoil) into a sketch, and then draw a line from the trailing edge to the leading edge. This defines the chord lenght. I constrain the leading and traling edge of the airfoil spline to the line, so that I can move the line around, and the airfoil moves with it.

From here, I can rotate the line and the airfoil rotates with it, defining the incidence angle of the airfoil. Then I can fix the line in space, and then define planes (to create additional sketches on) to the line, making all other geometry associative.

Anyway, I think I found the solution. You have to insert the spline by poles, rather than "through points" to get the spline to constrain to the line.



Regards,
Grant Wittenborn
Aerospace Engineer

http://www.darcorp.com

 

[Dave K]

I generally use positional constraints on sketchs with splines, esp. when I don't want the spline to be modified.

-Dave

-Dave

http://www.moslerauto.com

 

[mjcole]

Wittenborn,

You could possibly use an angled Datum Plane through the Stacking Axis as your Sketch Horizontal reference then by modifying the angle of the plane you could constrain the twist angle of the section with the Angle of the Datum.

You could name the planes and give them Parameters that you could easily modify from Expressions. For your chord you can just make the line collinear with the angled sketch reference plane.

Then you can modify the angle and the spline should stay oriented the same relative to the H ref plane for each angle. Give this a try and let me know if it works or not.

Michael
~~

 

[Wittenborn]

Michael,

Thank you for your responese to this question, however I think you may have mis-understood my problem. (But I wouldn't put it past me to misunderstand your solution, either.) Let me define it a bit more clearly:

I have an Excel spreadsheet, with a list of about 100 coordinates (X,Y,Z) that define an airfoil. I copy these coordinates into a .dat file so I can import them as a spline through points, either in modeling, or in Sketcher.

I have no problem getting the spline imported, but now I need to constrain it to something, but I don't want to constrain all 100 points defining the spline.

For instance, I want to constrain the leading edge of my airfoil to the horizontal and vertical datum axis I have defined. When I set the constraint on the point, it moves the single point on the spline, but leaves all the rest of the spline where it was, thereby screwing up my spline.

Basically, I want to constrain the ENDPOINT of the spline itself, not the defining coordinate point.

So I guess the question is: Is there a way to select a spline, without selecting it by its defining points?



Regards,
Grant Wittenborn
Aerospace Engineer

http://www.darcorp.com

 

[Dave K]

Use Position.

What version of UG are you using? in v.18 and lower, this would be the middle button, under "Sketch Options". I don't have a machine with NX handy, to tell you where it is there.

Basically, Position will locate the sketch in space, and not the entities within the sketch(point, spline, etc.). You can select your leading edge point, and set it to a distance of 0 from your datum, and the entire sketch will move to this location, leaving the spline in its original shape.

-Dave

http://www.moslerauto.com

 

[Wittenborn]

Dave,

So if I understand you right, you are creating the sketch, and then in modeling, you are positioning the sketch?

Ideally, I want to position the spline in sketch to some datum axis that defines the sketch plane.


Regards,
Grant Wittenborn
Aerospace Engineer

http://www.darcorp.com

 

[Dave K]

This is still within the sketch dialog, but you don't use the typical dimensional or geometric constraints. "Position" works just like when you position a feature.

Actually, if you create the sketch, and then exit the sketcher, and go to Edit->Feature->Positioning (referencing v.18 dialogs, hopefully NX is the same or similar) you can position the sketch this way too.

-Dave

http://www.moslerauto.com

 

[mjcole]

Wittenborn,

I understand what you are asking so Using My suggestions and Dave's you should be able to get what you want.

I tried this out and can send you a sample file with a sweep feature. The second section I made, in no way resebles a correct airfoil but you would be able to see how using offset datums and the Sketch positioning, you can control your section orientations.

If you have a feature for your stacking axis and a plane for the 0 degreee horizontal plane you can create offset datums for each section to constrain position along the axis. for each section create a plane through the axis at an angle from the 0 degree plane start with value of zero. then choose the sketch plane as an offset plane normal to stack axis.
Move your WCS Origin to the sketch csys location and do the spline import and create your sketch spline through the points.

Then use the sketch positioning to place your section point from Stacking Axis. Not if you name the Dimension for the angle plane something like SEC_AA_ANG and positioning dims SEC_AA_X&Y it will be easy to make changes to the section orientation and still have your spline points in the same locations relative to one another.

Send me an email at mikecolecole@excite.com and I can email you back a V18 sample file.

Michael