that's exactly what made me start a thread in the first place: if I could do this with 2D bonechains, I would. But HOW do you keep the first bone restricted on just Y-axis rotation and the next three restricted on just Z-axis? Can you give me that scene as an example perhaps? It may very well be a simple thing, but I'm just not seeing it - for five days now. It drives me crazy!
There are two chains in this example.
First chain is made of 1 bone (vertical) which rotates the base. This chain has an up-vector (orange null).
Second chain is made of 3 bones (horizontal).
The root of this chain is child of first chain end effector. So it always rotates with the base.
The end effector of this chain is position constrained to orange null.
There were many threads on xsi base about mechanic rigging, with scene files and examples.
I'm not sure if this is unbreakable rig, but at least it might help you.
robot turret rig.rar [87.42 KiB]
Downloaded 76 times