When animating a robot or other mechanical object, should I merge merge all objects into one, rig it with a skeleton and animate the skeleton. Or should I keep limbs separate, parent them to each other and animate them.

The robot will be very stiff. No bending limbs or cartoony squash and stretch. If I would use bones, each bone would have a very strict vertex group, exactly one limb of the robot. Limbs will rotate in sockets of their parents.

This is for creating animated movies, not for game animations.

Also, the robot has some symmetrical parts so it would be nice to copy and mirror keyframes between the sides.

I will probably want Inverse kinematics.

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    $\begingroup$ you can use both methods, either join all the objects into one, in that case you'll parent With Empty Groups, or keep them separate, in that case parent each object to each bone. I think the first method is more convenient, as it makes a single object, but I guess it depends on a lot of things $\endgroup$
    – moonboots
    Jun 12 '21 at 8:25

You definitely need a skeleton, maybe a Rigify rig, in order to have IK/FK switch, symmetrize pose, copy and mirror keyframes, and other valuable stuff.

The decision if to join into a single object or keep the parts separated is not crucial, but having a single object has no drawbacks and it's a cleaner solution: it's easier to manipulate and import/export into different files and/or applications.

In this case, as Moonboots said, you can parent the mesh to the rig using Ctrl P "with empty groups"; then select all vertices of a piece (shortcut L and Ctrl L), select the relevant vertex group and click the "Assign" button in the vertex groups properties panel; repeat for all pieces of the robot.

In the Rigify rig the limbs are controlled by 4 bones each (i.e. DEF-upper_arm.L, DEF-upper_arm.L.001, DEF-forearm.L, DEF-forearm.L.001). You may want to select all undesired bones (all thoose with .001 suffix, plus breast and pelvis additional bones) and uncheck their "Deform" option before parenting.

You can find all deforming bones on bone layer 29 (counting from 00 to 31).

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