Building new parts directly in the context of an assembly is a very powerful technique that has been available in SolidWorks for a long time. Watching such components update in sync as the user intended, just by modifying a few dimensions in the assembly, is magical. A properly built top-down assembly is a thing of beauty.
That being said, most users are apprehensive when doing in-context design, just because such a top-down assembly can be a minefield, waiting for the wrong step of an inexperienced user.
One of these very dangerous mines is the InPlace mate, which is used for locating parts created in the context of an assembly. The InPlace mate is a very useful tool but a very dangerous one. Some people believe that in order to move such a part, they need to delete or suppress the InPlace mate. Even the official Assembly Modeling manual published by SolidWorks teaches students to remove the InPlace mate when moving parts.
Do not do that! Never delete an InPlace Mate, if you do not want your assembly to disintegrate later on.
Fortunately, SolidWorks 2013 introduces new enhancements for the Envelopes functionality that can be used with great success in preserving the InPlace mates and, as a consequence, preserving your design intent when working with Top-Down assemblies.
As always, seeing is believing, so you better watch this video. As a bonus, I included my old techniques for dealing with this problem, techniques that can be also used in the previous releases of SolidWorks.
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