Summary

Duplo(TM)-compatible bricks with a ball-bearing swivel. Make a merry-go-round, globe, rotating castle, gun turret, or anything else that wants to turn.

Instructions

Requires a 608 ball-bearing set (~$0.60 on Amazon and other places if you buy ~10).

Tolerances may vary from printer to printer. Print out the bearing_test part to see if as-printed you get something that fits the bearing tolerably well. Then print out a bottom and your choice of top and press-fit the bearing to install.

The openSCAD file (requires Thing:2106, the Doblo factory) can produce the parts for the bearing_test, top, and bottom targets by commenting in and out the last lines. Likely adjustments are:
B608_play_outside, B608_play_inside: (more positive numbers give a looser fit on the bearing on the top and bottom half, respectively. Print out the bearing_test part for a more rapid iteration cycle than making the entire block each time.)
top_colrow: (rows and columns of nubs on top)
top_wheel: (if true, cut a circle for the top, otherwise leave square)

(I apologize for the crudeness of the code and any residual unused openscad from the development process. I tried adding little latching ears but it just caused the clips to break at the base. Maybe experimenting with latchdistance{in,out} will find a good compromise between breaking the part and having the bearing slip off.)

Doblo Spinny