Long time no post! In the middle of moving house so not much progress... I've 3D printed a small scale test for the grabber arm though to test the mechanism. It seems to work well and should be able to open/close rather quickly with reasonable torque.

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I'm using a beetle size gear motor in this test, but its a 100RPM one, so its extremely slow! That paired with the small pitch of the threaded rod makes for a very boring video, so I'll spare you all from that. Will post a video/pics of a full scale test once I get my hands on appropriate gear motors (Looking at you Ellis ).