Can't fully define a spline

[karlheinzstoeber]

The upper spline in the image below is not yet fully defined (blue). The remaining degree of freedom is the angle of the marked spline handle. When I try to dimension this angle, the message appears that the sketch is over-constraint. So, I can only specify a "Driven" dimension.

Even if I choose the “Driven” dimension, the angle remains changeable by turning the spline handle.

Does anyone have any idea why this is?

Here is the link to the project: https://a360.co/3KDkbBO

 

[johnsonshiue · Accepted solution]

Hi! I believe there is indeed a bug or a limitation here. You may assign horizontal constraint to the handle. This means, the rotational DOF is indeed recognized. However, there could be other DOFs preventing the angular dimension from being added. If you add a Fix constraint to the spline point (the handle center), the angular dimension can be applied also.

Many thanks!

[karlheinzstoeber]

Dear Jonson,

thank you for analyzing my problem. I think like you, that it is a possible bug, which needs to be regarded for future versions. I tried the fixed constraint and the spline changes to fully defined (black), but if I try to assign the angle, only a driven dimension is possible. 

Best Karlheinz

[g-andresen]

Hi,

It may be that there is a bug.
Nevertheless, I would like to point out that when several elements coincide in one point, an unintentional assignment of constraints can occur.

günther

[karlheinzstoeber]

And what is the difference between "unintentional" and "bug" in computer since?

Best Karlheinz

[g-andresen]

Hi,

1. If you assign a dimension to a construction line, you must still ensure that this line is coincident with its end points to neighbouring elements.
If other lines are then added, their relationships to all other participants must also be defined.
In the process, you can sometimes lose track of everything.

2. Even though it is always pointed out to determine sketches completely, in my opinion it does not make sense to strive for a complete determination with splines (especially with fit point splines = 4 dimensions).
With control point splines this is more successful because only 2 measurements are required for each C-point.

günther

[karlheinzstoeber]

Thanks Günther,

1. I know what you mean. I normally count the number of coincident symbols to avoid this situation.

2. From my experience, it is very helpful to fully define the spline in my case or lock the spline. The sketch displayed in the picture is one of the two reference sketches and any further object depends on this.

Regards Karlheinz

[g-andresen]

Hi,

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@karlheinzstoeber  schrieb:

2. From my experience, it is very helpful to fully define the spline in my case or lock the spline.

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1. That would be my solution if I want to make sure that the shape cannot be changed.
2. If you want to achieve fully defined sketches without fixing, you are better off using control point splines.

günther

[g-andresen]

Hi,

In my previous posts, I forgot to make an important note.
The 3 splines in the sketch have an optimal shape because they were perfectly traced on the canvas with a minimum of control points. Therefore, I would fix this condition to protect their shape against unintentional changes.

günther

[TrippyLighting]

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@g-andresen wrote:

..

2. If you want to achieve fully defined sketches without fixing, you are better off using control point splines.

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Indeed! I would argue that a single span control point spline is the proper spline for these curves.

[karlheinzstoeber]

Hello

in fact, there is more than one way to Rome. Each has its advantages and disadvantages.

I work with both, fit point splines and control point splines. In the case mentioned in this thread, I’ve been used a fit point spline.  The reason is that I was able to set the conditions at the nose (vertical), at the turning points above and below (horizontal) and at the transition to the tail tube (horizontal) very easy. The upper approximation consists of two splines because the contour is unsteady. Two three-point and one two-point spline are used.

Nevertheless, when I was still working as a software developer, I was always grateful when others told me about a problem in my software. Some rare events cannot be found even with the best test methods. No more and no less I intended with the post.

Since there are two specialists in this thread now, below is a simplified representation of the transition from the fuselage to the tail of the model. I did not succeed in creating a loft from the two profiles and using the edges of the upper profile as rails. To avoid the loft error message, I put two small lines at the tops. But the body did't really look like as expected.

 

Regards Karlheinz