Merge vs Union

Hi, Could someone tell me what the difference between these two operations is – and where/when to use or not to use them?


Merge is for when you have two objects that you want to combine but do not really overlap, or when you want to keep the overlapping faces.

Also, union requires two closed objects to properly work.

OK, thanks Richard – I’ve used the ‘Merge’ and ‘Union’ from time to time, but not on closed objects so hadn’t appreciated the difference. A ‘follow-up’ question if I may………

My current model attempt has a generous helping of nuts and rivets ( via Decapod’s plug-in ) on some of the faces. This either means lots ( and lots and….) of objects in the hierarchy, or , merging the nuts and rivets either with each other or onto the face they’re occupying. Would merging these relatively small objects affect the way 3DC handles the ‘auto’ LOD during export?

Thanks again.

Hi John – quick reply on auto LOD’s

Yes – merging can have an effect.
auto lod uses the bounding box of the object – merging all rivets into a single object increases the object bounding box.
I would either keep the individual objects or just merge small groups of them.


Hi Paul – thanks for that, I thought that might be the case. As it happens, I’m remaking the parts previously merged anyway, so I’ll deal with them differently this time.


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