Constraints or Fix only - Poll

[Deltech]

Hi - I am working for a Company who insist that all our assemblies (products) are constrained using only 1 fix & the rest of the parts constrained using other constraints (coincidence, surface contact, offset etc). There are a number of my colleagues (& myself) who have worked at other places where the method is to fix everything before production release. I would appreciate if I could get some feedback off you all telling what you do in your companies/industries

 

[jackk]

You've learned a bad habit, Deltech.

The Assembly workbench was intended to be used by fixing the main part, and constraining the rest of the parts to it. The benefit of doing this is when parts change, the assembly constraints will adjust the assembly to correspond to the modified geometry. The downside to this method is performance (contantly updating all the constraints) and broken constraints. Publishing the constrained geometry helps avoid the broken constraints.

In my opinion, the benefit of faster opening/update and few broken constraints is negated by having to manually adjust the assembly with every modification.

Unless you and your colleagues never had to modify a design at your former places of work.

 

[fighterpilot]

I agree w/ jackk. Fix the first one and then use constraints. Benefits far outweigh the negatives.

I've often wondered why people won't let the software do what it's designed to do.

--
Fighter Pilot
Manufacturing Engineer

 

[DBezaire]

Jack has it 100%


Regards,
Derek

 

[catiajim]

Constraints do not work well with VPM or LCA. Typically, we use them to locate the parts, then delete them.

 

[fighterpilot]

catiajim,

That sounds like a failure of the PDM system. Other CAD systems don't seem to have issues. In general, I think constraints are the way to go.

--
Fighter Pilot
Manufacturing Engineer

 

[jackk]

Question for CatiaJim:

If you're only using the assembly constraints temporarily, why not use SNAP instead? Seems like it would be less steps, and I think it's just as accurate.

 

[PeterGuy]

Snap can be a bit troublesome if you have to align more than one feature. DMU cumulative snap does not have the same issue, but this is under a DMU license so may not be available.
VPM and LCA are fundamentally different to systems (item driven) such as SmarTeam (document driven), as they don't store CATProduct documents unless they are specifically told to do so (Black box mode). They operate in structure exposed mode where product structure comes from the item structure stored in the system and the parts are located according to their instance positional matrix. This has a number of advantages particulary in large products such as planes and cars, such as zonal queries and product structure created dynamically. These advantages out weigh the disadvantges created by not using constraints. Which are supported but users don't understand how the context of a constraint is important in VPM and end up creating constraints in the wrong place, which results in very very long queries.

 

[catiajim]

We do use the Multi-Snap from DMU more often than we use Constraints.

The biggest problem with Constraints in VPM and/or LCA is that DS never supported Flexible Sub-Assemblies in the database. Hopefully, that is fixed in V6.

There are also performance problems when the Constraint in the Assemly tries to look for parts that are no longer there (because they were revised, and the constraint is looking for the older revision).

 

[Deltech]

Thanks to all for your input.
When working at the concept/ scheme stage of a design I use the constraints as intended (1 fix only) and this is when they are most useful as you may be changing parameters, material thicknesses etc - but when it comes to releasing the product I have been removing all constraints & then fixing everything. This is the only sure way of someone being able to open the product further down the line & seeing it presented in the way you intended. This was the methodology used in Airbus UK when i was there.

 

[jackk]

That seems to be a good compromise.

Where are you working now? (or what industry?)

 

[tooldeziner]

jackk said "Publishing the constrained geometry helps avoid the broken constraints." Perhaps this should be another thread, but could jackk or anyone else elaborate on that? Mainly, how does one publish the constrained geometry? I use constraints always, never snap. However, many times the constraints get broken and sometimes I don't know why. It just happens...
BTW, many of my colleagues who, like myself, have been using V5 since first release, "learned" way back then that constraints are to be avoided. Having used constraints in UG for decades, I was very happy when I saw that Catia had included those functions in their package.
Thanks for your input.

Peace Through Superior Firepower

 

[DBezaire]

Tool Deziner -
Tools --> Publications to publish the geometry. If you right click over a revolved face/solid you can choose face or axis. Inside the Publications -- Options tab check on Enable to publish the features of a body to publish out individual solid features like Patterns.

Regards,
Derek

 

[jackk]

Version 5 has improved alot since it first came out. It's now up to release 20 which operates alot better than the early releases did. I think the Publication feature was added around Release 10.

Derek told you how to Publish the geometry you intend to use for assembly constraints. When you create new constraints (or reconnect existing ones), make sure you select the publication from the bottom branch in the tree.

When the instance components are renamed or relocated, publications will help keep the effected constraints from becoming broken.

Also, try to modify constrained geometry instead of deleting nad recreating it.

Publication will not eliminate all broken links, but it will help.