[ART] Model acts up

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easiest way (What I always do) just combine and separate. You could alsoset the pivots to 0,0,0 but I find combine > separate is easier/quicker

Jup, it's quick and easy but take care, those two functions cause a helluva mess in the outliner, cluttering your scene hierarchy with groups and transform nodes and whatnot. Make sure to delete your objects history and clean up the outliner afterwards for a clean and bug-free scene!
Another quick way to move the pivot around is to just hit the "insert" key while in the move tool to switch to pivot-mode. You can then snap the pivot to the center of the grid by holding X for grid snap.