Well, lets say you've got a design where there are some variations and you'd rather just have two (or more) models instead of turning a bunch of features ON and OFF to represent the variations. What you could do is create the model up to the point where all the features are common to the variations. Then you do an Extract Body using the 'Fix at Current Timestamp' option so that you now have two identical models that from there you can add the unique features to either or both models (of course you'll probably have to do some selective Hiding and Showing to get access that only one model at a time but I'll explain how that can be easily done later on). Now when you've finished you'll have models which if you edit the features which were created before the Extract Body feature, it will cause BOTH models to update, whereas editing features created AFTER the Extract Body operation, they will only update their respective models.
Now you can get very creative if you wish, as I've already alluded to earlier, where you could have several different variations and the Extractions could have been performed at various points along the 'history' of the model so that you could have different levels of 'common features' for each variation. Now it's critical that you have to use the option where these extractions are defined as being 'Fixed at Current Timestamp'. Once you've got what you want you can assign each variations to its own Reference Set so that when this model is added to an Assembly you can pick and choose which variation that you want to use, and since ALL of these variations are collocated in the same place and orientation in the master model, even if the Components in the Assembly were not constrained, you could replace one variation with another and there would be no chance that they would move or be oriented differently.
Now this does bring up something that people may have been wondered about.
Have you ever looked at the Part Navigator and wonder why there's a 'Timestamp Order', or to be more exact, why would you want to toggle 'Timestamp Order' OFF? Well this is one of those situations where this is handy, when there are MORE THAN ONE model in the same part file, which would be the case if you were doing what I described above. When you're not in the 'Timestamp Order' mode you'll be able to see all the variations and by looking at how the features are related to which body you can get a better understanding of how all the variations were created. Also, while you've in this mode, it's very easy to toggle the display of the various 'models' ON and OFF (Show and Hide) which makes it very east to go back and forth to work on the variations, as shown below:
Anyway, I hope that helps to explain one the primary reasons why NX has this capability of Extracting Bodies yet leaving them collocated.
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
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To an Engineer, the glass is twice as big as it needs to be.