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sketch group constraint issue

Genius
Genius
As shown below, three connected lines (colinear) are packed into a sketch group with "Scalable Group constraint".
 
It can be easily deduced that the sketch already is fully constrained. But the message shows "Sketch needs 1 constraints".
 
Since I depend on the software message to evaluate my sophisticated sketch, I hope to get it correct. 
 
Hope to have your suggestion. Thanks!
 
Image.png
Image.png
 
12 REPLIES

Re: sketch group constraint issue

Solution Partner Genius Solution Partner Genius
Solution Partner Genius

Hi @surfactant

 

Your group contains three lines which are grouped using scalable group contraint. You need to specify atleast one dimension for them to make the sketch fully contrained. All three will increase at the same scale.

Attached is the file with the dimension.

May I ask why are you using Scalable Group for sketches?

 

Best Regards,

Paras Raina
Sr. Application Engineer | Solid Edge ST10 | NX 11
MSC Systems Pvt. Ltd. (India)

Re: sketch group constraint issue

Genius
Genius

Hi @Paras_R

 

Thanks for your solution! It does make the "Sketch needs 1 constraints" message gone, but does not work for my case.

 

I use Scalabe Group only to make the three connected lines able to be scaled at the same factor. I would need to constrain the end point (currently with Fixed constraint) to other element of my part. So when the point moves, the three connected lines would change their length at the same factor. If you dimention one of the three lines, the end point would fail to adjust.

 

ScreenClip.png

Re: sketch group constraint issue

Solution Partner Genius Solution Partner Genius
Solution Partner Genius

@surfactant

You need atleast one free end (unconstrained) for the scalable function to work. Try changing the dimension in my solved part file, and see for yourself if that does the change you want.

It WONT because you have fixed both ends (one is fixed using fixed constraint & other one vertically aligned with the origin). Delete one of those & the sketch will remain fully constrained and will do the changes as its supposed to do.

 

& In case you want both ends fixed on some curve/edge/part, why are you using scalable group (which brings me back to my first question; why exactly did you choose the Scalable group when you need both the ends fixed)?

Paras Raina
Sr. Application Engineer | Solid Edge ST10 | NX 11
MSC Systems Pvt. Ltd. (India)

Re: sketch group constraint issue

Genius
Genius

Hi @Paras_R

 

Please refer to the picture. I would constrain the end point to a previous object (here a block). Later the end point would need to adjust automatically with the block. If you fix the length of one of the lines, the sketch would fail upon modification to the block. Thanks!

 

ScreenClip [1].png

Re: sketch group constraint issue

Solution Partner Genius Solution Partner Genius
Solution Partner Genius

@surfactant

For this case, you would want to keep the sketch under-constrained as this will allow the geometry to adjust itself automatically with the changes.

This is why it shows 'Sketch needs 1 constraint' without a dimension as practically, it needs one dimension to control its length.

Paras Raina
Sr. Application Engineer | Solid Edge ST10 | NX 11
MSC Systems Pvt. Ltd. (India)

Re: sketch group constraint issue

Genius
Genius

Hi @Paras_R

 


@Paras_R wrote:

For this case, you would want to keep the sketch under-constrained as this will allow the geometry to adjust itself automatically with the changes.

This is why it shows 'Sketch needs 1 constraint' without a dimension as practically, it needs one dimension to control its length.


In fact before you add the additional dimension, the sketch is already fully constrained. The message

"Sketch needs 1 constraints" is only "fake news".
 
Thanks!

 

Re: sketch group constraint issue

Siemens Legend Siemens Legend
Siemens Legend

@surfactant

It is true that the curves are fully constrained within the group, however, once the group is deactivated, you now have one object which is totally free to move, so even though you fix one of the end points, the other end point (Overall length of the 3 lines put together) is undefined as far as the sketch is concerned.

 

So to do what you want to do you need to add a size constraint between the two open points. If you then set the value of that dimension to be equal to either:

  • The parameter you use to define the location of the block or
  • A measurement between the fixed point and the edge of the block

Then the size of the 3 lines will scale accordingly whenever you chage the location of the block.

Regards,
Abe

Re: sketch group constraint issue

Genius
Genius

Hi @Abeinjapan

 

Your solution works. Thanks! But the NX mechanism of handling sketch group constraint seems to be a little confusing, and very often try and error need be to done to get it correct.

 

... however, once the group is deactivated, you now have one object which is totally free to move, so even though you fix one of the end points, the other end point (Overall length of the 3 lines put together) is undefined as far as the sketch is concerned.

 


I can't understand the sentence in boldness. As shown below, "the other end point" (left) is in fact already fixed, not "undefined".

 

Image.png

 

Re: sketch group constraint issue

Solution Partner Genius Solution Partner Genius
Solution Partner Genius

@surfactant

What you are saying is indeed true. The sketch is fully contrained.

But one of the ends of the lines are 'free'. NX is preventing the change because of the scalable group. In reality, it does actually needs one distance value.

As @Abeinjapan pointed out, you can take the measurement value between one point and the edge of the block where you want its end to stick or be constrained to. This will also change the sketch to fully contrained.

 

Merry Christmas in advance!

Paras Raina
Sr. Application Engineer | Solid Edge ST10 | NX 11
MSC Systems Pvt. Ltd. (India)