Update: 1-28-14 Added .IPT files for your use and updated to v3 that includes some design refinements (tweaking tolerances) and set screw nut slot on input base. Recommend printing v3 files.
Update: 1-27-14 Added four holes on the CUP, just tap threads, insert bolts/screws, place washer inside CUP and add spring or small rubber piece, stack torque adapter atop spring, snap the base into the CUP, adjust bolts to set torque.
This is a motor coupling with 8mm shaft openings. This coupler is designed to slip when a certain torque is reached.
The torque limit is set by adjusting screws or installing different springs (size and strength) between the cup and adapter.
Install on 8mm shafts. Pick your spring (start with a lightweight one first) or small piece of rubber hose, pick out your bolts or screws, tap the four holes in the CUP, install screws, place washer in the CUP, install spring, insert adapter in the cup, Snap the cup assembly into the base and adjust screws/bolts to the torque needed.
I have tested with PLA using active cooling at 100% infill at .2 with supports.