Fixed and Touch/Align Constraints

[Recon1775]

Is there a way to have a Touch constraint and a Fixed constraint exist on a component without it considering it an error?

Technically a perfectly aligned hole between two components is not invalid if a fix constraint is applied.
I understand that it's basically telling me it's over constrained but that's actually what I want so that if the hole moves I get a flagged error saying the constraints are no longer possible or valid.

(Using NX 9)

 

[JohnRBaker]

To start with a 'Fixed' constraint is with respect to the Earth or Ground. It has nothing to do with relationships between parts. Generally speaking, you align holes by using an 'Align' constraint, selecting the center-lines of the two holes. Alternatively, if you're talking about two parts with holes which are placed face-to-face you could use 'Concentric' and select the circular edges of the two holes.

Now if you're talking about the 'Bond' constraint but said 'Fixed' by mistake, 'Bond' constraints cannot be used with any other constraint since it forms a rigid (immovable) relationship between parts even if they do not actually touch each other.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Digital Factory
Cypress, CA
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To an Engineer, the glass is twice as big as it needs to be.

 

[Recon1775]

When I say fixed I'm saying either using the Fixed constraint or even the Bond constraint. Either one might work.

When I'm building an assembly I want to assembly items using the provided bolt hole features but if another designer in the assembly structure changes a component that drives the placement of the mating component I wanted to have a CAD style flag to tell me that interface has changed.
I've used this style of configuration management on another CAD systems and the way it worked, was as long as the constraints were valid you could pile as many as you'd want into the mix. So if the holes are aligned the component can also be fixed at that location, both of those constraints are possible.
Now if a hole moves the holes are no longer able to be aligned because the Fix is holding the item stationary causing the hole alignment constraint to fail.

Is there a way to accomplish this particular style of "assembly monitoring?"
I've also used measurements in the other systems and output the values of those, say into a spreadsheet, and when a value was no longer at a default or zero value I knew something had changed and I wasn't notified by the individual.
That measurement setup takes a lot of time and maintenance which is why I liked using constraints to flag failures when things changed.

Thanks!

 

[JohnRBaker]

You have to understand that 'Bond' and 'Fixed' constraints are totally different and generally are not seen as simply a different way of doing the same thing. As I explained before, a 'Bond' constraint is between two or more components and what it does is 'fix' the relationship between the referenced Components but not relative to the space in which these Components are found. There is nothing preventing you from moving, as if they a single Component, even if they didn't physically touch each other, any one of the Components and the others will move as well.

As for the 'Fixed' constraint, it's generally considered a best practice to apply a 'Fixed' constraint (which prevents it from moving relative to the Earth or Ground) to at least one Component of an Assembly, generally the largest or at least the most significant in terms of how other Components may relate to it. Once 'Fixed', you can then proceed to add additional Components defining the desired positional relationships (i.e. constraints) as you go along keeping in mind that in the vast majority of the cases, you only really need to 'Fix' a single Component. Note that there is no problem if you apply a 'Fixed' constraint to more than one Component or if you decide later on to change which Component is 'Fixed'. Just keep in mind that all 'Fixed' Components are being held in a constant position relative to the SAME Earth or Ground.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Digital Factory
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[Recon1775]

I completely understand what you're saying.
With what I'm wanting to do, if you fix a component(to ground) and the component holes are physically in alignment to some other hole technically you could add an Align constraint and still be valid since the holes can align and the fix can still hold that component in that grounded location.

The problem though is NX sees the use of a fix constraint and another type of constraint as an error to the fix constraint.
Even if the align is perfectly in alignment via the constraint it flags that the component as fixed and cannot use the align without flagging an error within the constraints tab.

I'm not saying the fix or the align are doing anything they aren't suppose to do. I'm trying to figure out a way to do assembly interface monitoring as I've done in other CAD systems even if it means doing it another way in NX.
The way I used to do it was with constraints, but with NX the issue is it sees using a FIX and an ALIGN or TOUCH on the same assembled component as an error even if while fixed in that grounded location the holes are perfectly aligned.

 

[JohnRBaker]

This should only be an issue if BOTH Components were already considered to be fully constrained BEFORE the attempt was made to align the holes.

In order for a constraint to be considered valid, it must contribute something NEW to the spatial relationship between two components. In my above comment about two components already being considered fully constrained, the alignment of holes are contributing nothing new to the scheme of things and is therefore considered redundant and thus manifests itself by flagging one or more other constraints as not being valid. Perhaps we could do a better job in informing people as to why something like this occurred but that will be difficult since the constraint solver does what it does by solving simultaneous equations. This means that irrespective of the order in which constraints are applied, the solver will try and accomplish the same thing, create a network of constraints that either fully constrains the Components, or at least partially constrains them without creating any invalid or conflicting relationships. Now while it is true that NX will remember the order in which Components are added to an Assembly this in no way has any effect oh how the constraint solver goes about determining whether the constraints are valid or not.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Digital Factory
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[Recon1775]

Ok that's good to know. So there's no way to override and over-constrain a component in NX even if the constraints are technically possible as in aligned and mated perfectly.

For the hard question, given I can't really use the constraints as an interface checker since I can't apply more than what is necessary, is there something else others might have used to accomplish the task of catching changes in an assembly specifically in this case of interfaces between connecting components?

Thanks!

 

[JohnRBaker]

The only approach that I can think of, that would not involve developing custom applications using one or more of the NX Open API tools, would be to create a series of 'Requirements', which tracked the status of various 'Measurement' expressions created to show whether certain relationships have been changed, which could then be verified using either the HD3D suite of visual reporting tools or Check-Mate, that could generate reports about the status of the various 'Requirements' defined in your model/assembly.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Digital Factory
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[Recon1775]

Ok thank you! I'll look into that and see how it works out. Thanks again!