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overconstraining in sketch-II

Since the previous thread, I often ran into troubles with constraining sketch. Very often quite some time was spent on adjusting here and there without definite direction, and suddenly the issue was gone. This made me trapped in anxiety everytime creating a sketch with splines to be constrained.
Below is an example, in which I was told "Sketch nees 0 constraints" and in fact the angle (34.2°) is free. If the angle is fixed, the sketch is onverconstrained.
I found how to fix it after trial and error ---- simply moving the driving dimension "b=30.0" to other objects, e.g. the short line in the middle. But I don't know why it works. That means, I don't know whether I can have good luck to fix similar issue next time.
Hope to have your suggestions. Thanks!

Re: overconstraining in sketch-II

Valued Contributor
Valued Contributor

I have similar issues in sketching.

No real solution, that I found, other than trial and error.

IMHO sketching is worse in NX11 than it was in NX8.5.3, a definite step backwards.


Re: overconstraining in sketch-II

Siemens Phenom Siemens Phenom
Siemens Phenom

Hi Surfactant,


Sketch Constraints and Dimensions form the behavior and rule book of the sketch/cross section of the geometry you are forming and want a specific associative and often dimensional and constraint driven behavior of. There are some approaches to creating a Sketch.  You can either do it fully by your own rule book, semi system aided by the system or fully aided by the system. However since there are in most cases several solutions you need to tell the system the behavior you require – design intent. If you are a novice you might want to use the fully system approach – however since you are a programmer you understand that it’s like driving in traffic without rules – in most cases it just works but sometimes there are accidents where nobody knows who’s did right and who did wrong.


Without the design intent it is impossible to guide you correctly here. The lockup you are experiencing are due to over constraining the sketch – as you’ve seen. There are several solutions to your issue and they are presented to you by the system as it tells you that the sketch is over constrained when activating the angular dimension. The conflicting constraints are highlighted in red. You have to deal with them individually and decide which one is not acc to your design intent and delete it or modify it as some constraints control more or less depending on type of constraint.


You can use the Sketch Relations Browser to study your Constraints in order to deeper understand what relates to what.


You can also after the sketch have locked up (here change your dimension from Reference to Driving) turn on the “Continuous Auto Dimension” and delete one of the over constrained Constraints and Dimensions and study how the system now automatically constraints the sketch to keep it Fully Constrained.


You should also be aware of that Splines also have tacit “constraints” to it such as e.g number of poles required to fulfil C2 etc…. Sometimes it might make things difficult to grasp when using splines in a Sketch.


Have a look to the movie as there might be something in there that will guide you to optimize your sketch design intent and behavior. It would also be beneficial to you maybe to contact your Siemens contact as you are warming up and now working with complex stuff also given your previous posts, to set up maybe an onsite specific assistance to guide you through the optimal way of doing things.


I hope this helps


Best Regards


(view in My Videos)

Re: overconstraining in sketch-II

Thanks a lot for the very informative reply! The tricks for checking constraints are very helpful!
I must say that you are not only a Sandman (Smiley Happy) but also a superman in identifying the perpendicular constraint as the root cause of overconstraining.
Before the original post, I checked the sketch again and again by simplification at various sketching stages. As you can see in the simplified version below, either with the perpendicular constraint or not, the sketch is fully constrained. Here I thought everything is ok and went ahead confidently. But overconstrained soon.
Thanks again!
Image.png   Image.png