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Managing Assembly constraints

Valued Contributor
Valued Contributor

Are there any tips or recommendations for managing the assembly constraints?  I've been using NX for a few years now and it seems like the contraints are far more "fragile" than in other CAD softwares that I've worked with.  If I'm working in a medium size assembly with approx.  30 parts, I might be adding parts and they seem to be constrained OK.  Then I'll add another new part and the contraints get that red x by them.  

It's then difficult for me to see what constraints are interferring with each other, plus stumbling through that long list of all the constraints (and trying to see what part it's referring to) is quite cumbersome.  I've gone through a lot of the on-line training and have worked with assemblies quite extensivelly during these 2 years, but it still seems to be a lot more sensitive than what I'm used to.  Any tips would be helpful.  Thanks

16 REPLIES

Re: Managing Assembly constraints

Gears Phenom Gears Phenom
Gears Phenom

I find the constraints are very precise, sometimes more precise than the components you are assembling.  I don't mean that the data isn't precise, but if you have two faces aligned, and then try to constrain two more faces that look 90°, but one is really 89°, it throws a red X.  NX lacks the 'fuzzy logic' that many other types of software use, which makes them "easy" and "user friendly".

 

So....when I constrain my components, I think of it like I was taught geometric feature control.  Constrain the primary features, then secondary, and finally tertiary.  Usually before the 3rd, I'll apply, so if the 3rd fails, I can undo it, and interrogate as to why.

 

When using holes to align, and clock a component, rather than align two holes, align one, and use the angle constraint, with the "orient angle" subtype.  This gives it more freedom if the mating hole distances are off by a tiny amount.

-Dave
NX 11 | Teamcenter 11 | Windows 8.1

Re: Managing Assembly constraints

Valued Contributor
Valued Contributor

I'll try your recommendation.  Thanks.  At this point, I have all contraints OK, but then I try to add a distance contraint to an existing component.  The next thing I know, there are 17 contraints with a red X.  It doesn't seem to be easily apparent what the problem is.  Finding the two conflicting constraints that are causing the problem has been hugely time consuming.  

Re: Managing Assembly constraints

Gears Phenom Gears Phenom
Gears Phenom

I agree.  Debugging the assembly can be time consuming.

 

Probably preaching to the choir, but I'll usually go thru the assembly navigator, with the Dependencies window open and magnifying glass toggled on (Detailed View), and toggle constraints on/off, watching the behavior, seeing what makes it happy, and then hopefully narrow down the conflicting  constraints.

 

Multiple "fix" constraints can cause issue too.

-Dave
NX 11 | Teamcenter 11 | Windows 8.1

Re: Managing Assembly constraints

Siemens Phenom Siemens Phenom
Siemens Phenom

The other thing is to use the Constraint Navigator which has many options to filter what you are looking for. For example if you group by component status you can show inconsistently constrained and others. If you filter by status you get Error, Waring, Info and OK. 

 

Give it a try and see if it helps find the issues.

 

Scott

Re: Managing Assembly constraints

Siemens Phenom Siemens Phenom
Siemens Phenom

Let me give you two detailed tips on using the constraint navigator.

 

Use the creation date and the modification date columns
When you work on an assembly it can be useful to investigate how the thing was put together. Just like for features you can roll back in time and suppress all the constraints and then unsuppress them following the creation order.

For this first set the display the "Group by Constraints". Now sort on the "Creation Date" column (black triangle point up), the first constraint ever created is now on top. You are looking at the history of your assembly constraints. Now look where the first constraint with a problem is. Suppress all constraints after the first failure including that failed constraint. Unsuppress the constraint that failed previously, most of the time it will not have a problem. If it does have a problem fix it. Now unsuppress down the list, you will have to watch the graphics, to ensure nothing flips the wrong way. You will get the to root of the problem this way.

Another thing that happens is that you cause the problem yourself. You turned off constraint display and not you notice much later that you broke something. You can do the same thing as above by sorting on the "Modification Date". Now you will see your changes at the bottom and you can suppress your latest modifications and see where something went wrong.

 

Direction to Fixed
If there is a problem with a specific component then it can also be useful to sort the list by component. 

When you look at the direction to fixed column then you can identify which constraints position the component (green arrow to fixed) and which constraints position other components on this component (white arrow from fixed). It is usually a good idea to fix all the green ones. If there is a white one failing then jump to that component and fix the green ones there.

This method is not foolproof because there are sometimes multiple paths to the fixed component. If there is a long chain of components stacked on top of eachother, then this really helps.

 

Bonus tip
You can help yourself by creating a simple constraint system by trying to constrain each component to only one other component.

 

Enjoy using NX!

 

Regards, **bleep**

 

 

Re: Managing Assembly constraints

Valued Contributor
Valued Contributor

Thanks, everyone, for your insight and tips.  They have been very helpful.  Much appreciated!

Re: Managing Assembly constraints

Valued Contributor
Valued Contributor

I miss that when you move two or more components form one assembly to other the constraints between these components doesnt travel with the components.

Re: Managing Assembly constraints

Siemens Phenom Siemens Phenom
Siemens Phenom

You say that you "...miss that when you move two or more components from one assembly to another that the constraints between these components doesn't travel with the components".  Are you suggesting that NX used to work this way?

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA

Re: Managing Assembly constraints

Valued Contributor
Valued Contributor

No, I miss that because I was used to that behavior with other CAD softwares. Sorry about my english skills. hehe.