Any free toy articulation examples / guidelines?

Hi there,

This is my first posting so take it easy on me. I'm looking for examples or guidelines for toy like articulations. Basically joints I think, like arm+shoulder. I want to have a toy concept 3d printed and looking for moving hands or head. I have a few things in mind but looking for, perhaps, something with more authority.

As a concrete example: if I go for a ball joint type of articulation, how can I make sure that the hand will be able to snap into the torso, is there a rule for the radius of the socket in relation to the spherical joint?

Please excuse my engineering grade vernacular.

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