Spline Frustrations

[Arbiter]

Hey Folks,
It's been a while but I am working on a wildfire 4.0 sketch and I am frustrated as all get up! I am drawing the two sides of a fuselage looking top down and I am generating a 5 CP spline on each side. 2 of the points are colinear with the nose and tail datum points and are made tangent to the station datum planes at the nose and tail. The remaining 3 points are dimensioned hard to the centerline and the forward datum plane. I am using wildfire 4.0 student. Here's the problem frustration:

If I create the second side using the same techniques and dimensions, the splines do not turn out the same way (See image). I want to maintain the same shape on both sides and I remember hearing you don't want to use the mirror tool when trying to do this if possible.... What am I doing wrong? Both sides use the same dimensions (I checked multiple times)

Please help a pro/E noob understand what's going on here!

Insanely Frustrated:
Chris Z.

The X-location of maximum thickness in the pictures below are different despite the fact that the controlling dimensions are the same....


Is there a good way to dimension splines properly that I am missing? Thanks for your help!
Edited by: Arbiter

 

[kenppy]

[No message]

 

[orac]

If you want to control the shape of a spline by using dimensions, you should also control the first derivatives, namely the angles besides the regular dimensions.


E.g. activate dimensioning, select a spline point, then select the spline curve below the point, to end select the vertical reference and press the middle mouse button to place the angular dimension.


Hope this helps.


Best regards,


John Bijnens
http://cad2cam.khlim.be
Edited by: orac

 

[kenppy]

It's almost impossible to successfully mirror a spline within a single sketch, I've never figured out why this is. Even if this works changing it later affects each side differently.
Just do one side and mirror the geomerty in the next step.

 

[Arbiter]

Hey Orac and Kenppy,
I will give both of your suggestions a shot! It's worth a try at least! So frustrating! Thanks for the advice though!

-Chris

 

[Zestje]

To me splines in ProE are a bad thing, because splines should be dimensionably in an easy and most of all reproducable way. In ProE when you switch between the 2 editing modes of splines (control points and the polygon) information of the other editing mode is lost....

For a good spline definition I need the control point method to set point locations and the polygon method to control the shape, which can't be combined...

If there is any way to use normal curves instead of splines I will always do that.

 

[orac]

Hi,


"splines should be dimensionably in an easy and most of all reproducable way".


But they are. As I have already written earlier in this discussion.


Please don't forget that a spline (e.g. the NURBS)is not an ordinary interpolation function, but it is a composed (consists of multiple polynomials) approximation function. Where I can't stress enough the function is not one function but a number of functions (polynomials) usually one pro segmenti.e. one for the space between two control points. There is an exception to this and that is a Bezier spline which is one big polynomial where the coefficients of this polynomials are again polynomials ( so called Bernstein polynomials). But the B

 

[Zestje]

@orac: Although I don't understand all of your mathematical explanations I'll try playing around to see if I can get a better understanding of splines in ProE. I must admit my impression of splines might be based on a partial misunderstanding about how ProE handles them...

I have worked with CorelDraw a long time ago and loved the way splines work there:
Using points you can dimension and drag handles to determine 'how hard you pull the wire', which could also be dimensioned.

 

[orac]

If you want a similar approach to splines as in CorelDraw you can create some datum points and then use a style function (ISDX) to create a spline through them.


Best regards,


John Bijnens
Edited by: orac