I don’t see to find a way to key only one Axis. Even if we set Rotation as our main Key-Set, then all rotational curves are created: Qx, Qy, Qz, Qw. Even though I move my bone on the X axis only, Blender place Zero on the other fields creating those curves.
A work around obviously is to lock those unwanted channels, so no keys are written on them. But this process is tedious if you have many bones involved, because we need to create a first key, then lock the channels, and finally once we’re done then delete them to make those channels disappear. ( Well we do not need to lock if we are going to delete then, just to spot then easily by the empty spaces)
This is needed if you create different Actions constrains for the same bone that you would like to combine-MIX using drivers.:spin::spin::spin:
Does it make sense?