Also, assuming I can get it to work, is it possible to add a second constraint to the new welded body? Basically bring back the original constraint that is deleted once the bodies become welded.
Any thoughts?
I have selected two physics bodies in the PHAT editor, but the Weld option is grayed out. It works for some of the bodies, but for the specific combination I need it isn’t working. Any ideas?
Bump. Same here - 18.4.3. Nothing in the docs (which is a massive 2 sentences!) - what stops some bodies welding and not others???
6 years later - same problem in 5.3 
Anyone have any ideas why some bodies can’t be welded? Thanks!